How to build 12v-6v battery charger

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 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

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.

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.

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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