the only true test of a battery that you can do on an aircraft without removing the battery is to turn on the pitot heat and see how long it takes to get walm
No, the only true test is wot happens when you try to start the engine
A pitot draws only about 10A.
BTW one needs to be careful with in-situ charging. One has to make sure the battery can vent freely, and this is why most people remove them for charging.
I think the "gel" batteries (Concorde etc) vent a lot less (and they are much better batteries anyway) but one still has to watch it.