user8012: 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!...

user10984: 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.

8012: <p>I'm building a project which needs to fit in a wallet. I have an Arduino Micro hooked up to a <a href="http://www.adafruit.com/product/1697" rel="nofollow noreferrer">nRF8001</a>. I attempted three button cells wired in series, which provided a solid 7.5 volts, but only <code>20mA</code>. </p>

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

<p>Is there any battery I can use that's a reasonable thickness, and won't explode on me? Thanks!...</p>
 10984: <p>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.</p>
