Ni-Cd battery charger circuit

  1. Current Regulation: The circuit employs an automatic current regulator to regulate the charging current to approximately 4 amperes. When the charging current reaches 4A, the voltage across resistor R1 becomes 0.7V, turning on transistor Q1. This action, in turn, shorts the base of Q3 to ground, inhibiting the biasing of Q4 through which the charging is done. This mechanism effectively regulates the charging current to protect the battery from overcharging.
  2. Charging 6V and 9V Battery Packs: To use this circuit for charging 6V and 9V battery packs, a compromise on power efficiency is suggested. Presumably, the excess voltage (above the battery voltage) is dropped across Q4 to accommodate lower-voltage batteries. However, this approach is less efficient as the excess voltage is dissipated as heat across Q4.
  3. Challenges with Charging Lower Voltage Batteries: Charging lower voltage battery packs (6V and 9V) using this circuit comes with some challenges:

    a. Power Dissipation: Charging 6V and 9V batteries using a 12V-based circuit results in a larger voltage difference between the source and the battery, leading to higher power dissipation across the regulating transistor Q4. As a result, Q4 will generate more heat and a heat sink is advised to dissipate this heat effectively.

    b. Efficiency: Charging lower voltage batteries with this circuit can be less efficient due to the significant voltage drop across Q4. It might result in lower charging efficiency and higher power consumption.

    c. Component Suitability: The circuit's components, such as Q4 and the heat sink, must be adequately rated to handle the increased power dissipation during charging lower voltage batteries.

  4. Multiple Charging Modes: If you intend to use this circuit for charging both 12V, 9V, and 6V battery packs, you might need to implement a switch or a circuit modification to select the appropriate charging mode. This is necessary to ensure that the correct charging voltage and current settings are used for different battery packs.

Ni-Cd Charger Circuit diagram with Parts list.

Ni-Cd battery charger circuit

Download Circuit Diagram


  • The circuit can be assembled on a Vero board.
  • T1 can be a 230V primary, 12V secondary, 4A step-down transformer.
  • Bridge D1 can be made by using 1N5400 diodes.
  • Fuse F1 can be a 500mA type