Ni-Cd Ni-MH Adjustable Constant Current Battery Charger Circuit
This is a Adjustable Constant Current Ni-MH or Ni-Cd battery charger circuit. It can be used to get a constant current power supply. Here is the schematic diagram of the circuit:
This circuit can be adjusted to any value from a few milliamp to about 500mA. The max current is 500mA because it is the limit of the BC140 transistor in the current-limiting part of the circuit.
The input voltage has to be 5.25v above the required output voltage, because of 1.25v across the current-limiting section and Approximately 4v is dropped across the regulator. The LM317 3-terminal regulator will need to be heatsinked.
the LM series of regulator is suitable for this circuit because they have a voltage differential of 1.25v between “adj” and “out” terminals.
For example, to charge 4 Ni-Cad cells, just connect them to the output and adjust the 500Ω preset until the required charge-current is obtained. The charger will charge 4 Ni-Cad cells at the same current. But, don’t forget to turn off the charger before the cells are fully charged or the battery will be over-charged.