Yeah, I understand that, and I imagine it's a fairly tricky problem to overcome now that I think about it a little more.
I guess I always assumed if the power failed a battery took over (as you say it does), but when that drained all that was required was to reapply power and some the backup battery would be charged to a point where it could start up the main systems again
(I guess I'm thinking of the example where a phone's battery dies completely and plugging it into a charger won't power it back up immediately, but results in the phone charging the battery exclusively, before later powering back up)
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