Thin battery to power arduino micro with nRF8001? I'm building a project which needs to fit in a wallet. I have an Arduino Micro hooked up to a nRF8001. I attempted three button cells wired in series, which provided a solid 7.5 volts, but only 20mA. 

A 9V battery successfully powers it, but that's too big to fit in a wallet. I was considering using an A27 battery, but even that's thicker than I would like. 

Is there any battery I can use that's a reasonable thickness, and won't explode on me? Thanks!...
I think you should use a 3.7v mini battery that are around 200-300mAh capacity. These batteries are variable on online stores and are very thin. Now as you require 5v there are mini DC-DC boost converters available what you can use with them.