Generally it is noticed that while charging batteries people hardly pay any special attention toward the procedures. For them charging a battery is simply connecting any DC supply with matching voltage with the battery terminals.
How to Correctly Charge a Lead Acid Battery
I have seen motor garage mechanics charge all types of batteries with the same power supply source irrespective of the AH rating associated with the particular batteries.
That's gravely wrong! That's like giving the batteries a slow "death". Lead Acid batteries to a very extent are rugged and are capable of taking on the crude charging methods, however it's always recommended to charge even the LA batteries with a lot care. This "care" will not only increase the longevity but will also enhance the efficiency of the unit.
Ideally all batteries should be charged in a step wise manner, meaning the current should be reduced in steps as the voltage nears the "full charge" value.
For a typical Lead Acid battery or an SMF/VRL battery the above approach can be considered very healthy and a reliable method. In this post we are discussing one such automatic step battery charger circuit which can be effectively used for charging most of the rechargeable types of batteries.
How the Circuit Functions
Referring to the circuit diagram below, two 741 ICs are configured as comparaters. The presets at pin#2 of each stage is adjusted such that the output goes high after specific voltage levels are identified, or in other words the outputs of the respective ICs are made to go high in sequence after predetermined charge levels are accomplished discretely over the connected battery.
The IC associated with RL1 is the one which conducts first, after say the battery voltage reaches around 13.5V, until this point the battery is charged with the maximum specified current (determined by the value of R1).
Once the charge reaches the above value, RL#1 operates, disconnect R1 and connects R2 in line with the circuit.
R2 is selected higher than R1 and is appropriately calculated to provide a reduced charging current to the battery.
Once the battery terminals reaches the maximum specified charging voltage say at 14.3V, Opamp supporting RL#2 triggers the relay.
RL#2 instantly connects R3 in series with R2 bringing down the current to a trickle charge level.
Resistors R1, R2, and R3 along with the transistor and the IC LM338 forms a current regulator stage, where the value of the resistors determines the maximum allowable current limit to the battery, or the output of the IC LM338.
At this point the battery may be left unattended for many hours, yet the charge level remains perfectly safe, intact and in a topped up condition.
The above 3 step charging process ensures a very efficient way of charging resulting in almost a 98% charge accumulation with the connected battery.
The circuit has been designed by "Swagatam"
- R1 = 0.6/ half battery AH
- R2 = 0.6/one fifth of battery AH
- R3 = 0.6/one 50th of battery AH.
A closer inspection of the above diagram reveals that during the period when the relay contacts are about to release or move from the N/C position might cause a momentary diconnection of the ground to the circuit which in turn migh result in a ringing effect on the relay operation.
The remedy is to connect the ground of the circuit directly with the bridge rectifier ground and keep the ground from the R1/R2/R3 resistors attached solely with the battery negative. The corrected diagram may be witnessed below:
How to Set up the Circuit
Remember if you are using 741 IC then you must remove the red LED from the lower opamp and connect it in series with the base of the transistor to prevent permanent triggering of the transistor due to IC leakage current.
Do the same with the upper transistor base also, connect another LED there.
However if you use an LM358 IC then you may not have to this modification and use the design exactly as given.
Now let's learn how to set it up:
Initially keep the 470K feedback resistors disconnected.
Keep the slider of the presets towards ground line.
Now let's say we want the first relay RL#1 to operate at 13.5V, therefore adjust the LM338 pot to get 13.5V across the circuit supply line. Next, adjust the upper preset slowly until the relay just toggles ON.
Similarly, suppose we want the next transition to happen at 14.3V, ...increase the voltage to 14.3V by carefully adjusting the LM338 pot.
Then tweak the lower 10K preset such that RL#2 just clicks ON.
Done! your set up procedure is complete. Seal of the presets with some kind of glue to keep them fixed in the set positions.
Now you can attach a discharged battery to see the actions happening automatically as the battery charges with a 3 step mode.
The 470K feedback resistor can be actually eliminated and removed, instead you can connect a large value capacitor in the order of 1000uF/25V across the relay coils to restrict threshold chattering of the relay contacts.
Mudassir says
Thank you so much Sir,
Please can you help to understand how the charging current is calculated at three different levels.
Also most of the charge controllers today are using programmable IC, does that mean it is better with programmable ic , if yes can you please help us in the future to post charge controller design using such programmable ICs.
Thank you Sir may God bless you.
Swagatam says
You are most welcome Mudassir, programmable IC based chargers are used for enabling fast charging only, but normally these chargers are simply not required. The best way to charge a battery is through constant current and constant voltage for the full specified duration of time, which can be a bit slower but that guarantees longer life for the battery.
Sebastian Garrity says
Hi there!! Thank you so much for your design and taking the time to do this and read this comment. I am doing a project right now that is a battery charger for a robot and we would like to use the AC-DC converter of a laptop charger (19V 3A). Currently I have a DC-DC buck converter conected to it and it seems to charge fine but the batteries are AGM and I would like to extend the battery life as much as possible. Anyways I was wondering how this circuit would work if it was a DC input and a DC output and what changes I would need to make. Thank you for your time!
Swagatam says
Hi, yes you can definitely use the above design for charging AGM batteries. Here too the input is DC, from the bridge rectifier, so for your case you will just have eliminate the transformer and the bridge rectifier. The LM338 can provide upto 5 amp current, for higher currents you can replace it with a LM396, or upgrade the LM338 with an outboard transistor
Edmund Campbell says
Hi, just looking at your design it looks like it could work for me. I need to charge a 12 volt 75ah AGM battery, and need a charger that can be incorperated in a small portable inverter power supply unit. Could this circuit be modified for a maximum charge current of at least 8 amps?
Swagatam says
Hi, yes you can increase the current either by connecting two LM338 ICs in parallel over a common heatsink, or by replacing the LM338 with a single LM196 IC
or this one
https://www.analog.com/media/en/technical-documentation/data-sheets/1038fa.pdf
Edmund Campbell says
Thanks Swagatam, Re the LT1308 I see it is now an obsolete item. I will investigate further for a decent replacememt. Right now I am going for a more basic charger and as time permits I will put an improved one in the system. As I am using AGM batteries I need a charger that I can trust to give it a full charge and to keep it there, in a float charge function.
Swagatam says
No problem Edmund, the basic thing to consider is the over-charge cut off for any AGM battery.
Adeyemi says
I studied this article critically and I have a few questions
1. Why the need for the r1, r2… to reduce the current, since the current reduces as battery near full.
2. But at 13.5v the charging voltage should reduce from 14.4v to 13.8v for absorption mode
Swagatam says
Thanks Adeyemi,for studying the article deeply.
If R1. R2, are not used then the 3 step process will become meaningless, and the battery will get damaged due to high current.
Lead acid battery must be charged at minimum 14.2V to get an optimal back up. Absorption begins after 14 V mark. At this level the voltage does not need to be reduced, rather the current needs to be reduced to initiate the absorption and also the float charging.
Geo says
Hi Sir.
I want to ask some questions,
If I add 2 more steps, could it reduce the charging time?
If YES, could you design me for 5 steps of charging?
If NO, could you give me the reason?
THANK YOU, SIR!
Swagatam says
Hi Geo, No, adding more steps will not help to increase charging speed, and fast charging is anyway not good for any battery.
In fact, even a 3-step charging is not necessary for a Li-Ion. A standard 0.5C rate charging is quite enough and will ensure a healthy battery life.
Ultimately it’s the temperature level of the battery that becomes crucial factor while charging. Whether it’s step charging or normal charging the battery temperature must not exceed 37 degrees Celsius. However in step charging since the initial charging current can be significantly high (at 1 C rate) the battery may become quite warm which must be monitored and controlled either using a fan cooler or an auto shut down circuitry.
Therefore, in my opinion a simple constant current, constant voltage charger should be the recommended one for maintaining healthy charging and long life for the battery.
electronic_noob says
Hi, Swagatam
I am electronic noob trying to learning more. Sorry that some questions might look stupid to you. I am having difficulty in understanding the role or transistor (BC547) in the LM338 side. What is it for? If it is on, doesn’t it short Vout of LM338 to ground via 240 R? Moreover, why do you have diode in parallel with relay, is it necessary?
Thanks in advance
Swagatam says
Hi electronic noob,
Yes it will shut down the output when the current exceeds the set limit, and in this way it will keep limiting the current to the battery.
The freewheeling diode is for shorting the dangerous back emf from the relay coil each time it’s switched OFF, and for safeguarding the driver transistor from this EMFs
electronic_noob says
Swagatam many thanks for quick response. If you don’t mind, I would like to ask few more questions regarding to the circuit.
Is the current limit set by the value of R1? and i am not sure if i understand how to calculate correctly. So here it goes
To calculate R1, you stated that R1 = 0.6/ half battery AH. If battery is 100 AH, R1 = 0.6/50 =0.012 ohm? can you explain where 0.6 come from? and What would be power rating for this R1?
Really appreciate your time in teaching me.
Swagatam says
Hi electronic noob,
R1, R2, R3 are toggled sequentially by the op amp and the relays to regulate the current for the battery as the battery reaches toward the full limit.
The 0.6 is the optimal switching voltage level of the BC547
electronic_noob says
Swagatam Thank you so much!!
I understand, after reading through comments below, that this circuit is not really suitable for high AH battery. Actually, I am looking for a charger circuit that can fast charge lead acid battery with high AH. Do you think this circuit on your website (https://www.homemade-circuits.com/fast-battery-charger-circuit/) should work? If so, I will probably ask you several questions after I finish my reading on how the circuit works.
Best regards
Swagatam says
You are welcome e_n,
yes the fast battery charger circuit can be used for charging high Ah batteries in 3 steps.
lennox says
Is there any proof you have that this circuit works , if so could you point me in the right direction ASAP!? thank you sir
Swagatam says
I don’t have any proof, but I believe since it’s a simple and obvious design therefore will work without any doubts.
Glory says
Well done Swagtagam, I have a concern over best charging system, a friend bought a famous imported inverter but in 11months the 36v batteries went bad. They may have 3days or more uninterrupted power supply. What could be the cause of short life for battery because the charging has floating charging system as well.
Swagatam says
Thanks Glory, it could be either because the battery was originally a bad quality one, or the charging current was too high throughout its operational period ultimately destroying the battery, with float charge functional a battery can never get damaged…it will need to be checked thoroughly
Glory says
Good day sir, I read that safe charging current is 10-13% of the AH, what is the safe limit for discharge rate.
Swagatam says
Hi Glory, it’s exactly the same.
Glory says
Good day sir, please what’s your view on fast charging at 5C and cut off immediately when full. Is it OK. Most chargers now are fast charger, even in smartphone.
Swagatam says
Hi Glory, for li-ion it’s fine and recommended but not for lead acid batts.
Glory says
Thanks so much for your reply. Please does inductive load current draw have SIGNIFICANT effects on lead acid battery as per the battery AH -short burst. How
Swagatam says
It won’t have any effect on the battery but it can affect the mosfets or the sensitive electronic circuitry if the secondary section is not adequately snubbered
Glory says
Thanks sir, I meant I have 100ah battery, the inductive draw is 25amps but continuous current draw is 7amps, won’t the 25amps drawn have effects because discharge should be 10C of the battery AH
Swagatam says
Hi Glory, sorry I could not understand how can there be two consumptions separately, do you mean 25 amp peak and 7 amp RMS?
Glory says
25amps is the starting current for the inductive load while 7amps is the running current
Swagatam says
If it’s only for a couple of seconds then there’s no problem.
linx says
I have 1 electric car battery 12Ah, how to choose the voltage to charge it at first? Assuming I use 12vac transformer, how much should I adjust lm338 to voltage, and not use filter capacitor? thanks
Swagatam says
Without knowing the voltage rating of the battery it can be dangerous to apply the input
Glory says
Good day Swag, I saw the maximum charging current for a 200ah battery is 60amps, please how possible as against 10% ah
Swagatam says
Glory, the charging is relative to battery temperature, if you can keep it cool at 60Ah then it’s fine. So check the battery temperature while applying 60Ah if it heats up then it’s bad for your battery!
Glory says
Hi Swagatam, please how can one know quality battery, a friend got, an expensive highly acclaimed good quality battery. It is already going bad in 1year
Swagatam says
Hi Glory, there’s only one way of abusing and destroying battery life, it is by charging or discharging the battery with higher current than the recommended rate, so make sure the rating is never exceeded and your battery will live long.
Eric says
A very good method how to destroy a lead-acid batteri is to leave it (deep) discharged.
A flooded (or SLA) battery should always be stored fullt charged..
Glory says
Hi Swag, well done sir, please how can I build this circuit for my inverter/charger system. The inverter changes to charger when there is utility. Or any simple way.
Swagatam says
Hi Glory, you will have to replace your existing charger with this one.
Glory says
Sir, do you mean the inverter can not be automatic in changing from inverter to charger. I think The positive and negative line on the battery with this circuit serve for inverter during changeover. I see the the relays, how can it be compatible with inverter at same time.
Swagatam says
Glory, the relays in the above diagram has nothing to do with the changeover. I am not sure about your changeover circuit but I can suggest a separate method.
You will have to use another relay. Break the connection between LM338/240 ohm junction and the battery positive. Connect the pole of the relay with the battery positive, N/C with the inverter positive and the N/O with the LM338’s 240 ohm junction. Connect one end of the relay coil with its N/O and the other with the bridge rectifier ground. Connect the common battery ground line with the inverter ground.
Glory says
Good day sir, please how can i latch this circuit below. at preset battery full voltage the relay flickers
Swagatam says
Hi Glory, you an connect a 10uF/25V across base/ground of the BC547, and check the response
Glory says
Ok sir, done but no difference sir
Swagatam says
connecting capacitor across base/emitter of the BC547 will definitely prevent relay chattering, try increasing the value to 100uF…also the 470K should latch the relay, once the relay switches ON, decrease the 470k to 100K for quicker latching…another option to ensure perfect relay operation is to add a 10uF or similar capacitor across pin#3 and ground of the op amps.
Nikhil Mer says
Hi Swagatam , You always publish useful posts . Doing a great job everytime .
My query was between battery charger using pwm is better or the circuit above . which will yield more battery life .
Swagatam says
Thank you Nikhil,
The above charger will help to charge a lead acid battery relatively quickly and also maintain a reasonably good life, however charging a battery at a faster rate is never good no matter what technique is used. If long battery life is the main priority then a battery must be always charged at the lowest possible amp rate and for maximum possible time.
Using PWM will prolong the charging time, a continuous DC is a better option, PWM may be useful for preventing sulfation in a battery, in that sense it can be useful
Nikhil Mer says
One more question , I am using this charger circuit for my UPS , so can I use the same transformer for inverter when mains are not available (primary 24v to 230v step up) and for charging when mains are available (secondary 230v to primary 24v step down) . I am using H bridge configuration for onverter section .
Please help me with this I am stuck at this question .
Swagatam says
yes that’s possible my manually removing the transformer and configuring its wires accordingly
Tolu says
Thanks for the enlightenment, I have this questions.
1. Long battery life is my top priority. Please more information on this. What’s the lowest possible current and voltage for this priority.
2. Between the above circuit and auto-cutoff circuit which is better to achieve this priority.
Swagatam says
You can charge your battery with a current equal to 1/10th of its Ah rating for 12 hours manually, or if an automatic cut off is used set its cut off voltage at 17% higher than the printed voltage value of the battery, current should be always 1/10th of the Ah
You can just follow the above and achieve an ideal battery charging prcedure
Tolu says
Thanks sir, I made a normal charger with trafo and rectifier, I checked the rectified voltage after filtering with my inverter it showed 21v but with the ac mains it was 14v.
1.why
2. How can I check the voltage with my inverter accurately
Swagatam says
Tolu,
the AC voltage that you are seeing is the average voltage or the RMS voltage, after rectification it becomes DC and due to the capacitor storing voltage and discharging during OFF time of the AC cycles, you are able to see the peak voltage.
The formula for converting RMS to peak is root2 x AC voltage
therefore 1.41 x 14 = 19.74 (not 21V)
therefore at 14V AC you might see around 20V in your meter.
to charge a 12V battery you can use a 0-12V transformer which would enable you to get 1.41 x 12 = 16.92V ,and this can be used for charging your 12V battery if an auto cut off is included in the charger
Tolu says
Well done sir,for my inverter-charger as the battery is getting full, the charging current steps down to zero without any automatic charging circuit added, trafo just connected to mosfet with rectifier diode.what phenomenon is that. Please explain
Swagatam says
Tolu, all batteries will restrict charging as soon as the set maximum set voltage is reached, meaning when the terminal voltage of battery becomes equal to the charger’s maximum set voltage the pressure becomes zero, and no current is able to push. As long as this level is slightly lower than the full charge level of the battery it is fine, otherwise an auto cut-of must be employed
Tolu says
Thanks for the quick reply, does it mean the charging voltage should not be set at high level of 14.4v, or what voltage charging level do you advise for battery safety.
Swagatam says
if you have an auto cut off feature with your charger or if you are sure to remove the battery manually after the 14.4V is reached then it is OK, otherwise not
Tolu says
Good day sir, is it possible to remove lm338 completely and how can this be achieved, what is the disadvantage of this.thanks
Swagatam says
Tolu, LM338 is responsible for controlling the current so removing it may not be possible, unless we replace with some other alternate current control option.
Tolu says
I have completed building the design, but so difficult to test if it is working, and difficult to set the charging parameters, please help out Sir.
Swagatam says
Tolu, I have updated the setting up procedures at the end of the post do it exactly as mentioned.
Tolu says
Thanks so much for the update, as you mentioned can I just replace ic 741 with 358 with same circuitry.
Swagatam says
LM358 has two opamps in it, please check the pinout diagram of the IC and replace the pins accordingly. The pinout diagram can be found in the datasheet of the IC
Tolu says
Thanks for your response, I use ic741
1. Can I set it without a battery as load
2.can I use it for a 6v battery any modification
Swagatam says
It has to be set without a battery, if you connect battery it will give wrong results.
yes any battery between 6V and 15V can be implemented using IC 741…no modifications are required
Tolu says
Thanks for your response, the led intensity connected to the transistor base varies as I vary the 10k preset for relay #1, but the relay doesn’t click and the collector does not trigger, it reads in millivolts, please any solution.
Swagatam says
The LED should switch ON and OFF sharply, and not with dimming or fading effect, either your IC may be faulty or there could be some other fault in your circuit connections…It can be difficult for me to troubleshoot it from here
Tolu says
Yes, that was certainly how it did, the led light was on and went off sharply but the transistor collector reads in mV, and no trigger for the relay. Any reading on collector side.
Swagatam says
connect an LED with 1K series resistor across collector of the transistor and positive line and check the response, if the LED does not switch ON OFF in response to preset variations then your transistor may be faulty or wrongly connected
Tolu says
The led in series with the collector comes on and goes off sharply in response to the preset but the led in series with the base goes off completely. Please more advice. Please what is the function of 4.7k preset.
Swagatam says
The 4k7 resistor is for adjusting the output voltage to the battery from LM338 IC, you can use 10K also instead.
See the first circuit here to understand more about LM338 power supply:
https://www.homemade-circuits.com/ic-lm338-application-circuits-explained/
Tolu says
Thanks for your quick response, please according to the led response as above is the finding OK.
Swagatam says
If your collector LED is responding to preset adjustment then the opamp is working OK, now you must check why relay is not responding…
Tolu says
You said 741 ic can be used for voltages between 6v and 15v, I want to work with 24v battery how will do it. Thanks
Swagatam says
use LM321 or LM358
Tolu says
Hello Swagtagam, with a charger made from trafo and rectifier, and charging current tends towards zero, you said an auto cut off should be added, please i want to ask will zero current with further charging still harm the battery.
Swagatam says
Hello Tolu, yes it will harm due to continuous chemical reaction in the battery. However If you keep the overcharge voltage slightly lower the specified level then probably it won’t harm.
Tolu says
Thanks Swagtagam, please how is trickle charging system different from zero current charging with
Swagatam says
Tolu, There’s nothing called zero current charging, or may be it is another name for trickle charging.
Tolu says
https://www.homemade-circuits.com/current-servo/
For this circuit how can I convert it to use p channel MOSFET instead of n channel
Swagatam says
Tolu, Glory, favour, you can swap the input pin 3 and 2 with each other…the source and R3 will now connect with the positive, and drain/load with negative..
Tolu says
Hello Swag, please what is Voltage Controlled Current opamp circuit as battery charger, how does it work
Swagatam says
Tolu, please show me the link of the schematic I’ll try to guide you. Because opamp is used for cutting off supply at full charge, and current control is used for ensuring a safe charging for the battery as per its specification, so both elements work independently.
Tolu says
https://www.homemade-circuits.com/current-servo/
Swagatam says
It’s a current controller circuit, the resistor is the sensor and the opamp is use a comparator for cutting off supply when the current exceeds the specified limit
Tolu says
Thanks so much, which resistor is the sensor.
2. It is used for charging battery, which limit is exceeded sir, because it is used for trickle charge.
3. How is it connected to relay to cut off charging
Swagatam says
R3 is the sensor. When current exceeds, the voltage developed across R3 increases until the pin#2 potential goes above pin#3 reference level, causing pin#1 output to go low and cut off the mosfet. No relay is used her.
amal says
can you please help me to know much details as my battery is 24 volt 18000mah battery which is used for a electric cart.can you please give a circuit for that as well as the components specification.
Swagatam says
no need of making the above complex design, you can simply use the 1st design presented in the following article
https://www.homemade-circuits.com/how-to-make-current-controlled-12-volt/
just make sure to replace LM317 with LM338 and also make sure to adjust R2 to get a precise 28V (not more than this) at the output for charging the battery.
The input supply must be rated at 30V, 3 amps
amal says
can I know how to design a 3 stage battery charger for 24 v battery
Swagatam says
In the second circuit replace 4k7 pot with a 5.1k fixed resistor
Love says
Thanks for this 3-step automatic charger, how can I incorporate 3-step charging system for 48v charger
https://www.homemade-circuits.com/2012/08/make-this-48v-automatic-battery-charger.html
The above circuit is auto cut off.
I want 3-step automatic charger 48v. Please help out. Thanks
Swagatam says
For a 48V battery the LM338 section will need to be removed and replaced with a transistorized current controller stage such as the following design:
https://www.homemade-circuits.com/make-hundred-watt-led-floodlight/
If possible will try to update the complete diagram soon..
Love says
Please how will I calculate R1 in this circuit for the 48v charger
https://www.homemade-circuits.com/make-hundred-watt-led-floodlight/
Thanks.
Swagatam says
Formula is given in the article itself…
tinuke says
Hello, please I need battery charging balancer for better battery life for a VRLA battery.
Swagatam says
how many batteries do you have in series?
tinuke says
Four 200ah batteries
Swagatam says
you can try this circuit
https://www.homemade-circuits.com/lipo-battery-balance-charger-circuit/
for 4 batts, disconnect pin#15 of the IC from ground and connect it with pin#1, all the outputs after pin#7 can be ignored and removed since the first 4 will be only used
tinuke says
Thanks sir, how can I incorporate the balancer into my automatic charging circuit .
Swagatam says
tinuke, you will have to read the whole article carefully to understand the procedures.
dtorresreyes25 says
Hi Swagatam, I’ve searching for a while a circuit like this (3 stage charger)and i found yours very interesting thanks for sharing your knowledge with us, God bless you!, I’m wondering if I could have a version of this circuit but using a lm317(i don’t have any lm338, and is very difficult to get in Cuba) for a >40AH batteries and if I could replace the lm741 with lM319 o lm318 instead, thanks very much for your quick reply….
Swagatam says
Thanks dtorresreyes25,
LM338 stage can be eliminated If the transformer is appropriately rated, and the supply can be directly connected to the shown circuit.
LM317 cannot be used because it will restrict the current to 1.5 amps.
yes, you can use single LM319 for the design instead of 2 LM741 ICs
dtorresreyes25 says
Greetings my friend!, thanks so much for you quick reply,
yes effectively I was thinking on using the LM317 with Outboard Current Boost Transistor Circuit ’cause
my first battery to charge with this design will be a 12v40AH so I guess the 1/10th of battery total capacity current charge will be 4 amps and it exceed the actual
capacity of 1.5 amps of LM317, but I wanted to know if I could use this 7812 configuration as you describe here -> https://www.homemade-circuits.com/2012/11/high-current-transistor-tip36-datasheet.html ???, so that I can keep with me the LM317 cause I’d like to use its regulator capacity in another project, so I’ve got two more questions for you:
1- Why in R1 formula -> 0.6/ half battery AH , do you use the half battery capacity it shouldn’t use 1/10th battery AH?
2- Somehow it could have a way of regulate the charge current rate when i’ll go to charge another batteries with diferent capacities or I have to recalculate the R’s every time for limiting charge current for each new battery AH capacity.
Swagatam says
Thanks friend, yes you can use the 7812 configuration for your application, or a LM317 with a similar outboard transistor.
In the 3 step charger, the initial current can be high, therefore R1 is calculated to provide 50% AH current.
for different AH rating you will have to alter the resistor values accordingly, the same values will not work efficiently if the AH ratings differ widely.
dtorresreyes25 says
Thanks so much ! you’re the best. have a good day my friend.
Swagatam says
You are most welcome!! cheers!!
mark kim blando says
Sir swagatam good day, kindly please post a power bank circuit for my cellphone. Thanks in advance. God bless
Swagatam says
Hi Mark, you can try the concepts presented in the following article
https://www.homemade-circuits.com/2015/11/make-this-power-bank-circuit-using-37v.html
FREEDOM WORLD says
Hallo Swagatam sir,
I'm interested in your multi stage circuit and i have doubt realted
to filter capacitor design(1000uf).
sir i used some calculation as below
capacitance C = I * T/ delta V
where i choose
max load current, I = 5A
time period ,T = 10ms (for bridge rectifire 1/f =100hz )
for delta V,
first ,peak voltage for 12v transformer
= V rms *1.414 =12 *1.414 = 16.9v
after rectifire drop out it become =16.9 – 1.4 = 15.6v
and minimum voltage for 12v battery charging = 11v
then delta V = 15.6 – 11 = 4.6v (max drop between peaks)
so, C = 5 * .01 / 4.6 = about 10000uf
if it is wrong, please help me for design ,Mohammed danish
Swagatam says
Hello Mohammed,
I could not check the calculations due to lack of time, but most probably it is correct, I am assuming this since a filter capacitor value ideally may be huge when calculated, however this is not required, in fact even if you remove the capacitor still the battery can be charged effectively just with the Dc peaks.
just for sake of confirming your results you can also cross check with this article
https://www.homemade-circuits.com/2015/11/calculating-filter-capacitor-for.html
MUTAMAKKIN BILLA, Lc., M.Ag says
Dear Swagatam… Good Day!
I'm still wondering the value of R1, R2 and R3. I have here 45 AH lead Acid Battery and wanna try this circuit. please do me a favor, what value then for R1, R2 and R3? Thanks in advance!
MakinBill
Swagatam says
Dear Billa, the formula is explained in the article.
for example the R1 formula is:
R1 = 0.6/ half battery AH,
for your case this can be solved as:
R1 = 0.6 / half of 45AH
= 0.6/22 = 0.027 ohm
watts of this resistor wil be = 0.6 x 22 = 13.2 watts or 15 watts approximately
mark kim blando says
Hi hello Mr swagatam. I feel sori for my last comment. That was wrong. I meant, the 10k potentiometer in pin no. 3, what is the voltage output and also the other 10k potentio? Thanks in advance mr swagatam
Swagatam says
Hi Mark, the voltage at the relevant pin#3 should be just higher than the pin#2 reference voltages when the battery reaches the specified limits as per the user's preference….for example if the opamp needs the relay activation at battery 12.5V then the relevant preset should be adjusted such that at 12.5V the pin#3 voltage becomes just higher than the pin#2 voltage as set by the zener value
Mark says
Hi hello mr. Swagatam how are you?im back again..let me ask you. Can you give the exact setting of potentio from lm338 and potentio from relay 1 and potentio from relay2?thanks in advance
Swagatam says
Hi Mark, that can be difficult and is not recommended, for perfect results the adjustments should be done and verified practically as explained in the article.
mark kim blando says
Sir swagatam hi hello.. im back again..i almost finish my project..let me ask you sir.. what is tha maximum AH to charge this kind of circuit? Thanks sir swagatam in advance
Swagatam says
Hi Mark, the AH capacity can be customized simply by modifying the current limiting resistors, the relay contacts, and the LM338 current delivering capacity…and you can virtually use any AH battery with it…the present set up will not allow you to charge over 50 AH battery
mark kim blando says
Hi hello mr. Swagatam im here again.. can I ask you? Can I use this circuit for 12V and 24V except for the R1, R2 and R3?
Swagatam says
Hi Mark, it is possible, just make sure to use a 741 IC rated to work with 24V or you can replace it with a LM321 IC
mark kim blando says
Hi hello sir swagatam im here again..ask again.thank you..sir what is the best voltage regulator to use for 48V and also capabale of high ampere?
Swagatam says
Hi Mark, I have suggested the link in the previous comment, you can use that circuit for your application, it will regulate the voltage as per the charge level of the battery and never allow over charge for the battery.
mark kim blando says
Hi hello sir..im here again..can ask one more question? What if I will make a 48V charger with 365AH, what would be the IC needed to replace in order to have a capability to charge?
And one more thing sir. What was that above of the R1 with two black dots opened? Thank you sir
Swagatam says
Hello Mark, for higher voltage and current, you can try the circuits from the following article
https://www.homemade-circuits.com/2012/08/make-this-48v-automatic-battery-charger.html
mark kim blando says
Sir swagatam pls.dont get tired replying us. We need you especially me still learning in electronics..thank you alot sir
Swagatam says
Sure Mark, it's not an issue!! do keep posting
mark kim blando says
Hi hello sir swagatam.. im back to say thank you for your kind hearted..believe sir, you will reap what you sow..if you sow a good thing you will reap a good thing.. God bless you sir and your family
Swagatam says
It's my pleasure Mark, God bless you too!!
mark kim blando says
Hi hello sir swagatam..can I ask you? Can I ask thhe example formula of R1, R2 AND R3 if I am making charger 48volts 200Ah? Sir thank you for this big help
Swagatam says
Hi Mark, the formulas can be the same as given at the bottom section of the article:
R1 = 0.6/50% of battery AH rating = 0.6/100
R2 = 0.6/20% of the battery AH rating = 0.6/40
R3 = 0.6/5% of the battery AH rating = 0.6/10
Swagatam says
Hi Jade, sorry both will not do….but you can use LM338 in parallel as shown in this article,
https://www.homemade-circuits.com/2012/11/make-25-amp-solar-battery-charger-power.html
or go for an outboard transistor current booster as shown below:
https://www.homemade-circuits.com/2012/05/make-this-voltage-stabilizer-circuit.html
Jade Mark Talaboc says
Hi Swagatam,
Can you show me the modifications of this circuit if i'm going to charge 300AH lead acid battery? Please help me. Please show me also necessary calculations.
Thanks,
Jade
Swagatam says
Hi Jade,
The relay contacts must be rated at over 40 amps in your case, and the relay driver transistors must be upgraded to 2N2222 or 8050… also the input current from the transformer to the circuit must be around 35 amps.
Rest you will have do by yourself since I have elaborately explained the details in the article.
Jade Mark Talaboc says
hi swagatam,
thanks for the reply. can i use this to charge 2 X 6V battery?
Swagatam says
yes you can do it.
Jade Mark Talaboc says
Hello Mr. Swagatam,
Thanks for the very fast reply. Let me verify if i understood it correctly
R1 = 0.6/ half battery AH
R2 = 0.6/one fifth of battery AH
R3 = 0.6/one 50th of battery AH.
if AH is 4.5
R1 = 0R27
R2 = 0R67
R3 = 6R67.
By the way sir, do you have any calculations for the exact preset values of all the variable resistors so it wouldn't be a waste of time tuning up.
Does this circuit comes with an overdischarge protection sir?
Regards,
Jade
Swagatam says
Hello Jade,
those values are arbitrarily chosen, you can change them differently for getting the most optimal charging results, although you can stick with the mentioned values to start with, later on you can alter the values with some trial and error.
I cannot suggest fixed values and it will need to be confirmed manually…by the way the setting up procedure is a crucial part of the design..it cannot be considered a waste of time
the circuit includes a trickle charging facility which stops it from self discharging
Love says
Please which area is involved in trickle charging , which resistor
Swagatam says
R3 resistor is responsible for the trickle or float charging
Sandeep Singh says
Hello Swagatam,i am facing a problem with the the circuit above.
instead of using lm338 iam preferring lm317 as the current requirement is low( for charging 5ah battery)each time i try to adjust 4.7k pot to regulate voltage it burns out .i have changed 6 lm317 so far.using 18v-700ma transformer with 50v,100mfd cap across bridge rectifier.
could u pls help me finding out the bug.
Thanks in advance
Sandeep
Swagatam says
Hello Sandeep, there are two things that looks doubtful in your circuit. first is you might have connected the preset wrongly causing the the supply rails to short while adjusting it…secondly, LM317 have internal short circuit protection hence under any case it can never burn except until the input voltage is raised to above 40V.
so it appears your IC could be duplicate, and/or your preset is not correctly wired. please check these again and do it only once you are absolutely sure about the procedures
Sandeep Singh says
Thanks for such a prompt reply Swagatam,
as far as connection of preset is concerned it is ap per the schematics given in this article i.e pin-1 to adj of lm317,middle or pin 2 & 3 to ground.
in order to protect the ic from damaging effect of filter cap i have used IN4007 across Vin-Vout-Vadj pins.
the most surprising observation is that voltage regulation is perfect for 7-8 hrs & when the circuit is re powered; Vadj,Vout,Vinput readings become same.
If 4.7k preset is turned at this state ..it simply starts burning.
i also doubt genuineness of LM317 available here at rourkela,odisha @ Rs 10 per pc
Sandeep Singh says
May b this 50v,1000mfd capacitor is raising voltage above 40v…i am not sure,need your suggestion.
Swagatam says
Hi Sandeep,actually I thought you were referring to the opamp preset but anyway even it's the LM317 pot it should have no any effect of the IC no matter how its connected, so definitely its the IC that may be problematic…you can remove the diode across its output and input pins, its not essential and has nothing to do with the filter capacitor…the diode is required only if there's a situation were the output of the IC has a capacitor and the input of the IC is shorted with ground, which is mostly unlikely
Swagatam says
…18V transformer will never produce 40V at any cost regardless of the filter capacitor….but if the trafo is rated at 24V then it could be an issue and must be verified with a meter….
Sandeep Singh says
Thanks for your help friend..
Tulisan Org says
good day sir ! can i ask how to adjust the two 10k pot on the op amp? what is the proper settings? thanks!
Swagatam says
Hi Tulisan, adjust the RL1 preset to activate the RL1 relay at around 13.5V, and adjust the RL2 preset to activate it around 14.3V
this are for a 12V battery
luke opuri says
Hallo Swagatam, I'm interested in the switching modules of your circuit. How do the two opamps (ic 741) work?
Swagatam says
Hi Luke, I have explained it in the article, please read it again
luke opuri says
operation of the two comparators. I mean, what voltages do they compare so we can get the output that will switch on the transistor?? Could you please explain to me precisely.
Swagatam says
the tripping voltages can be approximately set, because it's not critical….RL1 may be set to activate at 13V or 13.5V, and RL2 at 14V or 14.3V.
This might need to be done by using an actual discharged battery, and by setting as soon as the battery reaches the preferred voltage levels or by using some kind equivalently rated dummy loads
luke opuri says
I read it, but I wanna know what voltages are compared. What's my reference voltage in this case? What adjustment should I make so that, relay one operates at say 12.8 volts and relay 2 at 13.3 volts?
Swagatam says
the zener diodes determine the reference voltages and the 10k presets needs to be adjusted for setting up the triggering thresholds
luke opuri says
Hallo Swagatam, that is a great idea!! I love it
I'm using a 30Ah battery, is it wise to use a 2.5 Amps transformer? Also, can I use a less rated adjustable voltage regulator?
I'm asking that, because I'm getting about 1.85 Amps (maximum current through resistor R1)
Swagatam says
thanks luke, in the above concept for a 30ah battery minimum 5 to 8 amp transformer would be recommended…and same with the IC, it should be a LM338 so that 3 to 6 amps can be optimally produced for the various stepped charging phases
luke opuri says
I have one last question, I've made the circuit and my output voltage from the regulator is 22.6 volts. Input voltage is 35.8 volts. How is this output connected to the relays, because I guess output voltage is too high and could cause damage?
Swagatam says
you must adjust the output voltage to the charging level of the battery and the relay voltage rating should be selected to match approx this voltage level…
the voltage can be adjusted by varying the the 4k7 pot (use 10K pot instead for a wider range)
luke opuri says
Hallo sir, I have one more question.
After putting up the circuit, I've realised that the voltage output to the battery gradually reduces from 14 volts down to 12 volts. What could be the posible cause for that?
luke opuri says
I realised the voltage drops after connecting the relay circuit to the output of my regulator. Also, I've found out that the output of the comparators ain't high to turn on the bc 547 i.e. when varying the voltages to pin 3
luke opuri says
Thank you so much Swagatam.
At last, I managed to build a small battery charger for myself using your designed circuit.
I would like to point out something, that it may require some advancement particularly, it should have a different signals to show connection of the battery to the circuit and also be able to indicate power supply. I mean, I noted that it was giving out the same signal either when power or battery is connected.
Swagatam says
hello Luke, something is wrong in your circuit, when you connect the battery to the 14V output, initially the 14v should drop to the discharged battery's voltage level but once the battery begins getting charged this voltage should gradually rise and finally attain the 14V mark at full charge level.
Swagatam says
remove the 470k while setting up the 10k persets of the opamps….connect it back once the setting up is completed….the setting should be done by connecting dummy loads selected to consume different current levels just as the battery would do.
Swagatam says
Hi Luke, by "signals" do you mean to say LED indicators? OK, I'll try to update the circuit with the LED indicators for the different changeovers during the charging process…
luke opuri says
Yes, I mean the change over from one stage to another.
Swagatam says
you can connect LEDs across the opamp output pin in the manner show here:
https://www.homemade-circuits.com/2012/07/make-6v-4ah-automatic-battery-charger.html
Swagatam says
Navjot, if you connect resistor with the ADJ pin, the whole circuit will become useless.
I think the transistor that you are using is faulty, change the transistor with a good quality one and then check the response, or there could be some fault in your circuit connections.
navjot singh says
helo sir…
thanku for your reply..
sir when collector of transisitor connected with adj pin of regulator that time output goes low…
but when i used a resistor of 120k between the collector and adj pin then output didnt gone low..
is it ok or not?
Swagatam says
Please explain the entire procedure and the current voltage parameters that you have utilized for the circuit, what you are suggesting can happen in a condition where the transistor may be faulty or if the circuit is configured wrongly.
Swagatam says
hello navjot, it's the optimal base trigger level of the BC547 transistor, so when 0.6V is reached the BC547 can be expected to conduct fully and ground the ADJ pin of the IC LM338
navjot singh says
hello sir…..
can you please tell me the meaning of .6 in the formula which we are using to find the value of resistors..?
Swagatam says
Hello navjot, the transistors are used for activating the relays in response to the different voltage levels during the charging process, no switch mode technology is used here.
navjot singh says
helo sir….
can you please explain the purpose of transistor in this circuit??
are we using as switch mode or any other?
Anandan Devendran says
Hi Swagatam,
Thanks for reply i will do the same 🙂
Anandan Devendran says
Hi Swagatam,
As suggested i changed the resistor on R3 but notice RL2 switch before it reaches 14.3v, i have aligned RL2 for 14.3 v properly.
when i checked voltage on LM324 VCC it was 14.3 and at battery it was 13.7. so as we have changed the circuit ground path to the output of transformer, thus the preset ground need to be changed?, i mean connecting the preset ground point to battery ground instead of transformer o/p as there is voltage difference cos of voltage drop on R3.
one more thing i noticed on both relay its get little hot when when voltage is 13.5 and above is it fine or we need to add a 12v zener ?
Thanks & Regards,
Anandan
Swagatam says
Hi Anandan, in that case you can simply readjust the preset such that RL2 cuts off when the battery terminal voltage reaches 14.3V, that would probably mean at 15v, or 15.2V on the circuit.
relay getting warm is quite normal, you can try adding a 1N4007 diode in series with the relay coil to keep it cooler
Anandan Devendran says
Hi Swagatam,
I was monitor the battery charging and noticed when the voltage reaches 13.5v it switch the RL1 and when it reaches 14.3 volt it switch the RL2 but when the RL2 is trigger the battery voltage start dropping and the RL2 goes again in normal mode (N/C) which disconnect the R3 and this keeps happening in every minute frequently RL2 On & Off, is this the normal operation expected from this circuit or do we need to make any changes?
Thanks & Regards.
Anandan.
Swagatam says
Hi Anandan, it could be happening due to a relatively higher R3 value, try other lower values and check until you find the right value which is able to hold the voltage in that position for the battery.
Anandan Devendran says
Hi Swagatam,
I cut track and change the ground link to the input of LM338 on the rectifier side now alignment is working fine :).
I will monitor the charging of the battery and update soon.
Thanks again for helping me.
Anandan.
Swagatam says
You are welcome Anandan, do keep in touch.
Anandan Devendran says
Hi Swagatam,
Thanks so fast reply helping me 🙂
Thanks & Regards,
Anandan.
Anandan Devendran says
Hi Swagatam,
Thanks for reply.
I have already tried putting a high value capacitor across the relay only changes it decreased the ringing frequency e.i if i put 100uf it switch on & off 5 time a sec and if i use 470uf it switch 2 time a sec but switching doesn't stop.
Now i added 2.2uf cap on base of the relay transistor it started ringing very fast didn't solve the issue.
I also noted while charging the battery through solar panel when it reaches 13.5 volt the RL1 start ringing for sum time like 8 to 10 switch On & Off and then come to normal possession.
Thanks & Regards,
Anandan,
Anandan Devendran says
Hi swagatam,
I tried shorting the black dot and emitter it stopped ringing so i traced the R1, R2 & R3 it connected properly R1 to RL1 N/C point R2 to RL1 N/O point and R3 to RL2 N/O point also the continuity is proper but still RL2 ringing 🙁
Correct me if i understood wrong when u said adjust the voltage to 13.5v and tune the preset does it mean adjust the LM338 output to 13.5v and tune the RL1 preset and then adjust the O/P to 14.5v and tune the RL2 preset ?
Thanks & Regards,
Anandan
Swagatam says
Hi Anandan,
yes, for setting up the thresholds the LM338 pot has to be used, so you got it right.
If RL2 is ringing then RL1 should as well ring, so that's confusing, and adding a high value capacitor across the relay should have corrected the issue
Try putting a capacitor across base and ground of each transistor, this should hopefully solve the issue.
The value of the capacitor can be anything between 1uF and 4.7uF
Swagatam says
…if the problem still persists then may be we'll have to change the relay contact configuration so that while the contacts operate the ground link to the circuit does not get disconnected even for a fraction of a second.
Swagatam says
Hi Anandan, I think I have located the fault, the ground of the circuit needs to be connected separately to the main ground which comes from the rectifier.
The ground link which terminates from R1/R2/R3 must go only to the battery negative.
I'll update the new diagram soon.
Anandan Devendran says
HI Swagatam,
As per ur suggestion i tried adjusting the circuit but when i increase the voltage to 14.3v and try to adjust the RL2 preset its starts ringing on & off, i have also connected the capacitor on both relay 100uf 25v .
kindly suggest what could be done.
Thanks,
Anandan.
Swagatam says
Hi Anandan, that should not happen, check R1, R2, R3 connections and there continuity, or temporarily short the emitter base black dots of the BC547 associated with these resistors and adjust again, if the ringing stops would indicate a wrongly connected R1,R2,R3.
krishna shah says
hi sir, you are doing the wonderful job ,i really appreciate your knowledge and work .i could not understand the use of bc547 located just above the R1 .will this affect the circuit ???
Swagatam says
thanks krishna, the indicated BC547 is responsible for interacting with the relay contact changeovers and implementing the subsequent current limiting actions….so in a way it's the heart of the entire design
Eddy Moulton says
I want to charge a 12v battery using a car alternator (so 14.4v). Is this circuit worth modifying for that? or should I be looking for some other solution?
Swagatam says
nothing will need to modified except the trafo which will have no role in your application…use a 10k pot instead of the shown 4k7 for setting up the LM338 voltage…
Eddy Moulton says
Thanks for the response. The datasheet for the LM338 says the dropout voltage is up to 3v, meaning the maximum output is somewhere around 11.4-12v for a 14.4v input, correct?
Swagatam says
according it has to be just 0.5V higher than the intended output voltage from the IC.
Anandan Devendran says
Hi Swagatam,
Thanks a lot for reply
I will do the same as you have mention above and let u know the result. 🙂
Thanks & Regards,
Anandan
Anandan Devendran says
Hi Swagatam,
For alignment of the preset for RL1 & RL2 i connect the discharge battery and kept the both the relay in normally connected mode and as the battery reached 13.5v i align the preset so the RL1 get trigger and waited till the batter voltage reach 14v and aligned the preset for RL2 so it get trigger. Battery gt fully charged up to 14.4v 🙂 but after i discharged the battery and started charging back again the RL1 started ringing after the battery reached 13.5v 🙁 please correct me if i did any thing wrong.
Thanks & Regards,
Anandan
Swagatam says
Hi Anandan, use the method as described in the previous comment for setting up the circuit and put a 100uF/25V across both the relay coils for eliminating the ringing sound.
Anandan Devendran says
Hi Swagatam,
Thanks for reply.
I have designed PCB for this circuit and all component are on it i have put a fix resistor in place of 4.7k preset (2.2k & 330ohms in series) i get 14.5 v at LM338 output.
Now i want to align the preset for RL1 & RL2, can u please explain in short how to align?
Thanks & Regards,
Anandan
Swagatam says
Hi Anandan,
Initially keep the 470k resistors disconnected, then feed 13.5V to the circuit and adjust the upper 10K preset such that the relay RL1 just actuates at around 13.5V
Next, increase the volts to 14.3V and adjust the lower 10k preset such that the RL2 just activates at this volts.
Now reconnect both the 470Ks, the setting is done.
Do the above without connecting any battery.
dtorresreyes25 says
Hi Anandan Devendran, sir can I have the PCB design you did of this circuirT? thanks for sharing it for all of us, have a good day!
Anandan Devendran says
Hi Swagatam,
Thanks for reply.
Currently i am using LM338 but i noticed when the battery was fully discharged to 10.5 v
and kept for charging even the LM338 on heat-sink was getting hot.
I see most of the people using LM317 for 7ah battery, will LM317 work for charging the 7AH battery or do u recommend to use LM338 only.
Thanks & Regards,
Anandan
Swagatam says
Hi Anandan,
LM338 is rated to handle upto 5amps and LM317 only upto 1.5amps…so if LM338 is getting hot LM317 will be even hotter…moreover LM317 will allow charging at the rate of 1.5amp max, therefore LM338 looks much better than LM317
Anandan Devendran says
Hi Swagatam,
Thanks for the reply.
I have 0.47 ohms resistor is it ok in place of 0.42 ohm for R2?
I will be using solar panel instead of transformer which is rated 17volt 25watts, as i see in the above diagram trafo spec is 0-24v will the solar panel rated 17volt will work?
Thanks & Regards
Anandan
Swagatam says
Hi Anandan,
yes 0.47 will do, the values are not critical because the recommended charging phases are not critical either.
17V will be enough to operate the proposed system…
Anandan Devendran says
Hi Swagatam,
I have assemble the above circuit, i am using 12v 7Ah battery. As per the above calculation for R1, R2 & R3 i am getting the (R1 value 0.17 Ohm). (R2 value 0.34 Ohm) & (R3 value 0.68 Ohm) please correct me if i am wrong in the calculation.
Thanks & Regards
Anandan
Swagatam says
Hi Anandan, as per the given suggestions in the article, R1 is correct, R2 should be around 0.42 ohms, and R3 = 4.7k
Joevil says
Hi sir swatagam,
is the circuit can be modify to charge a 12vdc 100ah deep cycle battery?
if ok, please tell what are the modification should i do.
thanks in advance.
Swagatam says
Hi Joevil, the above circuit is not applicable for above 50AH battery.
you can try the following one:
https://www.homemade-circuits.com/2011/12/high-current-10-to-20-amp-automatic.html
charu says
Hey Swagatam,please tell me how to make this 3 stage battery charger using microcontroller instead of using those 2 comparators?
Swagatam says
Hi Charu, I do not have the codes presently, if I happen to get it will post it for you.
Karthick M says
Hi, im interested in doing a simple project of controlling home electrical system fan, tube light through remote control. can you pls help me by your simple circuit diagram so that it is very helpful and easy for me to do.
Swagatam says
Hi, You can try and use the design which is shown in the following article
https://www.homemade-circuits.com/2013/07/simple-100-meter-rf-module-remote.html
It will allow you to control 4 different appliances separately
Soorya says
Sir can i reduce a 25 A dc current to 5 A or less using LM196 IC????
If yes how can it be done ?????'
Swagatam says
yes you can do it.
Swagatam says
…please refer to this post, it's explained there:
https://www.homemade-circuits.com/2013/06/universal-high-watt-led-current-limiter.html
Nazriya nazim says
Can you please suggest me a NTC THERMISTOR having resistance of atmost 500 ohm and power dissipation of 40 J SIR????'
Sir how can i depict the Joule rate in terms of wattage?
I have found many specified with only Wattage not with Joules.
How much wattage curresponds to a 40J rate. Is it 40 W or 40 mW????
Swagatam says
Hi Nazriya,
You will have to check the datasheet of an NTC thermistor manufacturer for getting the required value, I do not have the info right now.
Here's the formula for converting J to W
The power P in watts (W) is equal to the energy E in joules (J), divided by the time period t in seconds (s):
P(W) = E(J) / t(s)
Ronald Mugavana Mugavana says
By the way, am I right to assume that the same formulas above are to be used in computing the R1, R2 and R3 Resistor values, i.e.
R1 = 0.6/ half battery AH
R2 = 0.6/one fifth of battery AH
R3 = 0.6/one 50th of battery AH
Again I thank you.
Swagatam says
You are welcome.
Yes, The formulas will be exactly as shown in the article.
Ronald Mugavana Mugavana says
Thank you very much much for your quick reply. I will now embark on building the charger.
Your help is greatly appreciated
Ronald Mugavana Mugavana says
Good evening, sir, and thanks a lot for this very educative blog.
I would like to build a charger for a 12V-66AH Automotive battery based on the above schematic.
I have done the following modifications:
1. Replaced the LM338 with an LM196 Adjustable Voltage Regulator
2. Replaced the 4no. Rectifier Diodes 6A4 with a single Bridge Rectifier KBU1000
3. Installed a 10A05 Rectifier Diode across each of the Relay Coils
Please advise me if there is anything else I may have overlooked.
I thank you in advance.
Swagatam says
Thanks Ronald,
Everything's rightly done, just be sure you use the correct calculated value resistors for R1, R2, R3.
Devaraj Mohatha says
Sir a small request is there, please carry on reading…….
Sir i am very much tired of searching for those low value resistors ( of the order of 0.01 ohm ) in the internet. I need some few of them, but all of the websites that i searched want me to buy an atleast quantity which is predetermined by them and at the highest rates i have to pay. If anyone is gonna make this circuit, they will probably meet this dilemma for higher values of battery Ah.
SIR as per your design, the more higher the battery Ah, lower will be the resistance values ( R1, R2 and R3 ). So could you please suggest us a more better circuit utilizing almost all features of this one, but having no such low valued components included also having the feature of charging higer Ah batteries too.
( As per your previous reply to ma comment i will not get 0.01 ohm resistor with ten 1 ohm resistors. Please not it……. I will have to take hundred instead of ten. It will not be feasible sir )
I HOPE A QUICK REPLY SIR………
Swagatam says
I just forgot to tell you that LM338 wil not hold more than 5 amps, meaning more ta 50 AH batteries can't be charged with is circuit.
A transistorized current controller can be replaced for LM338, but even it will require the same low value resistors.
One solution is to use resistor divider network which will allow us to use higher value resistors.
I'll try to update the article soon.
Devaraj mohatha says
Dear sir,
Are those resistors marked as R1 to R3 are necessary in the circuit ? What are their real functions in this circuit ?
The 0.01 ohm resistors are not at all available in our area. Can you please suggest me a website from which i can buy that in smalk quantity ?
WHAT TYPE OF 0.01 OHM RESISTORS I HAVE TO BUY ?
I am asking this because i have seen different types of this resistance in internet such as wirewound, thick film, current sense resistor, surface mount type etc. Which should i prefer most. Please help me
Swagatam says
Dear Devraj,
R1, R2, R3 determine and confirm the 3 step charging process, without them the circuit will lose its purpose.
You can put 10nos 1 ohm 1/4 watt resistors in parallel for getting the 0.01 ohm value, by the way R1, R2, R3 will need to be calculated as per the battery being used….did you calculate them as per your battery specs, everything's comprehensively explained in the article, pls read it carefully.
Devaraj mohatha says
But sir, one doubt whether power will get added on paralling those resistors. I mean whether i will get 2 or more wattage rate for the entire resistor bunk by placing them parallel. Anyway that is very crazy to have such a connection. Anyway i am going to order for 0.01 ohm 2W current sensing resistors. Will that enough ?
Arun Dev says
Thank you sir for the advice. but a bit of confusion in it.
I am using three 12 V 20 Ah lead accid batteries ( 12EC25L SEALED ZERO MAINTENANCE RECHARGEABLE BATTERIES ) in series which have been taken from my old electric scooter. As per your suggestion 60 Ah can't drow current beyond 10 A. Then how it is possible for the four 20 Ah batteries in the scooter can run a motor rated above 600 W ?
Swagatam says
Is your scooter an electric bike? even if it's so if you check the current at any instant, it won't be above 6 to 10amps….the ideal rate of charging or discharging any lead acid battery is at 1/10th its AH rating…..you can draw higher currents from it, but that would only mean shorter backups from the battery. discharging current is inversely proportional to backup time
Arun Dev says
Thank you sir for the advice. but a bit of confusion in it.
I am using three 12 V 20 Ah lead accid batteries ( 12EC25L SEALED ZERO MAINTENANCE RECHARGEABLE BATTERIES ) in series which have been taken from my old electric scooter. As per your suggestion 60 Ah can't drow current beyond 10 A. Then how it is possible for the four 20 Ah batteries in the scooter can run a motor rated above 600 W ?
Arun Dev says
Hai sir, I am a sound engineer working in malayalam film industry, but an electronics enthusiast too.
It seems your project be very much helpfull to me in order to make my own inverter which is rated 300 w. but i have some doubts regarding the circuitry
1) First of all i am asking you that, will there be any problem in using a 300 W 12-0-12 V transformer in the circuit since it produces much current (25 A) than that given in the schematic.
2).' Installing a 60 Ah battery needs the addition of a TIP136 over LM338 '. I found this advice in this blog. So please help me to identify this modification correctly by providing me the schematic. Please help me quickly, becoz i am using three 12 V 20ah lead accid batteries in series…..
3).Is there any other modifications needed for complete safeguarding of the battery??
Reply me as earlier as possible to ma email id
dev.arun999@gmail.com
Swagatam says
Hi Arun,
If you are referring to the above circuit you can replace LM338 with LM196 and upgrade the circuit to handle upto 10amps.
Rest will remain as is, but the limiting resistors will need to be calculated and fixed appropriately.
Arun P.Dev says
Thank you very much sir for replying fast. I am now gonna take care of it. But a doubt still remains that whether the 60 Ah battery can run that much load ( 300 W ) in the inverter. If it can how much hours of back up will i get ?
And using TIP36 over the LM316 means replacing the regulator ic 7812 of the battery charging circuit with TIP36 given in the link
https://www.homemade-circuits.com/2012/11/high-current-transistor-tip36-datasheet.html
with the regulator ic LM196 ?
And also i want to know whether 0.01 ohm with 5w is sufficient instead of a similar one with 2 W, becoz 5w resistors are only available in our area. Can you please suggest me a site to buy these cheep resistance online ?
Swagatam says
Hi Arun,
60 AH will not support 300 watts full load, the discharging rate should not exceed 10amps for a 60AH battery, meaning 10 x 12V = 120watts max.
With LM196, TIP136 will not be required, just replace it with LM338 in the above step charger circuit, it will provide you with all the required protections and cut-offs.
Seok Sothea says
Thank you sir! I will try to build this circuit..
Seok Sothea says
Hi! Sir. I have a 15v 5a transformer. I want to use this circuit for charge 12v 50ah car battery. Can you guide me which components should be replace.
Swagatam says
Hi Seok,
you won't have to replace anything, you may proceed with the shown design.