Introduction: 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 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.

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.


JimK155 (author)2018-01-18

Hi I have just purchased a 12V ATV battery(AGM) .I have a older 10 amp battery 12V / 6V charger with overload protection. My new battery says to charge at 1.2A for 5 to 10 hrs .Do I need to purchase a new charger ?

Phil B (author)JimK1552018-01-18

You made me learn something. I found this page about charging AGM batteries by Optima. Low and slow is best, but they mention 1 to 10 amp. chargers as acceptable. Simple circuits for a current limiter using a regulator chip and a very few resistors and capacitors, maybe a diode or two can be used to make your own limiter. You could also contact the maker of your battery to see what they say.

NoskillsrequiredN (author)2017-12-05

Hi there, buddy, do you have any idea there is a relation between amps needed and the capacitor on the bridge rectifier side i mean how do you choose the capacitor there must be something...

optimusprime 17722 (author)2017-08-05

Phil if i may , . I have 2 battery chargers ,the one i use the most i have done a check on it,, and the output is aprox 12 volts . .But the older one is only puting out 10 volts aprox .Do you know what that reason would be?.I thank you for the diagram i will make one a soon as i can .

I am guessing there may be an internal short in the transformer that causes it to be less efficient. If that is the situation, extra heat in the battery charger could be expected. You could check two things. One would be the input current on the lines or mains. It would be higher than what the label says. Look at the label to see how many Amps. or Watts the unit is supposed to consume and check what it actually consumes. The other check would be for any leakage between between any of the leads and the frame of the transformer.

Or, a connection on the secondary side may be presenting a high resistance that is dropping the output voltage. The resistance could be corrosion or oxidation that you cannot see without taking a connection apart, especially if it uses a bolt or a screw.

Thank you for looking. As I said I am only guessing at possibilities.

ReynaldoB9 (author)2017-07-11

Can we use an ammeter after the bridge rectidier in serie with the load to monitor the level of the charge?

No amps consumption means the battery is already charged?

Phil B (author)ReynaldoB92017-07-11

I am no expert, but what you propose sounds like it should work.

ReynaldoB9 (author)Phil B2017-07-11

Thanks for your answer !

rrn1 (author)2017-06-03

Can we add capacitor ?

Phil B (author)rrn12017-06-03

If you want to use this as a DC power supply, a 1000 micro-farad capacitor is needed to smooth the ripples. But, I read that a battery charger works better when the current is not completely smooth.

horusapollo (author)2017-05-28

I have 12v to 17v dc generator can I use this charge a 12v battery and then the 12v battery to a invetor and then use 150 volts. Please help

Phil B (author)horusapollo2017-05-28

I believe you can. The inverter should have enough wattage to power the current draw of whatever you are using. So, a 3 Ampere current draw at 150 volts would require an inverter capable of handling 450 Watts or more. (Volts x Amperes = Watts)

syed subhan (author)2017-04-09

Tell the value of rectifier you used

Phil B (author)syed subhan2017-04-09

My bridge rectifier is rated at 4 amps and 50 volts.

syed subhan (author)2017-04-09


ManishJ24 (author)2017-03-21

Do I need to purchase a new transformer? I have the one with just two wires. Won't that work?

Phil B (author)ManishJ242017-03-22

A transformer with two output leads will work fine for a single output voltage. Mine had a center tap and that allowed me to make a dual voltage battery charger.

RickK60 (author)2017-03-14

Did everything identical to yours, i thought. When i plugged it ,transformer smoked. Transformer was a jameco 221356 -to a NTE 53016- OUTPUT. what might be wrong

Phil B (author)RickK602017-03-14

My first thought is the switch for selecting 6 volts or 12 volts is wired incorrectly and creates a direct short across the secondary of the transformer.

I observed a great trick when I saw a furnace repairman replace a transformer on the ignition system. He placed a fuse in line with the transformer. If anything went wrong, the fuse blew and protected the transformer.

RickK60 (author)Phil B2017-03-14

I dont have a 6 or 12 volt selector switch. Transformer only has 2 wires coming out, no center tap.

RickK60 (author)RickK602017-03-14

Tried again with new transformer, worked great. Im assuming first one was bad. Thankyou for your time and help

Phil B (author)RickK602017-03-15

I am glad it worked for you. I am sorry you had to go to the expense of another transformer. Thank you for the update.

Phil B (author)RickK602017-03-14

Sorry. When you said yours was exactly like mine, I assumed you had a center tap transformer and a voltage selector switch. I suppose I would disconnect the transformer from the rest of the circuit and then connect an ohmmeter to the rest of the circuit to see if it presents a dead short to the transformer. If you have a diode check function on your multimeter, you can check to see if the diodes in the bridge rectifier are funtioning as they should and not presenting a shortcut that allows too much current to flow where it should not. All of this assumes there was no problem with the transformer before you began.

gabrielg199 (author)2017-01-20

How can we add a automatic shatdown if the battery is fully charge??

Phil B (author)gabrielg1992017-01-20

That is beyond my knowledge. I believe you can find circuits for doing that on the Internet. It would be complicated. A simpler thing would be to use a common household timer on the power supply to the charger. Allow the charger to run the number of minutes you know will charge your battery.

I did once buy a battery conditioner for auto batteries for about $30. It automatically keeps a 12 volt auto battery fully charged.

gabrielg199 (author)2017-01-20

Dr. Bill

BaolongN (author)2016-10-14

Hello Dr. Bill
I have 12v schumacher battery charger, the transform inside as I look at, the secondary have 3 output tap, center tap go to negative output, both outer taps go to two diodes(half wave rectifier). The AC voltage from 2 outer taps I got 29volts . How can I wire this charger to become 24volts DC charger?

Phil B made it! (author)BaolongN2016-10-14

I made a diagram. Ignore the center tap. Use the two outside taps and a bridge rectifier or make your own from four diodes big enough to handle the maximum amperage you expect plus some for safety. The output voltage will be a bit larger than 24 volts, but not much. Check the polarity on the output before connecting to a battery.

Dr.Bill made it! (author)2016-10-04

I added a small value electrolytic capacitor across the output of the rectifier to smooth out any ripple and got an increase in voltage to 15 volts and on the 6 volt side to 7 volts. I also added an ne2 neon indicator for 120 volts and a momentary switch to discharge the capacitor. I find the charging abilities of this charger on a car battery to be better with the additional capacitor because the battery goes to full voltage. I use it to charge the batteries on both my portable solar carts when the sun is not out.

Phil B (author)Dr.Bill2016-10-04

thank you for sharing what you did.

amalik101 made it! (author)2016-08-22

Worked great.. This is exactly the schematic of lead acid battery desulfator having 16v output..
I have further three or four querries. we can boost the amps?*
2. Increase 12v to 16v without changing the transformer?*
3. Add trip logic when battery reaches 13.8v?
*Add a boost switch to increase the amps to 7 and 16vdc.
Kindly guide me further.
Thanks a bunch

Phil B (author)amalik1012016-08-22

I would like to be helpful, but your questions are beyond my knowledge. Perhaps someone else here can answer them.

amalik101 (author)Phil B2016-08-22

That's ok Mr. Phil. :) you have already helped enough.. Thanks again.

frank_anosike (author)2016-06-09

hello Sir,

please i would like to have a detailed circuit diagram for a 12v/75ah car battery charger. thank you.

Phil B (author)frank_anosike2016-06-09

From what I read, a battery should not be charged at a rate greater than one-fifth of its amp. hour rating. If you have a 75 amp. hour battery, the output of the charger should not be greater than 15 amps. There are a variety of circuits you can use. I have given a schematic in block form. If you do an Internet search, you will find many others. Some are more complicated and some are less complicated. The more complicated will require more expense and skill to assemble. But, they will also have more controls on the charger. A 15 amp. charger will require a transformer with an output of 15 amps. and a bridge rectifier capable of handling that much and more for a safety margin.

frank_anosike (author)Phil B2016-06-09

Thank You. will get back to you in case of any difficulty

alayinla (author)2016-03-15

Hello sir Pls help me out I want to build a 12v/70ah charger Pls how may I do that with transformer and diode

New_Educators (author)2016-03-10

hello sir

could this work for educational purpose as a source for car wirings circuits?

or even better buing one battery charger could make a wiring light of car with relay or flasher ( low light consumption) to work?

Phil B (author)New_Educators2016-03-10

Yes. The components I used are rated at only 3 amperes. The demands of devices you are demonstrating should not have a combined simultaneous current draw that exceeds the maximum output of your components. And, for the application you have in mind, I would add a 1,000 microfarad electrolytic capacitor at the output terminals of the bridge rectifier to smooth the current. That is good practice if you are using this as a power supply, not just as a battery charger.

DanishH3 (author)2015-10-20

Can I charge 12v battery with a 16v 4.5amp power supply ? It is a laptop chargers basicly.. I am using it to charge my battries.. will it damage my battries ?

Phil B (author)DanishH32015-10-21

I would want the voltage of the charger to be closer to the voltage of the battery. There may be a way to drop the voltage, but it might be more difficult and expensive than simply getting the right charger. Much depends on what batteries you are charging. A lead/acid battery would probably be more forgiving than a lithium-ion battery. Someone has done some Instructables on changing the output of switch mode power supplies. But, you would need to open the case of the laptop charger, find the correct resistor and change it to the correct value for the desired output.

Austine kauzeni (author)Phil B2016-02-19

Hi sir my name is Austine kauzeni from Tanzania i am a biginner to the electronics and i want to make an inverter than can also charge itself then i can also walkout with it anywhere and continue using without a problem of power drop . But kindly can you help me a circuits diagram?

sohail raza (author)2016-02-17

Hellow Sir,

I am making a 12 volt egg inkobeater and wants to run with 60 Amp wet battery, I want to make charger for battries need out put 65Amp please guide me While i have tried but that gives me just 7Amp

Phil B (author)sohail raza2016-02-18

In the end, the cost will be less if you can simply buy an automotive quick charger. Those have a high output like you need. I do not know where you would get all of the high current capacity parts needed to build your own. A 7 amp. Charger will eventually charge a battery that needs to put out 65 amps., but slowly.

itscipher (author)2015-12-31

Hello I have a 12 volt solar charger that I want to drop to 6 volts. I see you are able to do this pretty easy. But I am just not seeing how you can drop the voltage from 12 to 6 volts could you better explain this please?

Phil B (author)itscipher2015-12-31

The transformer I used has a center tap. The voltage between it and one of the outside taps is 6 volts. The voltage between two outside taps is 12 volts. Can you tap into your solar cell charger in a way that bypasses half of the cells?

Nono41012 (author)2015-12-09

hi sir

can this charger work with 12V 100Ah valve regulated battery? i bought an old battery from a junk shop nearby. i will use it as a backup power source when brownouts occur here in our place. i can't find removable caps on these batteries for adding water, its like a sealed type battery and only the two terminals are present on both ends.

Phil B (author)Nono410122015-12-10

I would think it could. You would want to avoid over-charging. Maybe you could run the charger through a timer. Have the timer come on for 30 or so minutes each day and see if that is enough to maintain a charge.

Nono41012 (author)Phil B2015-12-25

thank you very much sir, I'll try to make one. Uhmm.. any recommendation where i could add the timer in the circuit diagram you shown? I love electricals/electronics but I'm just a beginner. There are circuits I browsed on the net and this one you have is the simplest charger circuit I found, while the other ones have this IC parts that I was trying to analyze the works, you know.. kinda like catching a cheeta in the amazon!

Phil B (author)Nono410122015-12-25

ICs can do wonderful things, and are great, if they are really necessary. I do not understand much about most of them. I prefer to make things simple. If you wish to use a timer, add it to switch the AC mains.

There is a great story about Thomas Edison and Charles Stienmetz. Edison had a very large glass envelope for a special light bulb. He asked Steinmetz to calculate its internal volume. Steinmetz did a variety of calculations and took his results to Edison, but explained the uncertainty in his calculations. Edison asked him why he had not simply filled the glass envelope with water and measured how much water it contained. It is a lesson in simplicity.

About This Instructable




Bio: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying ... More »
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