Auto battery charger for 6 or 12 volt sytems

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 jambs. 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.
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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.

Step 2: Mount the transformer and attach the line cord

Picture of Mount the transformer and attach the line cord
Use screws to mount the transformer on the plywood base. Solder the ends of the power cord to the primary terminals of the transformer. I used a hot glue gun to cover the solder joints in order to protect against electrical shock. Hot glue is great for this. Be patient and let the first layer of glue harden so you can build the glue up for adequate protection.

From the photo you can see the details of the strain relief I made to hold the cord in place.

Step 3: Mount the switch and wire it.

Picture of Mount the switch and wire it.
Use the circuit diagram to wire the transformer secondary terminals to the switch. The aluminum angle is held to the base with screws. Label the switch positions for 6 and 12 volts.

Step 4: Wire the rectifier.

Picture of Wire the rectifier.
Of course you can use individual diodes to make a bridge rectifier. I found it easier to get a rectifier ready to use. I bent the input (AC) leads in one direction and the output (DC) leads in the other direction. This made a convenient base or support for the rectifier.

It is a good idea to use a heat sink when soldering diodes to protect them from too much heat. Put a rubber band on the handles of a needle nose plier and clamp the plier jaws on the lead you want to solder.

Watch the output polarity so the + terminal on the rectifier connects to the wire for the red aligator clip. I simply glued the bridge rectifier to the plywood with hot glue. Notice the strain relief for the output cord.

To use: Select 6 or 12 volts with the switch. Connect the red aligator clip to the red battery terminal and the black to the black. Set the base of the charger someplace safe. Plug in the AC cord. Disconnect the AC cord after 12 or more hours. Then disconnect the aligator clips from the battery. This prevents sparking that could possibly ignite hydrogen gas from the charging.

I have also used this charger as a power supply for things like a hot wire cutter. A smoothing capacitor is not necessary because batteries charge better with slightly choppy current.
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instped756 days 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)  instped756 days 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.1029723 days 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.
YousufS1 month 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)  YousufS1 month 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!2 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)  geolinl2 months ago
It will be safe.
ShajeehM2 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

Phil B (author)  ShajeehM2 months ago
We all begin someplace. I know a little, but there are many things I do not know.
There is a difference between the current rating of a battery and the current provided to the battery during charging. The current rating indicates how much current a battery can store, particularly for sudden high-demand requirements, like starting the engine in a car. A 26 amp battery can be charged with a 7.2 amp charger. If the current or amp rating of the charger is relatively low, it only means the time required for the battery to become fully charged will be extended.
ShajeehM Phil B2 months ago

this means that i can charge a 26amp battery with the your charger but the time required for battery to fully charged only will be extended. thank alot phil and one more question i have to ask does your charger has a cutt off facility i mean how many hours required for the battery to fully charged or at what point of time we have to stop charging to avoid explosion of the battery.

Phil B (author)  ShajeehM2 months ago

This charger does not have an automatic shut-off circuit. It charges slowly at a low current, which makes it unlikely that you will overcharge the battery and cause an explosion. I would not leave the charger on a battery indefinitely, but would remove it after the battery is sufficiently charged. By experience I learned charging a car battery overnight with this charger brings the battery up to a level that allows it to crank the car's engine and start it. From that point, I figure the car's alternator and charging control circuitry can take over. If you are charging a battery for use on something other than an automobile you can check the charge level with a squeeze bulb tester for specific gravity, if the battery is not sealed or a gel battery. A specific gravity tester draws fluid from the battery and something inside the glass tube floats. The float is calibrated in a way that indicates the degree of charge in the battery.

hi sir I have 1ph transformer 230/12 and 230/24 with 180-200 amp different current ratings controlled rectifier switch am I directly start car or heavy vehicles instantly with dead battery?

Phil B (author)  kulbhushan.tembhurne3 months ago
I am not an expert. From what I am able to read on the Internet, you should be able to crank your engine with your transformer. Someone suggested you should not continue to crank the engine for longer than one minute, or the transformer may overheat. The voltage required may be an actual 12.6 volts. 12 volts or less may be too little from what I read.
ata.mirza.713 months ago

hi sir i have a charger+32v-----375mA............... +16v-------500mA

and can i charge a 12v battery with this charger and i cant do this then pleas tell me how can i convert this charger to charge this battery .................................

Phil B (author)  ata.mirza.713 months ago
What you want to do is fairly easy. Is your charger based on a wire wound transformer, or does it use a switch mode power supply? (The wire wound transformer will feel heavy and will be larger than a cell phone charger. A switch mode power supply will feel very light in weight and be smaller like a cell phone charger.) If you have a wire wound transformer, you can use a simple variable voltage regulator chip, like an LM317 to reduce the output voltage to about 13 volts. (The voltage of a charger needs to be a little higher than the battery it charges.) An LM317 chip can handle up to 1.5 amps with a heat sink, but about 1 amp. without a heat sink. There are quite a few Instructables on power supplies using an LM317 chip. Most use a potentiometer for the resistor that sets the output voltage. It is much less expensive to calculate the value of the resistance needed and use a fixed resistor. Click here for an Instructable I did on a higher amperage power supply I made for my digital camera. It includes a link to an on-line calculator for the resistance. 

In my experience, an LM317 regulator chip does not work with switch mode power supplies. You can use common diodes in series to drop the voltage from 16 to about 13. Theoretically each diode will drop the voltage 0.6 volt, but when I tried it my results were more like 0.3 volt per diode. Here is an Instructable I did that uses diodes to lower the voltage of an old phone charger so I can power something else. A great thing about using diodes in series to lower voltage is that each diode can handle up to 1 amp. 

If you are charging a battery from an automobile, the charging times will be rather long because the amperage output of your charger is rather small. That is fine if you are patient.

I hope this helps.
hillmanie3 months ago

Hi - great topic. I've a powerful torroidal giving 18V AC open circuit and the PCB with diodes and charge indicator LEDs from a cheapie shop unit where its mains trfr burnt out after use. Can I use correct value (I over E = R) resistor say 10-20W to reduce the 18v to 13-14V? Do I put the resistor(s) in the AC or DC part of circuit?

Many thanks


ahmeddogar25 months ago

bhi ma agar transformer ki main wire ko direct laga du aur bhaki do ko direct battery ka sath laga du is this sutable

ahmeddogar25 months ago

bhi can you this in video

therahul6 months ago
therahul6 months ago
mel hadi6 months ago

is this charger suitable for charging 70-90 amps battery ?!

pmuhammad dwi made it!7 months ago

Is it OK to connect it only with 12v output pole?
I got 6 amp diode, can i charge my motorcycle battery with it?

but im not test it yet :(

greetings from indonesia

2014-05-01 05.19.41.jpg
Phil B (author)  pmuhammad dwi7 months ago
If your transformer has a center tap, you do not need to connect it if you are not using it. Is your motorcycle's electrical system 6 volts or 12 volts? The voltage of the charger should be very close to the voltage of the battery you are charging.

yes sir... i got 12 v
to make it more safer i put it in a wooden box

nikpav9 months ago

hi there mr. phil

i have an 12v 5000ma ac tranformer(it's is actually about 13,8v measure with multimeter) and i want to convert it to dc...

i saw your work and i think it is great.

with the characteristics i told you above of my tranformer,can you please advice me what components do i need to make change to dv 12v 5000ma

i am not an expert so please make me a list of exacly the parts(models) i have to buy..

like what type of brige rectifier,diode, etc.

the purpose of the project is to charge my motobike battery....

thank you in advance

Phil B (author)  nikpav9 months ago
5000 ma would be 5 amps. The bridge rectifier diodes should have a capacity a bit beyond 5 amps. to handle surges, etc. if you can, get diodes able to handle 8 to 10 amps. Thank you.
Phil B (author) 10 months ago
​it looks like you have everything you need. The voltage input is 230 volts, which I suspect is the normal voltage where you live. I would check the output wires with a voltmeter to see if you have about 12 volts DC (not AC).
allanjohn Phil B10 months ago
its DC sir..question sir is it ok to charge lead acid batteries with this kind of charger.?do they not harm or explode the batteries.?thanks sir..
Phil B (author)  allanjohn10 months ago

I have used similar chargers, including the one in this Instructable, to charge lead acid batteries. At 2.5 amp. output, it will not charge them too fast.

There are safety precautions to observe. Unless the battery is fully sealed with no venting, hydrogen gas will escape from the battery during charging. It is a good idea to connect the charger while it is not powered up. Connect the positive (+) lead to the battery terminal. Connect the negative (-) lead to a grounded clean metal object away from the battery (if you are charging a battery installed in a car). Power up the charger after the connections to the battery have been made. When the battery is charged, remove the charger from the power source. Then remove the connectors from the charger one at a time. This procedure prevents a spark that could ignite hydrogen gas.

allanjohn Phil B10 months ago

thank you very much sir..

allanjohn10 months ago

this is the other side..

allanjohn10 months ago

question sir..i got a transformer from altec lansing speaker..its a 12.5v has a rectifier and capacitors..can i use it to charge a 12v motorcyle.?do i have to add some resistors or any diodes.?or can already tap the + and - to the battery.?thanks sir..

russ_hensel10 months ago

I would consider a few minor revisions:

A bulb in series can be used to limit the current in cases of a short circuit ( like the two charging leads touch by accident ). Pick a 12 volt lamp with a current rating you like say 3 amp = about 30 watt bulb.

A fuse just in case, automotive 3 to 5 amp perhaps.

acelifted10 months ago

I forgot to add if the system is fully automatic i.e when the battery voltage reaches a preset point at which it will
switch to a very low current float charge. If the battery voltage drops
again the charger will begin charging until the voltage once again
reaches the cut off point. In this way it can be left connected to a
battery indefinitely to maintain full charge without causing damage. if not, what component can i introduce to make this happen?

Thank you


Phil B (author)  acelifted10 months ago

I would recommend shopping for a commercially available unit. You would to need to upgrade just about everything in this charger. Plus, you would need to add the auto shut-off circuit. I tried to repair one of those circuits for a friend by replacing a silicon controlled rctifier, but it did not work. Even if you were able to build what you want, I believe the cost of the parts could be as much as a a unit sold in a store.
acelifted10 months ago

Hi Phil,
Thank you for sharing this knowledge. Can you please help me with an information? I need a 12 volt battery charger with 12-14 amps capacity. from the schematic above, what are the components I need to change to achieve this (Transformer, diode specs etc)?

Many Thanks.


rasik410 months ago

sir ,i have 20 watt solar pannel and 20 amp car battery how can i charge my battery and light dc bulb ?

Phil B (author)  rasik410 months ago
First the solar panel needs to put out at least 12 volts. Most add a diode so the battery does not discharge itself through the solar panel. At 20 watts, the solar panel will put out about 1.4 amps, which is not a lot. I expect you can use the solar panel to charge the battery and run the light from the battery.
johannes831611 months ago

hi..i have an 220 vac to 24 vac with 6 ampere and i want to convert it to 24 vdc so my question is; how much current of the bridge rectifier should have ? is it ok that i use a 1 amp bridge rectifier?

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