I'm embedding the beagleboard for robotics, and I have enough room for
up to 4 NimH batteries (10000mAh). But :
1- Fully charged NimH battery voltage is ~1.40v, and since the
overvoltage protection circuitry is 5.3v, a fully charged 4x pack will
raise the overvoltage protection
2- Moreover, according to the TL1963A datasheet, the drop-out voltage @500mA is around 0.35v (worst case), and up to 0.45v for higher loads.
Since VBAT is 4.2v, we can't drop below 4.55v/4.65v while there's
still some juice into batteries at this voltage (~1.15v)....
It seems that NimH batteries aren't welcome... Any advice ?
The best thing to do is to boost the battery to 5V through a boost circuit. That way even if the battery drops to 1.15V it is still operational. The USB requires 5V so you have no choice but to supply the 5V. The board will actually work lower than the 4.2V, but of course the USB interface will not. If you are not planning to use USB at all, you have some more flexibility here on the operational voltage range.