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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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.tembhurne16 days 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.

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.7127 days 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.
hillmanie1 month 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


ahmeddogar22 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

ahmeddogar22 months ago

bhi can you this in video

therahul3 months ago
therahul3 months ago
mel hadi3 months ago

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

pmuhammad dwi made it!5 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 dwi5 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

nikpav6 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)  nikpav6 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) 7 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 B7 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)  allanjohn7 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 B7 months ago

thank you very much sir..

allanjohn7 months ago

this is the other side..

allanjohn7 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_hensel7 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.

acelifted8 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)  acelifted8 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.
acelifted8 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.


rasik48 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)  rasik48 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.
johannes83168 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?

Phil B (author)  johannes83168 months ago
I would want a bridge rectifier capable of handling at least 6 amps.
hi, i have a 12v DC Power Supply 2.9amps used to power a multi media speaker. can i use it to charge my car battery directly? or i have to add up a rectifier
oh sorry my bad.. its a 12v 2.9amps AC adaptor. can i convert it somehow to charge a car battery?

and i have a transformer with 12v output, can i use a 6A05 diode (i have 4 6A05 Diodes) to make a car battery charger?
Phil B (author)  RickyYardSales1 year ago
A 6A05 diode is rated at 6 amps. That will more than handle the 2.9 amps from your charger. When a current passes through a diode, the voltage drops about 0.5 volt. A bridge rectifier causes current to pass through two diodes at any given time. But, the output of your 12 volt charger is probably a volt or so higher than 12 volts, anyway.

A bridge rectifier smooths AC current when converting it to DC quite a bit. Batteries charge better if the current is a little choppy. Some early battery powered screwdrivers used an AC charging transformer with one simple diode between the transformer and the batteries. It was cheap, but it also lengthened the charging time by using only one half of the sine wave. You can assemble and use a bridge rectifier with four diodes and it will work.
aniq Phil B1 year ago
on my 6v battery its written max current 1.35 amps what do you suggest and by the way the transformer isnt a center tapped
Phil B (author)  aniq1 year ago
Did you see my response to you below on using a variable voltage regulator chip, like the LM317, and adding a heat sink? It will drop a 12 volt DC source to 6 volts (if configured properly) and will handle up to 1.5 amps with a heat sink.
this is the 12V 2.9amps AC adaptor, can i use this to charge a car battery?
Phil B (author)  RickyYardSales1 year ago
You can use this transformer to charge your automobile battery. Since it produces AC current, you will need to use your four diodes to make a bridge rectifier so the current is converted to DC power. Since the output current is larger than 1.5 amps, you will need a voltage regulator chip that can handle more current. I did that in this Instructable when I needed a power adapter for my digital camera. You will find information there about the chip to use.
or how can i convert it to DC?:)
Phil B (author)  RickyYardSales1 year ago
If it is DC it will work. Just get the terminals right: red to red and black to black. 2.9 amps will make a nice trickle charger.
aniq Phil B1 year ago
yes its 12V dc
Phil B (author)  RickyYardSales1 year ago
Adding to my comment below, red is + and black is -.
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