I'm working on a small project(hopefully small enough to fit in a necklace locket) that consists of a atmega85(using about 2mA) and one RGB LED(that I will probably try to get under 20 or 30mA for all three colors). A couple of stacked cr2032's would work but it would be really nice if I didn't have to burn through them. I'm looking at rechargeable 2032's but I figured instead of conforming to a standard size, I should be looking for best capacity/size.

So im looking for as small as possible, flat being a priority, and getting as close to 5v as I can.

Thanks

 

A tiny lithium ion will yield 4.7V.

 

I can only assume that you are attempting to create some kind of wearable device with LEDs. Your objective should be to keep the power consumption down to a minimum. Here is what I would do.

1. There are MCUs that consume very low power, in the micro-amp ranges.

2. Operate the LED and maybe even the MCU in a pulsed mode, sleep/wakeup in order to reduce the current drain.

3. Operate off a single 3V coin battery. This will simplify the physical assembly.

4. Use high efficiency LEDs that operate with less than 1mA.