This doesnt answer your question exactly but it is another possible solution.
Here is an Arduino with a built in Battery charger:
https://www.sparkfun.com/products/13614
It needs USB 5V input to charge a 4.2V LiPo, but if you hooked that to a switched power source, it would charge the battery so that it would probably remain powered for months without a drain on the battery.
Just an option