Where do you buy a battery checker?

Answer 1

auto parts store

it measures the voltage of the battery and has a load of a reasonable size

so voltage has some correlation to charge state.

when batteries had external posts you could check them one cell ata time and know which cells to add water to

now to test battery a cell at at time youll need to get a hygrometer theres one with assorted plastic balls of different densitys. about $1 and much better than electric test .

there are some batteries now that are sealed so throw those away if the auto parts place wont give you $5 for it

for a 9volt battery lil bity one you can get one at radio shack

this can also check cells, has load resistors for AAA AA C and D.