I took apart some ikea spoka night lights which have a 3.6V 550mAh NiMH battery pack ( which I can find for around $6 online ) and a 5V wall wart. They work pretty great for powering an atmega. I'm looking into putting it together from scratch because I ran out of night lights to cannibalize. It looks like a simple circuit to trickle charge the battery when the power supply is plugged in, 4 resistors, two diodes and a transistor plus a dpdt switch to turn it on and off. If anyone has any interest or experience with this, let me know.

I've done this. You really need to be able to monitor not just voltage, but current and temperature of the cell(s). Based on kind of battery, charge rate, and some other factors, the battery will get colder or warmer during charge. Together, these factors handled properly can virtually ensure you never damage a battery.

BM