How to Build a NiMH Battery Charger

Written by j.t. barett
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How to Build a NiMH Battery Charger
A simple circuit can recharge NiMH batteries. (Jupiterimages/Polka Dot/Getty Images)

You can recharge nickel-metal hydride (NiMH) batteries hundreds of times. Their light weight and decent current output make them a good choice for powering electronic equipment. The charger for NiMH batteries is a simple circuit that anyone with a modest amount of electronics experience can build in a few hours. The circuit takes direct current (DC) from a standard "wall-wart" type adaptor and manages the charging process with a single transistor. A light-emitting diode (LED) indicates when the battery is charged.

Skill level:

Things you need

  • Solder perfboard
  • BD135 NPN transistor
  • 120-ohm 1/4-watt resistor
  • 30-watt soldering iron
  • Electronics solder
  • 5mm LED
  • 18-ohm 1/4-watt resistor
  • 1N4007 diode
  • 220-microfarad, 35-volt electroltyic capacitor
  • 22-gauge solid wire
  • Wire strippers
  • Dual AA battery holder
  • Jack for DC power adaptor
  • 9-volt, 100 milliamp DC power adaptor
  • Heat sink for TO-126
  • Heat sink compound
  • Diagonal wire cutters

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  1. 1

    Orient the transistor just inside the perfboard's edge with the transistor's metal back facing away from the board. Set the transistor into the perfboard, letting its pins slip through the holes in the board. The pins should protrude through to the board's copper side.

  2. 2

    Insert one lead of the 120-ohm resistor near the transistor's collector and the other lead near the base. Solder these two connections. Place the LED's anode near the transistor's base and put the LED's cathode lead into a nearby hole. Insert one lead of the 18-ohm resistor near the LED's cathode. Insert the resistor's other lead near the transistor's emitter. Solder these connections.

  3. 3

    Orient the jack on the edge of the perfboard, connector side facing out. Insert the anode lead of the 1N4007 diode near the jack's positive lead and solder the connection. Insert the diode's cathode lead into a nearby hole. Put the positive lead of the 220-microfarad capacitor near the diode's cathode and solder the connection. Place the capacitor's negative lead near the LED's cathode and solder. Cut a 2-inch piece of wire and strip a quarter-inch of insulation from each end. Solder one end to the jack's negative pin. Place the other end near the LED's cathode and solder.

  4. 4

    Place the red wire from the battery holder near the 1N4007's cathode. Place the black wire near the transistor's collector. Solder these connections. Trim excess wire and leads on the board's copper side with the diagonal cutters.

  5. 5

    Put a small dab of heat sink compound on the metal back of the transistor. Slide the heat sink snugly onto the transistor.

  6. 6

    Place two discharged AA-size NiMH batteries in the battery holder. Plug the 9-volt power adaptor connector into the jack and plug the adaptor into a wall outlet. The batteries will take a few hours to charge.

Tips and warnings

  • You can use a capacitor with a higher voltage rating, such as 50 or 100 volts, if necessary.

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