I've always been bugged about the power for this. For a start, whether or not BLE will 'work' makes the decision between BLE and WiFi. But for now let's assume BLE is a goer. There's lots of talk of a one year battery life and that a CR2032 would be sufficient. An extra bonus from this is that there are a *lot* of CR2032 plus 'one or two buttons' cases being manufactured for the car security market. A better solution might be to use a AAA battery but that then gets really messy around nasty plastic cases screwed onto motherboards and all that yuckiness. I'm currently wondering about using Chinese 'promotional' keychain torches (i.e. http://bit.ly/1pHw4WV) and probably replacing the led end with the radio gear buried in injection moulded plastic. But still screw on/off so we can change the battery.

The other alternative is energy harvesting. Unfortunately this is not a cheap pastime ... TI publish a reference design for a BLE sensor with solar energy harvesting http://www.ti.com/lit/ug/tidu235/tidu235.pdf but it's *huge* and presumably very expensive. I was hoping that a small (25x10mm) solar panel with power management and a small LiPoly would be sufficient but it's beginning to look like (a) it might not be and (b) it'll be a *lot* cheaper to rely on replaceable batteries. I imagine a AAA stores many more joules than a CR2032 while still not being embarrassingly large (although any bigger may well be).