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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!...

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    You'd probably be better off replacing both chips with an nrf51822 both running your code, and containing the radio. Then you can run it on a single coin cell.
    – Chris Stratton
    Mar 17, 2016 at 4:03

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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.

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Either of these two are pretty good depending on how long you want it to live and how much current your device draws:

https://www.sparkfun.com/products/731

https://www.sparkfun.com/products/10718

You can still probably power the micro from these even though they're 3.7v. When charged they'll be closer to 4.2v and it will still probably run down to 3.5v anyway.

Or you could just put two in series to increase the voltage

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    Oof. Are these safe to keep in a wallet that I might fall asleep on? I feel like the heat and weight might be enough to cause one of those nasty lithium battery fires.
    – ollien
    Mar 22, 2015 at 15:24
  • I've never had any problems with these batteries overheating in tight spaces. The heat will depend on hw much current you draw from the battery. It sounds like you won't need much. Still though, I wouldn't suggest sleeping on it.
    – user2525692
    Mar 23, 2015 at 16:28
  • This is the same battery in smart watched and cellphones. If enclosed they are fine.
    – futurebird
    Aug 20, 2015 at 13:00
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I run my projects directly from a 3v CR2032 lithium cell. Bypassing any voltage regulator. You could use something like a I'd get a 3.3v arduino, something like a Arduino Pro Mini. (You can also use your micro, but you'd have to "underclock" them, as the ATMega32u4 isn't necessarily capable of running 16Mhz at 3V.)

By bypassing the voltage regulator, you don't get any of the losses.

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  • Will the button feels provide enough current for everything? The arduino and the nrf8001?
    – ollien
    Mar 22, 2015 at 15:22
  • nRF8001 peak current is 12.5mA. Atmega will be around 5mA. Which seems to be fine for a CR2032. Note that you only have 240mAh of capacity.
    – Gerben
    Mar 22, 2015 at 17:43
  • So in other words, this would be a battery replacement every 12 hours. Fantastic.
    – ollien
    Mar 22, 2015 at 18:37
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    That is just peak current. Nominal power is lower, and you can put them both into sleep mode most of the time (depending on your application).
    – Gerben
    Mar 22, 2015 at 20:43
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You can use rechargeable Ni-Cd batteries of 3 AAA size cells with wired connection.There are also small 4V rechargeable lead-acid batteries,which are mostly used in led torch.

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  • I think the problem there would be 3xAAA are larger than a 9v battery so its probably going to cause more size issues. Also NiCad is not good to have in your pocket where coins might be, in fact a I can't think of a battery that is good for use in a pocket without a proper case.
    – Code Gorilla
    Jun 12, 2017 at 10:59

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