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.
nikpav1 month 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)  nikpav1 month 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) 2 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 B2 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)  allanjohn2 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 B2 months ago

thank you very much sir..

allanjohn2 months ago

this is the other side..

allanjohn2 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_hensel2 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.

acelifted2 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)  acelifted2 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.
acelifted2 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.


rasik42 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)  rasik42 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.
johannes83163 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)  johannes83163 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)  RickyYardSales7 months 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 B7 months 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)  aniq7 months 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)  RickyYardSales7 months 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)  RickyYardSales7 months 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 B7 months ago
yes its 12V dc
Phil B (author)  RickyYardSales7 months ago
Adding to my comment below, red is + and black is -.
aniq7 months ago
The transformer looks like this:
Phil B (author)  aniq7 months ago
Are you certain the output current is DC? It looks like an AC transformer, which means you would need a bridge rectifier. The 4007 diodes might work as long as the current draw does not exceed their limit. Still, you have the problem of getting the output voltage down to around 6 or 7 volts. Just looking at the transformer in the photo, I would guess its output might be around 2 to 3 amps.

I see three tabs with solder. Is the transformer a center tap style that would produce a lower voltage between the center tab and one of the outside tabs? (Check with a voltmeter.)
aniq7 months ago
nice instructable
i m new here
i wanted to ask that i have a 6v 4.5 ah SLA battery,can i charge it using 12 v dc transformer with a bridge rectifier?
Looking forward for your suggestion.
Phil B (author)  aniq7 months ago
Welcome to Instructables.

If your transformer's output is DC current, you do not need to add a bridge rectifier. A bridge rectifier converts the current from AC to DC, and your transformer's output is already DC.

But, 12 volts is too much for charging a 6 volt battery. You need to reduce the voltage to 6 volts or 7 volts. (I think someone said batteries charge better if the charger's voltage is just a little above the battery being charged, but not too much.)

You can reduce the voltage to the battery in several ways. One would be two resistors in series to make a voltage splitter. But, the resistors will need to be able to handle the power to be dissipated without becoming so hot that they fail. You could also make a voltage regulator from an LM317 variable voltage regulator chip. But, this chip can handle only 1.5 amps at most, and that is with a heat sink. There are many Instructables on using an LM317 chip. A lot depends on the current output of your 12 volt transformer. If it is too high, an LM317 chip would burn up and fail. You could also drop the voltage by stringing together a chain of diodes properly aligned for polarity. Each diode will drop the voltage around 0.5 to 0.6 volt. But, the current flowing in the circuit would need to be lower than the maximum tolerated by each diode so that they do not burn out.

The best would be to find a transformer with an output of about 6 to 7 volts DC.
Dr.Bill9 months ago
I put one together like yours switched for 6v and 12v.
I added a switch to the line cord so I can turn on/off the transformer as well.
I have it hooked up to a 2 speed 12v fan I got in K-Mart.
This allows me to get 4 speeds out of the fan.
Fan set to high, transformer set to 12v then 6v so that's 2.
Fan set to low, transformer set to 12v then 6v so that's 2 more.
Its nice to have air moving when ya sleep.
Phil B (author)  Dr.Bill9 months ago
Thank you for telling me. I have used mine for all sorts of things in addition to charging a car battery.
iconrl10 months ago
Could I use a transformer like this one? It's out of an uninterruptable power supply (UPS).
Phil B (author)  iconrl10 months ago
Probably. You need line voltage inputs and outputs of about 12 to 14 volts. You probably will not have any leads in the 6 to 7 volt range for 6 volt batteries. The current output will determine how much time is needed to charge an auto battery.
iconrl Phil B10 months ago
Thanks for the reply. I probably won't have trouble connecting the wiring, but how can I tell which connections are input and which are output? I have a multi meter, but am not sure how to test this.
Phil B (author)  iconrl10 months ago
You may be able to tell by taking resistance readings between sets of leads. What is the resistance between the blue and brown wires? I am guessing they are 120 volt inputs. I am guessing the black and white wires or the black and red wires are the 12 volt output wires. What is the resistance between black and white or black and red? I am guessing the brown and blue wires might draw an amp or two. Divide 120 volts by the resistance shown between brown and blue. Is the answer anywhere close to 1 or 2? There will be some additional resistance in an AC circuit due to impedance, and that will affect the mathematical answer.

If you have a low voltage AC transformer you could connect its output to one set of leads and see what you read out on the other set. That would give you a pretty good clue about which set of leads goes to the primary and which goes to the secondary.

I cannot see the end of the red word in the phot to know what it's end looks like. Is it like the black and the white?
iconrl Phil B10 months ago
Blue - Brown = 1.6
Black - White = 1.2
Black - Red = 1.2
Red - White = 1.2
The red, blue, and brown all have female spade connectors, but they have a clear boot on them.
I just realized my multi-meter is just like your red one used in some of your instructables. I had it set on 200Ω.
Thanks for your help. I really appreciate it, especially since I'm extremely an amateur at this, but I like to learn.
Phil B (author)  iconrl10 months ago
Thank you for your reply and your efforts. I had hoped for more difference between the readings would be greater so there would be an easier way to differentiate the windings. I still think the blue and brown wires are probably the 120 volt inputs. They appear to be thinner wires than the red, white, and black wires. A transformer for a UPS would not need a lot of current from the line voltage supply. The output would be at a lower voltage, but a higher current; and would use heavier wires.

I did find an article on the Internet for people who work with antique radios and may have several old unmarked transformers in their parts bin. It gives some guidance, although there still may be some guessing involved. Just click on the hot link in this reply.

I think the safest thing to do would be to find an AC transformer for charging an old battery powered tool that uses 120 volts in and gives out around 9 volts AC. Attach the 9 volts AC to the blue and brown leads. Take a reading on the red and black leads. See if it is around 1 volt. That is not enough to hurt you or the transformer. (Of course, if my suspicions are backwards, the red and black wires would have around 90 volts coming out of them.) The test transformer needs to be AC rather than DC because transformers do not work with DC current.

tutdude9810 months ago
whats the voltage on the output? shouldnt be there 1v drop on bridge rectifier? and also shouldnt car battery be charged with 13-14v

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