Picture of Auto battery charger for 6 or 12 volt sytems

It is handy to have a small battery charger for your automobile, especially if someone parks it in your garage for the night with a door just slightly ajar. In that case, the dome light will remain on all night and the battery will be very low in the morning.

I made this charger when I ordered some electronic parts and received a 120 to 12 volt AC transformer with center tap. Sending it back would have cost as much as the transformer. Similar transformers can be found at places like Radio Shack. The transformer puts out about 3 Amps., so it is ideal as a trickle charger. A charge will require about 12 hours. But, it has gotten me out of several jams. Some friends have also borrowed it when their batteries were dead and it worked for them.

The center tap allowed the output to be either 6 or 12 volts. At the time I made this charger I helped to maintain an older farm tractor with a 6 volt electrical system.

Note: The output of the transformer is actually about 13.4 volts. When the voltage passes through the diodes in the rectifier it drops 0.6 volt for each of two diodes to roughly 12 volts. If you look for a 12 volt transformer you may at first be frustrated because you can find only 13.4 volt transformers.

Step 1: The circuit and what you need

Picture of The circuit and what you need
The circuit is very simple.

I used a piece of plywood for a base. I already had a double pole toggle switch. I used a piece of scrap aluminum cabinet angle to mount the switch. Any piece of aluminum or steel could be bent to do the job. I used a 4 Amp. bridge rectifier from Radio Shack. I also got aligator clips already connected to a cord to connect to the auto battery. This was also from Radio Shack. The AC cord and plug came from a neighbor's discarded televison set that was put out on the curb on garbage day. You will need some screws, soldering iron, and a hot glue gun.
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Luthfi Adam13 days ago

Just did it, 3A trafo i bought for Rp50000 (about $4), and 4A Brectifier. Used it to charge my motorcycle accu which previously has dropped to 11.4v (holy-S!). For now it's still working just well, 30 mins has passed and the accu voltage raised to 11.7v. It's just a small accu (40Ah?) but i guess i have to wait for more hours.
My question is, what voltage do i have to stop charging? I think i'll stop the charging when it reaches 13v. I kinda afraid if it would blew up if i crank it to 14v.

*excuse me bout my english, not a native*

Phil B (author)  Luthfi Adam12 days ago
Your English is much better than a lot of people who claim it as their first language. I have worked at learning a second language and know how difficult it is to use it well.

If your battery is sealed, it is difficult to know when it is fully charged. You could do some research on the Internet to see if you can find any data on your battery's voltage at a full charge. If your battery has removable caps so you can add water to the cells, any sign of boiling or of depletion of the fluid are indicators of over-charging. To be safe, I would charge until the battery has enough power to crank the engine. I expect the motorcycle has a charging circuit that will maintain the battery as you ride it.

If the battery does not take a charge, it may need to be replaced.

Congratulations on a job well done,

AbdulH91 month ago
Sir , how to make a permanent auto charger for 12v
Phil B (author)  AbdulH91 month ago
By "permanent" do you mean one wired directly to the battery so it is never removed? You might want to check the cost and availability of a battery conditioner. These have smart circuitry to avoid over charging the battery.
mmikid2 months ago
Sir, can a 9v charge my car battery
Phil B (author)  mmikid2 months ago
No. A 12 volt auto battery needs a charger a little larger than 12 volts by a volt or two.
PramodL3 months ago
sir, can i use this circuit for 6volt 700mah battery?
this is a rc car battery i want to use that in my project so plz help me
Phil B (author)  PramodL3 months ago
I suppose you could. It seems you could use something less, like a power adapter from a device you no longer use. That would be free. If the voltage on the power adapter is too high, a string of diodes will bring the voltage down to the desired level. Be sure the diodes are able to carry the current load. Basic diodes can handle one amp.
student2189433 months ago
sir how to made simple home made 12v Dc to 230v Ac inverter? sir plz tell me
Phil B (author)  student2189433 months ago
This is a bit outside my area. In theory, a transformer will not work with DC current, but needs AC current. A circuit that rapidly reverses polarity in the 12 volt portion of the circuit is needed. That is then fed into a transformer that converts 12 volts to 230 volts. Here is a so-called simple inverter circuit for 12 to 120 volts. Just use a transformer capable of putting out 230 volts. You will need to consider the maximum current load at which you want the circuit to perform and make certain all components are able to handle that.

SharanS3 months ago
How to 100Ah battery Charge
Phil B (author)  SharanS3 months ago
That is a small battery. I would use an old power supply for some household device. The power supply should have a low current output. If it is not already DC rather than AC, add a bridge rectifier. If the voltage is too high, use a variable voltage regulator and configure it for the charging voltage you need. You will likely need to experiment to determine the proper amount of time required to charge the battery.
SharanS3 months ago
How to 100Ah battery Charge
SharanS3 months ago
Sr plz 100Ah battery charge
tsaleem5 months ago
sir how much ampere per hour by this charger
Phil B (author)  tsaleem5 months ago

Batteries produce a number of amps per hour. This charger produces 3 amps. at 12 volts continuously. A larger capacity transformer would produce more amps.

sir, if i increase the transformer to have more amps, should i also need to increase the rectifier amp rating?

Phil B (author)  geoaldous.agunias5 months ago

Yes. The rectifier will need to be able to carry the maximum output expected from the transformer.

rj7366 months ago
Phil B (author)  rj7366 months ago
If you intended to send a comment, the text did not register.
rj7366 months ago
rj7366 months ago
rj7366 months ago

sir how can we charge only a 12v battery without a double switch for 6v nd 12v battery... i just need to charge a 12v battery ...so can u plzz give me a circuit diagram without double swith...

Phil B (author)  rj7366 months ago
Just ignore the center tap on the transformer. Use the bridge rectifier and connect it to the end taps on the transformer.
Phil, I have slightly different use but need your advice. I use electrolysis in the restoration process to help me bringing old machinery back to life. My old manual battery gave up the ghost on me and i desperately looking for a replacementthat isn't too smart like all the new ones are. I would actually like to make about two or three that will cook at varied amperages. Maybe 25, 75 and 150 amps. What would you suggest?
Phil B (author)  jonathan.farmer.9256 months ago
Thank you for your inquiry. I have no experience with electrolysis. I can not make an informed recommendation.
12-0-12 transformer
12-0-12 transformer
Which tool u used to shift over to 6V or 12v
And sir how to switch over to 6V and 12V
Sir is this possible to get charged 9V and 6V AA RECHARGEABLE BATERY with this circuit
instped758 months ago

so, if you have a transformer, a 230/240V AC to 12 v AC, or even a 14 v and you need to get a rectifier to convert the output to DC, which one do you need to get? is that depending on the output Amps. What specs does it need to have?

Phil B (author)  instped758 months ago

The transformer may be marked to indicate the output in amps. You may need to calculate the current based on the wattage of the transformer, assuming wattage is marked on the transformer. Wattage is volts multiplied by amps. Because transformers are usually highly efficient, the output wattage should be very close to the input wattage. So, if the input voltage is 240 volts, and the input wattage is 100 watts, the input current must be 0.4 amp. That means the output current would be 100 watts divided by 14 volts, or about 7 amps. You would need diodes capable of handling 7 or 8 amps.

when our battery fully charge this cur cut will spout to auto stop charging or not so what we can use auto stop charge

Phil B (author)  faisal.iqbal.102979 months ago
It charges slowly enough that shutting it off at a precise time is not critical. But, you may be able to find plans for auto-shutoff circuits and build your own.
YousufS9 months ago

24 volt battery with 6 volt battery and 180 amp only how can do this please solve my problem with circuit diagram

Phil B (author)  YousufS9 months ago
Just connect your two batteries to a charger in series. Make the voltage of the charger slightly larger than the combined voltage of the two batteries. The current output of the charger will determine how much time is required to charge the batteries. If the 6 volt battery has a lower current capacity than the 12 volt battery, it will likely reach a full charge before the 12 volt battery. Watch for signs of over-charging on the 6 volt battery, like bubbles foaming out of the vents on the cell caps.

Every automotive battery is made of smaller cells joined in series. You can join two batteries in series and charge them simultaneously.
geolinl made it!10 months ago

Dear sir.
It will be safe to use Coleman Adaptor #5232E640T
INPUT: 120VAC 60HZ @ 1.4 AMPS
To Charge a 12 Volt - 700cc Car Battery.
Thank You.

Phil B (author)  geolinl10 months ago
It will be safe.
ShajeehM11 months ago

how can we make a circuit according to the current requirment of the battery for example if i want to charge a 12 volt 7.2 amp battery with this circuit does this circuit works?? and if i want to charge a 12 volt 26 amp battery will this charger circuit works basically i want to ask what changes we have to make in the circuit if the current rating is different.may be you find this a dumb question but as i beginner i want to remove my misconceptions

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