How to Build a DC Battery Charger

by William Kinsey

Battery chargers are used to recharge rechargable batteries. These chargers come in a wide variety of sizes and power output. As more power is needed, the battery charger will grow in side due to the size of transformer needed for the given power requirement. A typical battery charger consists of a transformer, rectifier circuit, and any necessary voltage regulation. The transformer can reduce 120 volts of alternating current (AC) down to 12 volts of alternating current (AC). The 12 volts of alternating current is then sent through the rectifier which converts it to 12 volts of direct current (DC). Voltage regulation can be added to keep the output stable.

Look at the transformer. The transformer should have two terminals on one side labeled primary, The other side of the transformer should be labeled secondary and it should have three or more terminals. Connect the two wires of the power cord to the two terminals on the primary side of the transformer.

Look at the secondary side of the transformer and look at the documentation that was supplied with the transformer. Going left to right, label each terminal A, B, and C. To get 12 volts as an output, connect to terminals A and C. If you connect to terminals AB or BC, you will get an output of 6 volts. Connect two 6 inch wires to the 12 volt terminals on the secondary side of the transformer.

Connect the other two ends of your wires to the alternating current (AC) input side of the bridge rectifier. Connect one end of a red wire to the positive terminal of the bridge rectifier. Connect one end of a black wire to the negative terminal of the bridge rectifier. Strip 1/2 of an inch of insulation off of the other end of the black and red wires. Attach one large alligator clip to the end of each wire.

Plug the power cord into the transformer. Set the digital multimeter to read DC voltage. Clip the red positive alligator clip onto the red positive digital multimeter probe. Clip the black negative alligator clip onto the black negative digital multimeter probe. Observe the voltage reading on the digital multimeter. It should be reading between 11 and 13 volts.

Items you will need

About the Author

William Kinsey lives in Concord, N.C. He started writing articles in March 2009, which have appeared on and He currently holds a Bachelor of Science in electrical engineering from the University of North Carolina at Charlotte and a Master of Business Administration from the University of Phoenix. He also has several years experience as an outside plant engineer and planner with AT&T. He also currently owns and operates Sophisticated Curves, an online fashion mall that caters to the needs of plus size women.

Photo Credits

  • photo_camera power supply image by pmphoto from