Even with low capacity, these old batteries are good for mouse etc low power devices. So instead of throwing them away, they are charged with a slow constant current charger. Here is a simple design of such charger.
- The design is powered by +5v. I usually use a phone charger or a computer USB port
- The yellow LED has a +2v drop. With a 0.7v Base-Emitter voltage of BC337, that means there is 1.3v across R2
- With R2 = 12 ohm, the charging current is around 1.3v / 12 ohm = 0.108A
- With a 1000mAh battery, the charging rate is around 0.1C. Good for slow charging
- Simple to make. I just used components I found in my DIY junk box. Resistors and transistors can be replaced by other parts. Just make sure you calculate the charging current correctly
- No terminate logic. Batteries will be charged until removed. Make sure you don't overcharge your batteries
When the +5v power source is removed, the battery will be powering the LED. Adding a diode could solve the issue. But to keep it simple, I left that out and simply remove the batteries after charging