Self Regulating Lead Acid Battery Charger

The post explains how a neat little self regulating automatic battery charger circuit can be made using just two inexpensive transistors.

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.


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,
P1, P2 = 10K PRESET,
T1, T2 = BC 547B,
C1 = 2200uF/25V
C2 = 47uF/25V (Please connect this capacitor across the relay coil)
D1---D4 = 1N5408,
D5, D6 = 1N4007,

self regulating battery charger circuit lead acid

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:

setting up the presets for the battery charger

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.

self regulating battery controller prototype images

self regulating battery battery connection prototype images

red led cut self regulating battery controller prototype images

green led off controller prototype images

Need Help? Please send your queries through Comments for quick replies! And please Bookmark my site :)


Swagatam said…
yes, that's why I have attributed the word "self regulating" to it, meaning as soon as the battery voltage drops at a certain value the circuit will again start the charging process and never allow the battery voltage to fall below a specified lower level.
Muhammad Awais said…
Hello sir,

Can i use this circuit for my UPS battery charge control. It charges at 10amp but there is a problem in the UPS built-in charge controller and it does not trip automatically when the battery is fully charged. I'm asking if i can install this circuit in this system.

Swagatam said…
Hello Muhammad,

Yes you can use it, but you will have adjust the presets very carefully for getting the desired cut-offs.
Devashis said…
Swagatam, i have a 1000VA IPS that uses 24v, 14AH sealed lead acid battery. It was brought for backup computer. Is it possible now to use the IPS with a 24v, 70AH battery for longer backup as I want to drive it a fan and 2 light?
Pls suggest me what can i do at this time.
Swagatam said…
Hi Debashish, yes it's possible, as long as the voltage is not altered or exceeded beyond the specified limit AH won't matter, it will only help to provide higher backup time.
Swagatam said…
yes it will work
Swagatam said…
thanks! presently i do not have 0-1000 degree circuit, if i get i'll surely post it for you.
NEWTON said…
@Swagatam Majumdar,
good work...
my question is this....
will this circuit diagram cut-off the voltage when the battery is fully charge?
thks. hope to read your reply soon.
Swagatam said…
thanks! Yes the relay will cut-off as soon as the voltage reaches the set upper threshold and stop further charging of the battery
Flhex Aplaon said…
Hi swagatam
Thanks to your post i make this circuit and its can i add 2 led bulb for charge monitoring..i plan to place 1 red led for charging status and 1 green led for full charge can i connect this to the circuit?

Swagatam said…
Hi Flhex,

Thanks, however here only one LED would work effectively across D5.

Connect it with a 10k series resistor, it will indicate "battery full and disconnected", another "power ON" indicator LED can be added directly across the positive/negative line with a 10k series resistor.
suresh aprs said…
Hi sir,

I want made 12v & 24v single transformer. Variable current rating. How I design cut off circuit. Please help me.

Thank you
Swagatam said…
Hi Suresh,

Try the first circuit from this link:
ogunyemi gbenga said…
pls i dont understand that relay part. NO and COMMON are nit joined inside relay but you joined them in ur diagram. i understand that NC is not connected. pls clarify the connection of NO and COMMON. thanks for your response!
Swagatam said…
The diagram shows the relay in activated mode while charging the battery, therefore the N/O is connected to pole, when deactivated it will connect with N/C
Mega blessing said…
Hello Swagatam, please can dis circuit output voltage be regulated so as to charge the battery at thesame rate as it is discharged?
Using d appropriate transformer amperage how long will it take to charge a 12v 100amps battery?
Mega blessing said…
Please can this circuit output voltage be regulated so as to charge the battery at thesaurus rate as It's discharging?
Swagatam said…
Hello Mega blessing,
yes it can be done simply by using an appropriately rated transformer.

for any lead acid battery the ideal charging time is around 10 to 14 hours.
Hello swagatam, i like ur circuit,easy to understand, i want to charge 3v battery(2x1.5v) can i change D5,D6 to 3v zener diode,to trigger the relay.hope u understand
Swagatam said…
hello mohammad,
all the diodes shown in the diagram are 1N4007, these are not zener diodes but ordinary rectifier diodes, you can use them as given.
dear sir 1 want to charge 12v45ah battery i have 5A20v ups transformer. this will work?
Swagatam said…
Dear Jayanth, use a LM338 IC based charger to drop the 20V to 14V for charging your battery.
I have published many LM338 IC chargers, you can search them through the search box at the top
ok sir,,,,How to Make an Automatic 12 volt Battery Charger Circuit Using IC LM 338
im going to make above one. please answer my question on that comment forum.
sskopparthy said…
sir, for c1, can i use a 1000v,25uf capacitor? and for c2, can i use a 100uf 25v capacitor? and for d1-----d4, can i use 1n4007 diodes, sir? please tell me sir....
sskopparthy said…
sir, i want to use this circuit with a 12v 7ah battery. can i use a 12v, 1amp transformer, sir?
Swagatam said…
yes will do...
Swagatam said…
yes, it's fine, you can proceed.
sskopparthy said…
sir, i assembled the circuit and it switches on relay when i connect power supply. and when i adjust the presets, the relay stays in on position only(in both cases i.e., in adjusting cut off and adjusting switch on of relay). please tell me whether i should connect power supply including bridge or directly to capacitor? also please tell me if i should connect battery while setting the circuit, sir? please help me...
sskopparthy said…
sir, please tell me where i should connect variable power supply when setting up the preset resistors and the circuit before connecting the battery and fixed transformer? thank you for your help.......
Swagatam said…
Battery should not be connected while setting up the circuit.

you must use a variable power supply for setting-up the circuit as instructed in the article.

I'll update the diagram with the required information shortly.
sskopparthy said…
sir, thank you very much for your help. im sure this would help others the way, this circuit WORKED on my breadboard..thank you again.......
Swagatam said…
you are welcome!
sskopparthy said…
Dear sir, ive been using this circuit for more than a month great I sent the photos of circuit to your mail(homemade
Swagatam said…
That's great SS, thank you, I'll check them out soon.
Arun Dev said…
Sir, could you please help me in designing an automatic 24 V battery charger circuit having the following spec.

- it should be capable of charging a 24 V 80 Ah battery bank

- higher voltage cut off and lower voltage turn on facilities

I have a 300 W inverter transformer with me which is a 230 V / 15-0-15 type... So the primary winding can handle a maximum current of 20 A........ So is it possible to use the primary terminals of this transformer ( end taps excluding the mid one,showing a voltage of 30V total ) to charge this battery ?????
Swagatam said…
Arun you can try the second circuit from the following article:

You can use the 30V/20Amp transformer as the input, but don't use a filter capacitor, just connect a single rectifier diode for rectification, and use it as the input with the above linked circuit.
Tim Petersen said…
Is the second schematic (testing setup) accurate?

It seems to conflict with text as to the connections around D6.

The text indicates the +VDC to be connected to the anode of D6 but the schematic shows it connected to the cathode and the coil and NO connection of the relay.
Swagatam said…
Actually D6 is not crucial and could be removed, I placed it initially just for a psychological comfort, practically it serves no purpose.

Therefore the circuit actually becomes very simple now, remove the entire D6 link and connect the R1/R2 junction with the positive line.

The setting up procedure may be done by supplying the external variable voltages across the positive/negative lines of the circuit, without any battery connected and also without the mains input to the circuit transformer.
Tim Petersen said…
Thanks for the info.

I have completed the circuit without a D6, but used PN2222 NPN transistors instead, as I had them here and their characteristics are similar to the 547B ones.

I can setup the charge on cycle, but I cannot get the off adjust to work at all. The relay stays locked on through the entire range of P1. In fact once the relay engages, the only way to get it to go off is by removing the power. Once power is restored, it immediately engages again. If I drop P2 back to center, it will stay off until I adjust it as per spec.

Any thoughts on this?

Tim Petersen said…
Additional info:

I found one of the resistors I used was way out of tolerance, down to about 700 ohms or so.

I replaced it and the circuit seems to work as stated, but I am having a lot of diffiuculties getting the levels dialed in as both affect the other. Adjust one to the proper level will bring the opposing one off of it's level and vice versa.

It seems to me the critical one to get accurate is the off limit, as this one is the key to stop over charging.

I found the point was so finely tuned that just touching the pots would cause them to flip the relay state. Maybe multiturn pots would make setup easier.

I also found that the lower setting has little effect. When the upper one is set at say 13.8 volts (normal for a fully charge 12V lead acid cell), any voltage below 13.3 will cause the relay to engage and start the charge cycle again. The voltage would never get down to 11.5 volts, even though that level had been set first.

I gave up tweaking after 45 minutes, as it is just too sensitive to get dialed in properly.
Swagatam said…
I agree with you, setting up the lower threshold could be a bit difficult with this could probably try one simple modification for achieving the desired lower threshold activation to a reasonably accurate extent.

Try adding a high value resistor from N/C of the relay to the base of T1....the value of the resistor will need to be experimented, may be a 1M could be the starting value and lowered down until the right one is found.

The mod will basically add a bit of hysteresis to the design which is actually missing in the here and needs to be added for the intended results.

you can also add a capacitor in parallel to the relay coil to stop the relay from chattering at the thresholds
Tim Petersen said…
I have the 47uf cap already in place.

I will try the resistor mod when I get back to bench later on, and let you know hos it goes.

Tim Petersen said…
It ended up being an 820k resistor that gave the best results.

It is still very sensitive to tweak, but it got easy enough to tune it in quite close to were I wanted it to be.

There is still a fair amount of variance in the cut in voltage, but the cut out is quite stable.

Thanks for the help!

Swagatam said…
That's great! Thanks for confirming it, this update will be quite helpful for the other readers.
Tim Petersen said…
I just noticed something about the circuit. When you said to remove D6 and connect the R1/R2 to +, was that correct?

It seems it will not detect the battery state unless the relay is enaged, so the charger will never start when hooked up to an actual battery.

I think the R1/R2 should be connected to the COM (Armature) contact of the relay that is connected to the battery.

Swagatam said…
Oh yes, I seemed to have missed it completely.

R1/R2 should be connected with the relay pole, as rightly indicated by you.
Willy LAI-TIONG said…
Hi M Majumdar,

Please could you help me !!! I can not make the setting of your circuit. I don't understand what you means with " connect this point temporarily" on the second drawing? Is there 2 points for the positive input for the variable supply? The line in dot ???
And where do you set the resistor of 820K in the circuit?
Swagatam said…
Hi Willy,

" connect this point temporarily" means join those points together with a wire link (wire link shown as red dotted line)

Join 820 k resistor from the "normally closed" contact of the relay to the base of T1

Tim Petersen said…
Well, I could never get this circuit to work properly. Even if I got it to calibrate, in the real world when connected to a battery, the system either does not start a charge or hammers the relay like a buzzer. I am going to have to scrap it and find another charger. This one is just too finicky.

- Tim -
Swagatam said…
As per the title, the circuit is specified to keep the battery always in a topped up condition by regulating its voltage within the top 5% margin, meaning a 12V battery would never be allowed to fall below 13.5V and rise above 14V, so I think the circuit should be allowed to work under these specs only and nothing more should be expected from such a simple design.
Ahmed Sabra said…
hi Swagatam again..
I have a sonnenschein a612/150 gel cell battery.. I charge it using a "SUOER MAD-1220A" 20amps charger that should have 4 steps charging modes..
the question is.. when a green light turned on that means floating mode is on.. but when I apply multimeter on the battery during floating mode charging.. it gives 14.3-14.5v.. is that good?? because I know that floating mode volt should be 13.8v..
is my charger stop charging when green light turns on ?
or this green light means that floating mode is started?
or is my charger is broken and it's never stops charging and never enter floating mode ??

thank you for your patience
Swagatam said…
Hi Ahmed, applying float charge at 14.3V is OK, but whether the charger is getting into a trickle charge or not at this volatge can be confirmed only by practical testing.

if your charger is specified to produce float charge once the green light comes ON then it should be initiating the process with the green light turned ON.
in 12 v 7ah lead acid battery charge which ah transformer is suitable
Swagatam said…
between 1 amp and 2 amp will be OK
inayat kheshgi said…
sir if I use 12v 10amp transformer what will be the maximum current in output?
Swagatam said…
It will be 10 amps only
ningrat_edan said…
hi sir,..
nice to meet you, I'm achamt from Indonesia sory my english is so bad
how to setting P1 and P1 to get voltage suitable.thank you so much
Swagatam said…
hi ningrat, i have explained the entire process in the article and also have discussed it elaborately in the comments, please go through them for understanding the procedures.
ataull hakim said…
circuit is not work
Aju said…
Hello sir i hve made this and working perfectly.
I hav one doubt.i hope ucan clear it.
Can i connect a 12 volt5 ah battery and a 12volt 7ah battery parallel to get 12 volt 12 ah ?? Is threre any problem connecting 2 batteries with different specs ??
Swagatam said…
That's great Aju, I am glad to know this.

No, you cannot charge two batts with different AH specs in parallel, that can be harmful for the lower AH batt.
Good day sir..i have made this circuit to charge a 6v 4ah battery..i used 6v relay..i followed the instructions into how to set the p2 and the p1 for the thresholds..i initially set the variable supply to 4.8v for my lower threshold voltage but somehow the relay won't work..please help me sir..
Swagatam said…
Good day Mary, the setting-up procedure of the above circuit might require some skill and some thorough experimentation.

It would be difficult for me to troubleshoot your circuit since I do not what exactly know what might be causing the problem...but I can assure you that the circuit will definitely work if done correctly.

If you are having difficulty with the above circuit you can try an opamp based design
Swagatam said…
Hi Papan, the two resistors which I suggested are different.

The relay pin configuration will depend on its type and can refer to the following article which shows the pinout configuration of one of the common types of relay, which you can also use

HI friend do you think that 1n5408 bridge is capable of delivering 20A for charging or should we use 10A10 diode bridge for that
Swagatam said…
1N5408 is rated to handle a maximum of 3 amps (possibly with heatsink) pwerhaps you shoul go for a higher rated bridge
Yes exactly, I read your conversation and seen that someone is charging a 100Ah battery so that made me confused that how? Coz 1N5408 is a low ampere diode. Connecting 10a10 diode or using a 30A bridge (are easily available everywhere) would be good.
Swagatam said…
yes, for 100 AH battery the transformer, the diode and the relay contacts all will need to be changed accordingly, that's left up to the readers to take note and do the needful.
ilias.epam said…
Very nice circuit. Cheap, easy to make and it really works !!! The only part that needs attenmtion is the setup of the cut off voltages ... but if somebody reads carefully the instructions and try 1 - 2 times then it is a success !!!

Thank you very much for this post! I wonder if i can make it work for charging 24V 14Ah batterfy ... but i think it worths a try ...

Many greetings and thank you again !!!
Swagatam said…
Thank you very much Ilias, I am glad you could understand and adapt the design correctly. I appreciate it very much.

yes definitely, through minor modifications this design could be used for 24V or in fact any other preferred voltage operations.
Kesava Raj said…
Hai sir....
I have some ?

Cut off voltage of 12v,6v,4v and 24v lead acid battery....

For 12v I'm using 10+3.6v=13.6v zener
For 6v I'm using 6.8v zener
But 4v and 24v battery i do no the cut off value pls guide me sir...
Swagatam said…
Hi Kesave, which circuit are you referring to?

anyway for 24V you can use a 28V zener

and for 4V use a 4.7V zener
Atinuke Funke said…
Hi engineer, pls how can I integrate this charger into automatic changeover inverter because the positive line of the battery is connected to relay. when it trips off, there can't be contact on battery.also, at 13.5v the battery can't be fully charge because within 30mins of charging the battery will get to 13.9v with a good charger. what is the meaning of the self regulating. thanks for all your invaluable assistance.
Swagatam said…
Atinuke, for automatic mains to inverter changeover you will more relays attached in the supply lines.

you can chage the battery to 14.4V also, there's no restrictions.

self regulating means the relay will go on tripping ON/OFF between two small thresholds say 13V and 14V to keep the battery perfectly charged and in topped up condition.
untung suharto said…
Hi, friend this ia Gery simple but good idea and very Cheap self charging controller.
Swagatam said…
thank you friend!
Manvir Singh said…
Can it be use for 9v battery?
& what is the output voltage and output current
Swagatam said…
yes, but the relay will also need to be 9V rated, current and volts will as per the input supply
Faith Jumbo said…
Thanks for reply actually i did not press the Load more button. sir concerning this circuit can i use it for a 12v@600AH if yes how do i increase the transistors in other to increase the current

If this circuit wont work for that can you please give me a good and working circuit i can use for 12v@600AH battery i really i want to make a good charger for my batteries
Thanks for help
Swagatam said…
Faith, the above circuit is extremely good and fully automatic, here the relay charges the battery not the transistors, transistors only control the relay operations. you can use it for your battery, but make sure the relay is able to handle 100amp current...and use a TIP122 for the relay driver transistor T2....other designs are also present in the website which use opamps and can be even more difficult for you to make and adjust...
Faith Jumbo said…
I wil like to make this one but getting a 100amp relay is the problem here while i wil keep searching for a 100amp relay can you kindly give me the address to others. I think it won't be difficult for me

Thanks being regard
Swagatam said…
if 100 amp relay becomes problem, you can try a mosfet for the cut off, as presented in the first diagram of this article
Faith Jumbo said…
Sir please i want ask is there to regulate the voltage going to relay because if 16v which is coming from my transformer gose to relay won't it burn the relay
Faith Jumbo said…
I got 70amp relay today can be ok for the 12v@600amp
Swagatam said…
Faith, 16V could be a little high, you can put a few series diodes with the relay coil to drop it, however the connected discharged battery would quickly get it down to its charge level so ultimately it won't have any impact on the relay coil....
Swagatam said…
you can try should work...
Faith Jumbo said…
Sir i hv building but setting it is a little problem when i set one side it off set the orther

I fail to mention the relay i bought is 70amp 4pin it normally open relay can i use it in place of 5pin i.e one N/C and N/O
Swagatam said…
Faith, Please keep trying you will succeed...refer to the comments for the details...

make sure the T2 is appropriately rated to handle the 70 amp relay coil....use a TIP122 if possible for T2
Faith Jumbo said…
In one the comments i read u said they should connect 820k from the N/C open to the base of T1 and also connect high value from the base of t1 to ground which of them should i try because my relay is 4 pin relay i.e its only normally open there is no normally close please advise me what to do
Faith Jumbo said…
And yes but T1 and T2 are Tip22
Faith Jumbo said…
Sir honestly speaking i have try the settings for the past 2hrs now, one setting always offset the other what I'm looking at doing is to use two relay one will be for cutoff while the other will be to trigger will this work please your advice is needed

Thanks and best Regards
Swagatam said…
It can be implemented to latch the circuit and create a desired gap between the high cut off and low restore points. But it is not required if the setting is done correctly through the given presets, and this is not difficult not at all if done with proper understanding and using good quality parts...I have tested this circuit, and for me it was just a matter of 10 minutes
Swagatam said…
use any small NPN for T12 such as BC547..and use a higher amp NPN for T2 such as a 2N2222 or D880 or 8050 etc so that it is able to handle the high current for the 70 amp relay coil....

TIP122 may not be required actually
Swagatam said…
Faith, try the last circuit from this article, for a low voltage actuation:

and connect a feedback resistor from the collector of BC557 to the base of central BC547...this will determine the high voltage actuation of the relay...

remember here the BC557 will need to upgraded for your 70 amp relay or if possible try with a smaller relay and confirm the results first
Carl Corbeau said…
Could someone re-work this circuit so that one could plug any 12v charger into it and have it sense the battery voltage and turn the charger on and off as per the hi/low presets.

Swagatam said…
you can use the above circuit with any 12V charger input, once the preset is set it will respond to all charger inputs identically
Carl Corbeau said…
Swagatam my man, do you build each of these circuits or do you run some of them on a simulator program?
If you run a program which one would you recommend, also is there a good free one?
Carl Corbeau said…
BTY in Canada we call those presets, cermets
Carl Corbeau said…
I got this circuit working.
It seems this device would be best used as a battery shut off if one didn't want one to discharge below a fixed voltage.
To be any use as advertised it would have to latch off at the high end and come on again at the low setting.
Used on a working battery it would spend most of it's time rapidly turning on and off maintaining the high value. Might as well use hydro in that case.
It might be useful providing a float at a lower voltage, would keep the relay busy with all that on and off business, though.
"Make this 48V Automatic Battery Charger Circuit" might fill the bill. I'll give it a try.

Swagatam said…
Hi Carl, I never use simulators for checking my designs, to me those are just useless...I rely entirely on my mind simulation...which has been so far 99% correct, although the final result on some occasions require a bit of serious tweaking.
Swagatam said…
Thanks for the update Carl, appreciate it a lot!!

yes, it will keep ticking at the threshold within a margin of 0.5V. that's the reason I named it as "self regulating" :)

it can be used at the lower switch-OF threshold or at the upper full charge threshold, depends on how the relay contacts are wired.

however to introduce a hysteresis feature you can experiment by adding a resistor from one of the relay contacts to the base of already discussed with Mr. Tim in my earlier comments...
Carl Corbeau said…
Mind simulation is what Tesla used....good for you.....It sure served him well.
Swagatam said…
yep, thanks Carl...
Carl Corbeau said…
Swagatam, when you have the time could you advise on how to make this circuit more robust. I have a 25amp, 12 volt relay/contactor and some 2n3055 transistors.
I'm guessing this would call for different values for the resistors.
How would I calculate the new values?
Thank you in advance.
Swagatam said…
Thanks Carl for posting this interesting question, if you are intending to use 2N3055 transistor then the relay won't be required at all...and that would allow you to get a perfectly solid state version of the above...if possible I'll try to design it soon and post it here with the required hysteresis feature.
Swagatam said…
Battery life is shortened if it's charged or discharged beyond it's safe limits, not due to charging it at 70% or 80%. Moreover here the charger is designed to self-regulate meaning it would keep switching On and OFF within a range of 0.5V to 1V, in other words between 14V and 14.5V for a 12V battery...and that's a good way of keeping the battery topped up.

A lead acid battery life can be maintained at maximum level as long as it's charged and discharged at 1/10th of its AH rating and within 11 to 14V for a 12V batt

I hope this enlightens you regarding the facts.
Hi sir, can you please provide me an automatic 12v lead acid battery charger without using relay, I have right now tip 122, TIP 127,and other components but I don't have relay so can you help me.
Swagatam said…
Hi Aabhishek, you can try the following design

Hi sir, I want an auto cutoff lead acid battery with relay but I don't want ic and also I didn't get the common pin of relays in upper diagram and explain them please.
Swagatam said…
Abhishek, you can use the circuit which is explained in the above article. Just connect a 10K resistor from N/C to the base of T2, this will latch the relay as soon as full charge is reached.

the contact which is connected with D6 and the battery positive is the common pole
manash Athparia said…
I have a 12v 7ah battery but I don't have proper charger right now to recharge it so I am connecting the battery directly to my home inverter to recharge it. Will it recharge correctly with proper current and voltage or it will destroy the battery.
Swag said…
a 12V/7ah battery will need 14V, 1 amp to charge correctly for 10 hours continuously...if this is not followed your battery will get damaged.
manash Athparia said…
What if I charge it with 20v - 24v 1A instead of 14v 1A
segun said…
Good day, pls Intend to use this circuit for charging battery up to 200ah. My transformer is 255v/30A, my bridge rectifier is 50A, my relay is DC 10A/28v will that be ok?.
(2) what determines my max charging current.
(3) what fuse rating to use to protect the circuit from a load drawing more than design current
Swag said…
Segun, for a lead acid battery the ideal rate of charging is around 1/10th of the battery's AH rating, meaning your 200AH battery should be charged at the rate of 20 amps for around 12 to 14 hours, for allowing 95% charging.

that also implies the 10A relay will not be enough, you must change it with a 30A relay, and use a current controller in the middle to make sure only 20A is able to pass into the battery
segun said…
(1) Do you have any current controller for me to use.?
(2) can the circuit also be used for lower battery say 7Ah
(3) hope the 30A relay will last?
Swag said…
you can use the circuit for any battery, by selecting a matching relay as per the battery specs.

for current control you can use the following concept
segun said…
Thanks so much for your response.
Pls can you be specific to help me for the current control circuit of 20A and how to connect it to suit my design. Thanks
Swag said…
you can connect it between the main DC supply and the battery control circuit. the transistors can be TIP35 for the upper transistor and 2N2222 for the lower one.

resistors can be calculated as per the given formulas.
segun said…
I'm very grateful.
But more questions: At 1threshold current=20A
I have calculated my
(a)R1=35.75ohms (approx 36ohms)/6.25W or 6W
(b) R2=0.04ohms /14Watts
Q(1) where will my TIP35 collector be connected in my self regulating charger circiut
Q2(2)will 15v DC be able to drive the TIP35 bcos Vcc is 20v from datasheet or it that the max
Q(3) how can I use MOSFET to replica the work of the relay in case I can't find 30A relay
Thanks for your timely response
Swag said…
TIP35 and 2N2222 are for the current controller stage, not for the above circuit.

Th above self regulating circuit will remain as is.

for a mosfet version you can try the second design from this article

In this design an external current control is not required since the BC547 stage will to do it for you
segun said…
Hello Swag it seems you have lost track of my discussion. The immediate questions below come up as a result of the quoted reply from you.
I want to include the current controller in the "self regulating charger circiut", is it possible? Or are you saying I should just convert the "48v charger circiut to 12v and limit the I=20 or 25A
" At threshold current=20A
I have calculated my
(a)R1=35.75ohms (approx 36ohms)/6.25W or 6W
(b) R2=0.04ohms /14Watts
Q(1) where will my TIP35 collector be connected in my self regulating charger circiut
Q2(2)will 15v DC be able to drive the TIP35 bcos Vcc is 20v from datasheet or it that the max
Q(3) how can I use MOSFET to replica the work of the relay in case I can’t find 30A relay
Thanks for your timely response"

This is your previous reply before the above questions.
"you can connect it between the main DC supply and the battery control circuit. the transistors can be TIP35 for the upper transistor and 2N2222 for the lower one. resistors can be calculated as per the given formulas."
Swag said…
Hi Segun, I have not lost the track,

I am replying you correctly.

Earlier you asked me to provide a current control stage for adding to the above circuit right?

So I suggested you the following

So the TIP35/2N2222 was referred for this linked circuit.

and yes the 48V charger concept can be converted for your 12V, if a mosfet version is required instead of a relay.

In the 48V version, just remove all the 22K and replace them with a link.
remove the IC pin#7 zener diode, and also remove the 4k7 at pin#3 of the IC
solomon said…
hello sir, i need a help from u i need an automatic 12v battery charger with following features;
1. battery level indicator
2. during charging process, the indicators (LEDs in series) will be blinking one after the other according to the level of the battery during charging
3. and when the battery is full the charger cut the charging process and the level/status indicator continue to work
Swag said…
Hello Solomon, I have updated the diagram in the following article

for the cut-off facility you can further modify in the following manner
rajesh said…
hai sir

i want to change my 12v 17 ah battery for my bike. what is the trsformaer sec voltage. i had a 12-0-12 3A trsformer is it enough for that and if i want to put low adjust from 11.5 to 12.5 what i want to do. please advice
Swag said…
Hi Rajesh,

12.5V is not the optimal charging level for a 12V battery, it should be 14V.

You can adjust the circuit in the following way:

In the second diagram, keep both the presets to ground level initially.
Adjust the variable supply to 11.5V, and adjust the right side preset such that the relay just switches ON.
Now increase the voltage to 14V, and slowly adjust the left side preset until the relay is just switched OFF.

seal both the presets with superglue.

Yes the transformer would be fine to charge your battery
rajesh said…
hai sir

i meant that the battery comes 12.5 the charging needs to starts and stoping with 13.8 . for this adjustment what i need to do. can i use bd139 instead of bc547 because bd 139 is ready available with me.
do have any other same circuit using with out relay. i mean using other kind of active switching devices.
with the same starting and stoping voltage range.
Swag said…
OK, then you can try the first circuit from this article:

but it does not have a low restore selection, the battery will be cut of at the high 13.8V and restart at 13V, this will go on continuously. BD139 cannot be used because it won't handle 2 amp.
rajesh said…
hai sir
last time I tried with bc 547 and it is getting very I thought it requires high current transistor..
Swag said…
Hi Rajesh, none of the circuits which I referred use BC547 for charging the battery, BC547 is used only for sensing and cutting of signal to the power transistor or relay. It is the power transistor or the relay which actually handle the battery supply
rajesh said…
yes ..I am with you in your circuit you are using two bc 547b transistor one is for sensing and other one is driving the relay ..I am taking about the second one which one is driving the is getting hot .. that y I asked
Swag said…
that could be because you may be using a high current relay, make sure the relay coil resistance is above 200 ohms, and make sure to use a diode across the relay coil.

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