The post explains how a neat little self regulating automatic battery charger circuit can be made using just two inexpensive transistors.
This circuit will automatically regulate the charging supply to the battery depending on its charge level, by switching the input supply ON and OFF periodically.
How it Works
As can be seen in the diagram, this auto-regulating battery charger circuit utilizes just two transistors for detecting the charging thresholds, and cuts off the process as soon as these limits are detected.
Using two transistor actually makes the design hugely sensitive compared to a single transistor charger circuit.
The indicated preset is set in such a way that the T1 is just able to conduct at the specified full charge threshold of the battery.
When this happens T2 begins switching OFF, and ultimately at a point it is unable to sustain the relay conduction and switches OFF the relay, which in turn cuts of the input charging source with the connected battery.
Conversely, when the battery voltage begins dropping, T1 gradually deprived of its adequate conduction voltage level, and ultimately it ceases to conduct, which quickly prompts T2 to initiate its conduction and trigger the relay into action,
The relay now reconnects the charging input supply with the battery, and restores the charging process until it yet again reaches the full charge threshold, when the regulating cycle repeats itself.
How to Sep Up the Circuit
Setting up this battery charger circuit for automatic regulation is very simple and may be done in the following way:
- Initially, do not connect the fixed transformer power supply; instead connect a 0-24V, variable supply voltage to the circuit.
- Remove the anode of D6 from the relay contact and connect it to the positive of the power supply.
- Keep both the presets somewhere at the center position.
- Switch ON the power and adjust the voltage to 11.5 volts or lower.
- Adjust P2, so that the relay just activates.
- Now increase the volts to about 13.5 volts, and adjust P1 so that the relay just deactivates.
The setting procedure of the circuit is now complete.
Check the whole procedure by continuously varying the voltage up and down.
You may now remove the variable power supply and connect the fixed transformer, bridge power supply to it.
DON’T FORGET TO RECONNECT THE ANODE OF D6 BACK TO THE RELAY CONTACT OR THE BATTERY POSITIVE.
The battery connected to this circuit will be charged only as long as its voltage is in between the above "window" level.
If the battery voltage crosses the above "window", the relay will trip and stop the battery from charging.
Parts List
- R1, R2 = 10K = 2
- P1, P2 = 10K PRESET = 2
- T1, T2 = BC 547B = 2
- C1 = 2200uF/25V = 1
- C2 = 47uF/25V (Please connect this capacitor across the relay coil) = 1
- D1---D4 = 1N5408 = 4
- D5, D6 = 1N4007 = 2
- RELAY = 12 VOLT, SPDT = 1
- TRANSFORMER = 1, (AS PER THE CONNECTED BATTERY AH (DIVIDE BY 5)
The following diagram shows the instructions which needs to be followed while setting-up the circuit with the desired cut-of thresholds, using a variable power supply unit:
The above self-regulating battery charger circuit was successfully built and tested by Mr. Sai Srinivas, who is just a school kid but nevertheless has an immense interest in the field of electronics.
The following images were sent by him which displays his talent and intense dedication in the field.
For One Shot Operation
If you want the above circuit to lock itself into a permanent cut off position when the battery is fully charged, then you may modify the design as shown below:
Note: To ensure the relay does not latch itself quickly on power switch ON, always connect the discharged battery first across the shown terminals and then switch ON the input power.
In order to indicate the charging status of the battery, we can add a couple of LEDs to the above design, as shown below.
The above circuit was also successfully built and tested by one of the dedicated electronic enthusiasts from this blog. The following pictures verify the results:
Warning: Make sure to test the preset setting thoroughly before you leave this application unattended. And be sure to use only 14V as the input supply for a 12V battery.
Just stumbled upon your very helpful circuit of self regulating battery charger. Please what is the value and type of the diode used in the alternative circuit without relay I mean the one connected to the base of TIP122? Is it a silicon diode? The circuit was last updated on May10, 2021. Thanks for your kindness.
Hi, thanks, can you please specify which circuit diagram are you referring to?
I cannot see any circuit without a relay…
Thanks for responding. I am referring to the reply you sent to Vee on May I 2021 where you posted an alternative circuit diagram using a diode connected to a TIP 122 transistor to replace the relay and the 2nd transistor in the original circuit, thus making it a solid state self regulating auto cut off battery charger. I tried to copy and send it you but didn’t succeed. Thanks again.
OK, understood. However, after a deeper investigation I realized that this circuit had some issues, so I replaced it with a better alternative.
Here’s the link for the new solid state self regulating battery charging circuit, which you can try:
https://www.homemade-circuits.com/wp-content/uploads/2022/04/simplest-bench-power-supply.jpg
Thank you very much Swagatam. Very happy that people like you are still on this earth. I wish you more knowledge and strength.
Thank you Robert, I am always glad to help!
What is the purpose of C2 ? Will this circuit work without it, and why isn’t shown in the circuit schematic ..?
It is to ensure that the relay does not chatter while switching….it is important to attach this capacitor parallel to the relay coil.
Thank you Swagatam, I thought that’s what D5 was for, but I can assure you I will install that capacitor for good measure ..! Thank you so very much for sharing this circuit, I was trained as a technician and have very limited knowledge of circuit design, thanx again for your help, best regards-Mark
You are welcome Mark, D5 is for shorting out the reverse voltage spikes generated by the relay coil and thus prevent the transistor from getting damaged. For C2 please make sure to use a higher value capacitor such as a 220uF or a 470uF which will ensure smoother switching of the relay. Wish you all the best!
Thanx for the capacitor values and I really appreciate your lightning ⚡️ fast response ..!
I am glad to help!
greetings…!!!!
Hi dear Swagatham,
Its so inþeresting your circuit.Regarding the first circuit once the max chaeging voltage reach the relay will cut off then when will start the charging automatically again?ie..when 13.8volt set to cutoff then termiñal voltage become low then will it start immediately hence chance for a chattering relay.can we adjust the lower threshold (the point which charging voltage to start again )in any way?expecting your valuable advice.
from Kerala with ❤❤
Thank you Sison, The relay will not chatter actually, rather switch ON/OFF within a span of maybe 4 or 5 seconds.
The switch ON threshold can be perhaps increased by adding a hysteresis to the circuit.
I have tried this through a feedback link, as shown in the second last schematic.
Owkey sir thanks to reply me!!!
But could u send me your whatsap contact so that u can see easy what I did. Because I used 10K pot which is 103 is real it is exact pot needed? If possible please
Sorry Kadama, providing watsapp number may not be possible. 10k pot will not work correctly, instead you must use a 10K preset, preferably a multiturn type.
you ask me: how many volts and amps is the input source, how do I connect the capacitor C2 that I don’t understand, can you guide me? I really need this, thank you
Input V and I of the supply will depend on the battery specs. Connect C2 in parallel with the relay coil or D5….positive of the C2 will go on the positive supply line, and negative of C2 will go to the lower terminal of the relay which is connected with the transistor collector
my battery is 12 volt 7.5 amp, so what is the V and I I use, I drew the circuit, I sent it to your email, can you check it for me?
You can use 12V, 1 amp transformer, which will produce around 16 after full bridge rectification. Sorry, I do not communicate through email, if you are having problems with the above schematics please specify about it here.
hi, I drew a printed circuit using Proteus software based on your schematic, I want to send it to you to check for me but it doesn’t work, my source is 15 volts 7.5 amps, now I want to charge it what to do for my battery, thank you
Hi, I cannot suggest about the Proteus results, but practically the circuit works nicely, I have tested it, and many others have also tested it successfully. However if you want an easier op amp based design I can also provide that
I drew the circuit of the battery charger to disconnect automatically according to your diagram, can you check it for me, my battery is 12 volts and 7.5 amps, thank you
C2 must be connected parallel to D5, not D6….the relay connections look messy so I cannot comment on that!
Na lista de peças fala em C2, mas não é indicado no esquema. Agradeço indicação como ligar.
Obrigado
You must connect C2 across diode D5 or the relay coil. positive of the capacitor will go the cathode of the diode D5, and negative of the capacitor will go to the anode of the diode.
Hi Swagatam
I managed to play around with the above circuit and got the switching – off to on – to about 1 minute 35 secs approximately by adding a 2200 mfd capacitor across the relay and a 1000 mfd across the base emitter of T1 and a 100 mfd across base emitter of T2. So now its not like fast flip flop.
From a discharged level battery it charges up and cuts off at about 13.0 volts and then cycles between 12.55v to about 12.92v.
And I also changed the connections a bit on the Relay. I connected the Common to the positive of the relay and NO to the battery, works fine.
I tried sending you pictures of the project to your email but got a failure reply saying this email does not exist.
Thanks for all your guidance and help, hope you find this ok
Please reply
All the best till the next time
Regards
Vee
Thank you Vee, appreciate your kind feedback
Glad to know the circuit is working OK for you now!
You can send the images to hitman2008 @ live . in
Thank you very much Vee…got all the images, and I will soon publish them all in the above article.
Hi Swagatam
I must thank you immensely for all the help and guidance you have given me and for your precious time and patience.
Im so blessed to have you as a friend in this field of electronics for which I only as a hobbyist have a passion even at this old age,to pass my time in these crazy times bless you,
Thanks for also sending me an alternative circuit which I would try when I am able to get the TIP 122 . Is this also a SELF REGULATING or AUTO CUTOFF
Thanks again Swagatam
All the best to you
Can I send you a picture of my project by email to you, please let me know ok
Vee
It is my pleasure Vee, I am always happy to help.
Yes the transistor circuit would work exactly like the relay circuit, but without making any noise, it would silently keep doing the flip flop switching at the battery full charge threshold, and make sure the battery never gets overcharged…
Yes definitely….you can send the pics to my email…hitman2008 @ live . in
Hi Swagatam
Good news
I managed to get the circuit working finally the only thing is when adjusting the cut off voltage and cut in voltage it keeps switching on and off like a flip flop so fast when I switch off the power supply and back again then its stable but it happens now and then
I added the caps between the base and emitter of the transistors
I know it takes very minute adjustments of the presets
I has to discharge the battery several times down to about 11 volts and try and it recharged quite fast with the power supply I have, recharges at 1.15 to 1.2 amps and goes down to about 250 ma in about half an hour to 13.8 volts
Thanks again for all your help
Regards
Vee
That’s great Vee, at least you could get the circuit to work ON/OFF at the threshold.
If you connect a 1000uF right across the relay that might prevent the relay from self regulating too fast.
Alternatively you can completely get rid of the relay and use only the transistor stages to charge the battery in a solid state manner….here’s the modified diagram for you:
You can replace the T2 with a TIP122
Hello swagatam.
Can you write how to make the settings of the transistor circuit?
Hi Suat, the setting procedure is the same as described in the article.
Hi Swagatam
I know you said the relay connections are exactly as shown in the diagram but I am a little confused with the labelling as you only show NC & NO & not the C
So I want to know if the connection to the battery and the anode side of the diode is on the NO side of the relay and the Common is to the positive side of the relay
Please reply
Thanks
Vee
Hi Vee, the common pole of the relay is connected with the battery positive and the anode of the diode D6, the N/O of the relay is connected with the positive supply line of the circuit. The N/C point is blank.
Hi Swagatam
Thanks for your reply shall do as you have told me
The car battery is 640 Cold cranking amps Reserve is 85 Amps
The Power supply is a variable 4.5 volts to 24 vdc 2amps I also have a 3 Amp one Its set to 14.0 volts
Are these ok or do I have to have higher amps power supply
I tried these power supply with a Auto cutoff circuit of yours using a BC547 or 2N 2222 and the battery charges well intially at about 1.2/1.3A and slowly reduces to about 200 ma and cutoff when the battery is at 13.8 volts , works good
But I am curious to see how this SELF REG. circuit works …… anyway shall try again aa you instructed me
Thanks Swagatam for your help
Vee
Hi Vee, for a 85 Ah battery 2 amp current is too less, and it might take at least 24 hours for the battery to get fully charged.
The above circuit was tested by many of the visitors successfully so the circuit will work although it might require some skillful adjustment of the presets.
Hi Swagatam
I tried the above circuit (self regulating charger) but I still could not get it working
Is the common contact of the relay connected to the anode of the diode and the positive power supply connected to the NO of the relay
When I tried doing the initial settings as per your instruction there is no response on the relay.
I am using a 12 volt cube relay whose coil resistance is 410 ohms
I also tried transistor BC 547 and also 2N 2222 ( pinout is opposite to 547)
Power supply is from my variable bench supply set as per your instructions
Please help
Vee
Hi Vee, yes the relay connections are exactly as shown in the diagram.
You can set up the circuit in the following way.
Keep the slider of both the presets to ground.
Switch ON transformer supply, and then connect a discharged battery to the relay contacts as shown in the diagram.
Now slowly adjust P2 until the relay clicks ON….now the battery will start charging.
Monitor the voltage with a voltmeter until the battery is fully charged.
As soon as the battery is fully charged, slowly adjust the P1 preset until the relay just clicks off.
Your setting up procedure is now complete.
Connect a 100uF capacitor across the base/emitter of both the transistors to stabilize the relay contact operations.
Remember the transformer current must be 10 times less than the lead acid battery Ah rating.
Hi Swagatam
Sorry for troubling you, I was thinking if I could use a bypass transistor like the MJE2955T with the LM 317 to get a higher current output and can i put them on the same heat sink or should they be isolated this 2955 is rated 60V /10A in a TO-220 package will this work for a variable voltage of 1.5v to about 24.0v and 10 amps . I have this transistor.
Appreciate your reply
If I want to send you the circuit which you would definetly know just to be sure I have it right how do i do that .
Thanks
Vee
Hi Vee, It will work but not with 10 amps rather only 5 amps max.
And please, I want you to ask these question under a LM317 or any power supply article, because the above self regulating charger is about battery charging so our discussions are off-topic, right?.
So please ask under a LM317 article or any power supply, I will try my best you solve your queries.
Hi Swagatam
Sorry about being off topic I was not aware how this works I shall pic as you say the related topic and ask my doubts
Sorry about that
Regards
Vee
Hi Vee, you can put your LM317 questions under any of the following articles:
https://www.homemade-circuits.com/?s=LM317
Hi Swagatham
I told you earlier I had made a Variable voltage supply using the LM317 and that is working fine but can only handle 1.0 amp so I tried using an LM338 which I got from China as no store has it locally. Using the same circuit as for the LM 317 I tried replacing it with an LM338 using the same power supply as in the earlier LM317 circuit but there is a bit of flicker and the IC fails and pin 2 & pin3 get shorted out. I put back the LM317 and it worked fine with the same load of about 600 ma I tried again with the LM338 and this to burnt out in seconds
I would like to send you the circuit so you tell me it it’s suitable for both IC ‘s ……. should I send it by email to you please advise
Thanks
Vee
He Vee, that clearly indicates that your LM338s are not good, they may be faulty internally or maybe duplicates.
Instead of using LM338, you can try putting 2 or 3 LM317 in parallel, and mount them over a common heatsink and close to each other. that should effectively work like an LM338 equivalent.
Hi Swagatam
Thanks for your reply, I was considering doing the same as you advised about paralleling 2 or 3 LM 317’s do i have to have any resistors small value at the out puts, can i do 4 of them and if possible can you send me the diagram and this will need a real big heat sink. Do I have to have a minimum load connected to it to avoid over voltage at startup
Thank you very much Swagatam fir all your help if possible you can email me the parallel circuit, I know how it should be but just to make sure Im on the right path and is it ok if I am using a 24 volts /6 amps SMTP adapter or should I use a conventional transformer
Thanks
Vee
Hi Vee, there’s nothing you have to do in the circuit, except connecting the IC pins in parallel, but make sure to connect the ICs over a common heatsink, very close to each other.
Thanks Swagatham
I’ll do as you have advised me
All the best to you
Regards
Vee
You are welcome Vee!
Thanks for your knowledge shared. I will try it. But sir, can it be used on a 12v 200ah inverter battery? Please answer me thanks
Are you referring to the following new post:
Simple SCR Battery Charger Circuit
No sir I’m talking about the Self Regulating Battery Charger Circuit
You can use any battery for charging with the above circuit, regardless of the Ah value, you will only need to suitably upgrade the relay specifications in accordance with the charging current of the battery
Hi Swagatam
Thanks immensely for your prompt reply and help in providing me with some long duration timer circuits, shall try them out and let you know if i did it right
Thanks again
Vee
No problem Vee, I am always happy to help!
Hi Swagatam
I wrote to requesting you to give a circuit for a delayed relay timer of 10 minutes to 2 hours, I have made one from 10 mins to 1 hour with IC 555 but this has I understand a limitation of capacitor of upto 2200 uf
I would appreciate if you could give me a circuit whenever it’s possible by you
Thanks
Hi Vee,
you can try one of these circuits:
https://www.homemade-circuits.com/how-to-make-simple-versatile-timer/
Hi Swagatam
I need some help with converting an ATX to a bench power supply.
I have written to you earlier about your selfregulating battery charger etc and have followed your instructions and got them working and thank you for the same also built a variable power supply and other projects & working good
I am a 70 plus years older hobbist and love doing small electronic projects to keep myself busy and pass my time usefully, I have been interested in electronics for many years and happy to get to know you on the net.
The present problem with the ATX is I have tested it with the power ON switch and the little push switch which turns on the fan and the voltages at the several plugin terminals …….
Orange PG 3.3 v ……. Red + 5.1v ……. Yellow +12.2 v ……. Blue — 12.2v and White — 5.1 v ….. blacks …return/ grd.
When I tried to see if power was really there with a 12 v /10w bulb it did not light up so I added a load resistor 10 ohm 10 watts between the Red and white I guess by mistake then the light came on the resistor got hot and the ATX went off
I should have connected the Red and black to the Resistor I think. Now when I switch on the Power the fan just jiggles and nothing happens . Is there anything I can check or do to get it working or have I made it a paper weight ????
Please reply ……. sorry for the long story
Thanks in advance
This is an old SAMSUNG Model PSCD151404A ATX
Hi Vee, I appreciate your efforts and your interest very much in the field of electronics. I am glad you could build a few of the projects from this website.
For converting your ATX into a bench power supply, you just have to feed the bench power supply output to a regulated bench power supply module. This may be done through a rotary switch for selecting the different ranges of input to the bench power supply module.
Here is a link to the construction of a bench power. The transformer and the bridge section could be eliminated, and replaced by the various outputs from the ATX via a rotary switch:
How to Design a Stabilized Bench Power Supply Circuit
Hope this idea proves useful to you!
Dear Swagatam
Thank you very much for your immediate reply which I appreciate.
I do know I have to either put a rotary switch or have multiple binding posts for the different voltages etc.
If you read the last portion of my message you will see what happened when I was testing it , the ATX Power went dead and when I tried to switch on the power again the cooling fan just jiggles (shakes) but does not run and there are no voltages at any of the leads . So I need to know if any of the IC’s would have shorted out or what should I check or do to get it working again . I checked the 4 amp fuse is ok. So please see if you can guide me where to check etc.
Thanks for your kindness and help, may you be blessed in all your endeavours
Regards
Vee
You ae welcome Vee…
ATX involve SMPS circuits which can be difficult to troubleshoot without a proper test equipment or without practically viewing the circuit.
If you think your ATX is burned then it can be quite difficult to diagnose the fault, unless you are well versed with the working of AC SMPS circuits.
Most probably the MOSFET might have burned, so you can try checking the MOSFET or any big power device associated with the circuit, and see whether it is alright or not…