Obviously it wouldn’t have enough capacity to be a proper battery bank, but would it be feasible to make a battery expansion card with an on/off switch that could hold just enough to get like 15 minutes of extra charge for an emergency scenario? Would probably be way too niche to be a viable product, but I’m curious if it’d theoretically work?
Maybe if you put it in limp mode for those 15min, 15min running 5W is 1.25Wh which may fit in an expansion card but would leave very little space for the electronics. There is also the point that high capacity batteries don’t like putting put all their capacity in 15min so you’d need high drain optimized ones and that would reduce the volumetric efficiency even more.
Edit: browsed through a few micro-drone batteries and it looks like getting 0.5Wh in that size may be possible even if tricky, 1 almost definitely not and that isn’t even taking into account the space needed for the electronics. The micro-drone batteries won’t have much issue giving up all their capacity in a few minutes though so at least that would not be a big issue.
Edit2: found some smartwatch batteries that have more capacity that fit but they aren’t able to discharge fast enough for this to work.
All in all, possible maybe, viable or practical probably not.
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I think you’d be far better off getting an external battery, and carry that and a USB-C cable around.
I use this: https://www.amazon.com/gp/product/B0BD45R61W
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pretty much any form of power-bank would be better, though much bigger and not integrated.