A Maxell 2032 rechargeable button cell has a capacity of 70mAh to 140mAh. You might have to drop the Neopixels if you must use a button cell. I haven't looked at the current consumption of the Flora, BLE module, BME280 sensor or pulse sensor, but for 8 hours operation dropping only the Neopixels might not even be enough.
george_i:
Flora draws 10 mAh.
BLE draws 27 mAh.
Pulse sensor draws 4 mAh.
BME280 draws 4 mAh.
Those aren't current values you're quoting, they're charge.
For example, the BME280 chip configured to run with the throttle wide open draws about 0.65mA of current. The value 4mAh would correspond to running at that current level for about 4mAh/0.65mA = 6 hours. Is that really what you meant?
If you add up the total average current for your entire system (mA) and multiply that by the number of hours (h) you would like the battery to last you'll have a first order estimate of the battery capacity (mAh) you'll need.
george_i:
This means a constant draw of 70mAh.
For at least 8 hours of functionality, what's the smallest battery I could use?
I have the feeling is 560
Even bigger than 560mAh. The 'true' capacity of a battery is related to it's rate of discharge. You need to check out the graphs in the datasheet of any battery you're considering.
If your figures are correct, you'll need a battery rated to have a capacity ≥560mAh 'at 70mA' current draw.
That will sometimes mean using a battery with a 'label' capacity 2-3 times the capacity that you need. That's worst-case.Luckily, lithium cells are better for high discharge rates than some other battery chemistries, so you probably won't actually need 2-3 times the capacity, but you'll almost certainly need >560mAh. Again, check those datasheets.
george_i:
If I put 2 or more rechargeable coin batteries in series, can I recharge them as they are wired, or it can be done only one by one?
Putting batteries in series does not, by itself, provide more milliamp-hours.
You will need to do more.
The difference between the stronger cell and the weaker cell
will increase with every charger / discharge cycle
unless you balance the two cells during recharge.
LiPo cells come in all shapes and sizes. (Well maybe not shapes.)
As others have pointed out putting rechargeable batteries in series makes them more difficult to charge. LiPo cells can be charged in parallel. There are a bunch of articles from reputable sources about charging LiPo cells in parallel.
I don't really see a need for parallel batteries. Just find a LiPo the size you need.
Eventually I've decided that for the first version of this device to make possible to attach it to a belt instead of bracelet. So I can put a larger battery there. Perhaps will become a belt buckle
