DIY Constant Current Charger for AA / AAA NiMH Battery

Normally I use my MH-C9000 to recharge and condition my AA / AAA batteries. However, it wont charge some of my older batch batteries. Maybe the internal resistance is just too high.

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

Pros:

• 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

Cons:

• 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