Air Console battery gave up the ghost. Then it failed to start up at all. So I took it apart and found that the dual-18650 battery pack could be disconnected.
I was surprisingly able to run it with no battery (plugged into USB power), so I reassembled it without the battery and will just use a battery bank from now on.
I have no idea whether the batteries themselves or the charge control circuit failed, but with the internal battery connected, it won't boot.
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