We need to develop a circuit to charge two serial-connected rechargeable AA batteries (NiCd/NiMH) from 5V USB power source. Can someone please advise a low cost charging IC, and a overcharge protection cirucuit? Thank you,

 

I've been mulling over a problem similar to this, putting a timer on the charger is what I was leaning towards. But that may not be the best solution.

The Battery University

 

We need to develop a circuit to charge two serial-connected rechargeable AA batteries (NiCd/NiMH) from 5V USB power source. Can someone please advise a low cost charging IC, and a overcharge protection cirucuit? Thank you,

This is a very good option if you can stick to NiMH:

https://datasheets.maximintegrated.com/en/ds/DS2715.pdf

 

This is a very good option if you can stick to NiMH:

https://datasheets.maximintegrated.com/en/ds/DS2715.pdf

DS2715 seems like a very good IC. Not sure how likely a charging IC's protecting mechanism might fail? Wondering if additional overcharge protection circuits are needed in the rare case of a charging IC failure? And if there are existing samples?

Provided below is what I know about NiMH rechargeable batteries:
1. Suggested charging voltage is 1.45V
2. Voltage at full charge is 1.4V
3. Voltage holds at 1.2V for 5% ~ 95% capacity

A charging IC should
1. Calculate how much current has been input into the battery, and cut off at some pre-set value to avoid overcharge
2. Auto cut off if the input current suddenly increases dramatically

Not sure
1. What happen when a NiMH overcharge occurs? Will the batteries exploded? Or the circuit eventually gets shot-off and the batteries just become unusable?
2. If additional overcharge protection circuits in addition to a NiMH charging IC are needed?
3. Are there simple NiMH overcharge protection circuit samples available?