4 USB Battery Charger for NiCd

This is a simple charger for nickel-cadmium battery (NiCd) that can be connected in your USB port. The charging scheme employs only a constant current charging. The two NiCd cells are connected in series and charge at 200mA current. Charging current is controlled by zener diode D1 and resistor R1. You can increase or decrease the current by using the approximate equation:
Current = VD1/R1; where VD1 is the zener voltage.
The use of D2 is to limit the voltage across the battery, thus protect it from damage. Charging time of the fully discharged battery is given by the formula:
Time = Battery rating / 200mA ; where Battery rating is in mAH.

Example: battery rating 220mAh;
Time = 2200mAH / 200mA = 11 hours; meaning do not let battery charged more than 11 hours.

Charger Schematic

Part List:
D1, D2 - BZX84C2V7, 1N5223B, or any 2.7V zener diode 1W
R1 - 10 ohms  1/2W
R2, R3 - 2 kilo ohms 1/4W
Q - 2N2907, 2N4403, or any similar PNP transistor

2N2907, 2N4403 pins

Battery holder

Usual wire colors in USB cable

USB pins

Note: Improper connection or shorting the USB ports may cause damage to your PC. Do not use the charger when lot of devices are connected in the USB ports. Check the connection before inserting or using it. Do not leave the battery charged for a long period.