Auto Battery Charger for 6 or 12 Volt Sytems





Introduction: Auto Battery Charger for 6 or 12 Volt Sytems

About: 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 posting things I have learned and done since I got my first ...

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

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

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.

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.

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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232 Discussions

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 ?

1 reply

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.

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...

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 .

1 reply

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.

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?

2 replies

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

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.

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

1 reply

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)

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

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

1 reply

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.

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

2 replies

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.

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