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.
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.
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.
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!
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
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?
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
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.
No problem Edmund, the basic thing to consider is the over-charge cut off for any AGM battery.
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
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.
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!
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.
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
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
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.
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
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
You are welcome e_n,
yes the fast battery charger circuit can be used for charging high Ah batteries in 3 steps.
Is there any proof you have that this circuit works , if so could you point me in the right direction ASAP!? thank you sir
I don’t have any proof, but I believe since it’s a simple and obvious design therefore will work without any doubts.
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.
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
Good day sir, I read that safe charging current is 10-13% of the AH, what is the safe limit for discharge rate.
Hi Glory, it’s exactly the same.
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.
Hi Glory, for li-ion it’s fine and recommended but not for lead acid batts.
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
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
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
Hi Glory, sorry I could not understand how can there be two consumptions separately, do you mean 25 amp peak and 7 amp RMS?
25amps is the starting current for the inductive load while 7amps is the running current
If it’s only for a couple of seconds then there’s no problem.
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
Without knowing the voltage rating of the battery it can be dangerous to apply the input
Good day Swag, I saw the maximum charging current for a 200ah battery is 60amps, please how possible as against 10% ah
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!
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
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.
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..
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.
Hi Glory, you will have to replace your existing charger with this one.
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.
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.
Good day sir, please how can i latch this circuit below. at preset battery full voltage the relay flickers
Hi Glory, you an connect a 10uF/25V across base/ground of the BC547, and check the response
Ok sir, done but no difference sir
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.
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 .
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
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 .
yes that’s possible my manually removing the transformer and configuring its wires accordingly
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.
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
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
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
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
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
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.
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
Good day sir, is it possible to remove lm338 completely and how can this be achieved, what is the disadvantage of this.thanks
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.
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.
Tolu, I have updated the setting up procedures at the end of the post do it exactly as mentioned.
Thanks so much for the update, as you mentioned can I just replace ic 741 with 358 with same circuitry.
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
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
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
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.
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
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.
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
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.
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/
Thanks for your quick response, please according to the led response as above is the finding OK.
If your collector LED is responding to preset adjustment then the opamp is working OK, now you must check why relay is not responding…
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
use LM321 or LM358
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.
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.
Thanks Swagtagam, please how is trickle charging system different from zero current charging with
Tolu, There’s nothing called zero current charging, or may be it is another name for trickle charging.
https://www.homemade-circuits.com/current-servo/
For this circuit how can I convert it to use p channel MOSFET instead of n channel
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..
Hello Swag, please what is Voltage Controlled Current opamp circuit as battery charger, how does it work
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.
https://www.homemade-circuits.com/current-servo/
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
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
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.
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.
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