I have a Lenovo T440s laptop, which has two batteries: an internal one, and a user-replacable external one. The external one can be hot-swapped at any time. I do this regularly, switching between a light battery when I know I won't be away from power for long, or a larger battery when I need longer running time.

When I switch from small->large, upowerd reports:

Jun 24 10:13:10 sporall.goop.org upowerd[3621]: (upowerd:3621): UPower-Linux-WAR
NING **: energy 68.470000 bigger than full 18.580000

But then seems to update its state and carry on.

However, when I switch back from large->small, upowerd reports that as a sudden loss of capacity and shows a reduced battery percentage and pessimistic lifetime prediction. I need to restart upowerd to get it to report correct battery capacity/lifetime.

The batteries do have distinguishable manufacturer/model/serial number, so in principle it should be possible for upowerd to notice and account for the battery change.