Making a Simple, Smart Automatic Battery Charger Circuit – Universal Battery Charger Circuit

The following automatic smart battery charger circuit was exclusively designed by me in response to requests from two keen readers of this blog, Mr. Vinod and Mr.Sandy.
Let’s hear what Mr.Vinod discussed with me through emails regarding the making of a smart battery charger circuit:

“Hi Swagatam,
My name is vinod chandran. Proffessionaly i am a dubbing artist in
malayalam film industry but i am an electronic enthusiast too.
I am a regular visitor of your blog. Now i need your help. I just
built an automatic SLA battery charger
but there is some problems with that. I am attaching the circuit with
this mail. The red LED in circuit is supposed to glow when battery
is full but it glows all the time .(my battery shows only 12.6v).
Another problem is with 10k pot. there is no
difference when i turn the pot left and right. . So i request you to
either correct these problems or help me to find an automatic charger
which gives me a visual or audio alert when battery is full and low .
As a hobbyist i used to make things from old electronic appliances.
For the battery charger i have some components.
1. Transformer from an old vcd player. out put of 22v, 12v,3.3v. And i
don’t know how to measure ampere.
My DMM has only the ability to check 200mA. It has a 10A port but
i can’t measure any ampere with that.(meter shows “1”)
So i assumed that the transformer is above 1A and below 2A with
the size and requirments of the vcd player.
2. Another transformer -12-0-12 5A
3. Another transformer – 12v 1A
4. Transformer from my old ups(Numeric 600exv). Is this tranformer’s
input is regulated AC ?
5. couple of LM 317’s
6. SLA battery from old ups- 12v 7Ah. (Now it has a 12.8v charge)
7. SLA battery from old 40w inverter – 12v 7Ah. ( the charge is 3.1v)
One thing i forgot to tell you. After the first charger circuit, i
made another one (i’ll attach this too). This is not an automatic one
but it is working.
And i need to measure the ampere of this charger. For that purpose i
googled for an animated circuit simulation software but didn’t get one
Atlast i found one in this link
( But i
can’t draw my circuit in that tool. there is no parts like LM317
and LM431(variable shunt regulator). not even a potetiometer or led.
So i request you to help me to find a visual circuit simulation tool.
I hope you will help me.

vinod chandran”

Hi Vinod, > > The red LED should not glow all the time and turning the pot should change > the output voltage, without the battery connected. > > You can do the following things: > > Remove the 1K resistor in series with the 10K pot and connect the pot’s > relevant terminal directly to ground. > > Connect a 1K pot across the base of the transistor and ground (use center > and any one of the other terminals of the pot). > > Remove everything that’s presented at the right side of the battery in the > diagram, I mean the relay and all….. > > Hopefully with the above changes, you should be able to adjust the voltage > and also adjust the base transistor pot for making the LED glow only after > the battery is fully charged, at around 14V. > > I don’t trust and use simulators, I believe in practical tests, which is the > best method of verifying. > > For 12v 7.5 ah battery, use a 0-24V 2amp transformer, adjust the output > voltage of the above circuit to 14.2 vollts. Adjust the base transistor pot > such that the LED just starts to glow at 14V. Do these adjustents without > the battery connected at the output. > > The second circuit is also good but is not automatic….is current > controlled, though. > > Let me know your thoughts. > > Thanks, > Swagatam

Hi Swagatam Your Idea is graet. The charger is working and now one LED is glowing to indicate the charging is in progress. but how can i configure the charge full indicator LED. When i turn the pot to ground side (means lower resistance) LED starts glowing. when resistance goes high LED will be off. After 4 hours of charging my battery shows 13.00v. But that charge full LED is off now. Plz help me.

I am sorry disturb you again. The last email was a mistake. i didn’t
see your suggestion correctly. So please ignore that mail. Now i adust
the 10k pot to 14.3v(it’s quite difficult to adjust the pot, because a
slight variation will result a bigger voltage output. ). And i adjust
the 1k pot to glow a little. Is this charger supposed to indicate a
14v battery?. After all let me know the danger level full charge of
the battery. Once again thanks.
vinod chandran
Hi Swagatam,
As you suggested, everything was alright when i test the circuit from
breadboard. But after solder into PCB thing are happening strangely.
The red LED is not working. charging voltage is ok. Anyway i am
attaching the image that shows the present condition of the circuit.
plz help me. After all let me ask you one thing. Could you please give
me an automatic charger circuit with a battery full indicator. ?.
Hi swagatam,
Actually i am in the middle of your automatic charger with hysteresis
feature. I just added a few modifications . i will attach the circuit
with this mail. plz check this out. If this circuit is not ok then i
can wait for you to tomorrow .
vinod chandran

I forgot to ask one thing. My transformer is about 1 – 2 A. Idon’t know what is the correct. how can i test with my multimeter?.
Besides if it is a 1A or 2A transformer, how can i reduce the current
to 700mA.
Hi Vinod,
The circuit is OK, but won’t be accurate, will give you a lot of trouble
> while adjusting.
A 1 amp transformer would provide 1amp when short circuited
(check by connecting the meter prods to the supply wires at 10amp range
and set to either DC or AC depending upon the output). Meaning the
maximum power of is 1amp at zero volts. You may use it freely with a 7.5
ah battery, it won’t do any harm, as the voltage would drop to the
battery voltage level at 700ma current and the battery would get safely
charged. But remember to disconnect the battery when the voltage reaches
14 volts.
Anyway, a current control facility would be added in the circuit that I would be providing you, so there’s nothing to worry

Hi Vinod,

I’ll provide you with a perfect and easy automatic circuit, please wait
>> > until tomorrow.
Hi swagatam,
I hope you will help me to find a better solution. Thank you.
vinod chandran

In the meantime, another keen follower of this blog Mr.Sandy also requested a similar battery charger circuit through comments……

So finally I designed the circuit which will hopefully satisfy the needs of Mr.Vinod and Mr.Sandy for the intended purpose.
The following figure shows an automatic 3 to 18 volts, voltage controlled, current controlled, double stage battery charger circuit with standby charging feature.

For circuit explanation please read the comments…..