Battery in the saddlebag now, and I can carry plain water again as well as Isostar.

Trouble is that the bag has a hefty moment of inertia and makes the bike feel heavy: yet the new battery only adds 0.8 kg.

I eventually gave up on the new battery and bought a Decathlon el cheapo to use with my old 135mm-axle
motor so that it wouldn't be wasted. Took the Trek back to the bottle-battery system.