Power source, GPIO current output

Is there a strict limitation to 3.0V or does the puck accept other batteries, ie 3.2V (LiFePo4) or 3.7V (LiPo)? And what is the maximum GPIO output power?

It'll run off 1.7v to 3.6v, so LiFePo4 would be perfect (I had no idea the voltage was different on them).

Unfortunately that means it won't work out of the box with LiPo - which is annoying because you can buy LiPo CR2032 batteries. I've actually just finished tweaking the PCB for this though... I added a part outline for a diode. If you cut the trace and solder a diode in, that'll drop the voltage enough that you can use a LiPo.

Also, the absolute maximum rating is 3.9v, so if you were willing to push it, you could use a LiPo directly - just not when charging! And not when you turn the magnetometer on, as that really does only support 3.6v :)

IO is 10mA typically, 15mA max - so not as beefy as the Pico, but you'll struggle to draw a lot of power out of a CR2032 anyway.

You're probably best off looking at the chip specs for other info - the document is pretty good: http://infocenter.nordicsemi.com/pdf/nRF52832_PS_v1.1.pdf

If you cut the trace and solder a diode in, that'll drop the voltage enough that you can use a LiPo.

I like this option.

A typical 1N4148 type, would that work? ...because when (deep)sleeping, drop may be as low as 0.5V. Is that good enough?

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Yes, should be... as long as you're not charging the battery so it's below 4.1v.

As you say, diode choice is important as some won't drop enough voltage when you're drawing small amounts of power.

Any chance to get a CR24xx mounted on the puck? (i.e. additional mounting holes/pads for a 24mm holder or a battery with solder pins). CR2450 is 620 mAh, CR2477 is 1000 mAh, see http://www.digikey.com/product-detail/en/panasonic-bsg/CR-2477-HFN/P126-ND/107129 .

The problem is the NFC ring around the outside, but then also the room for the pivot point and the button :(

If you didn't care about that you could probably drill through the NFC ring and fit a bigger battery holder though.

Thing is, CR2032 are still almost certainly cheaper per Watt and easier to buy since they are produced/sold in higher numbers. I was considering adding a bigger battery but in the UK at least you can buy CR2032 from many shops on the high street. Good luck finding a CR2477 anywhere offline :)