I am struggling to find a highly efficient power source for my XBee Pro 900 Digimesh. They have incredible low sleep drains but supplying them with power going through a regulator still kills the battery even with a "really" efficient one like a Dimension Engineering 3.3v - uses 15ma at no load. So would this work - The XBee is powered by 2 alkaline batteries during sleep, when it wakes up pin DI-9 (sleep indicator) powers a transistor or relay that cuts power from the alkalines and turns on a larger lithium battery which goes through a 3.3v regulator to power the XBee while its awake. When it goes to sleep the alkalines are put back online. I assume I would need a capacitor to provide power during the switch over too. Is there an easier way and i'm just overthinking this or is this a reasonable solution to use the least power possible? I haven't tried using just a single cell LiPo connected directly to the XBee, I wasn't brave enough to test the XBee max V input. Thank you for any help or links you can provide.