If you open up a battery you will find cells. That is the definition
of a battery; a group of cells. I expect there are instances where
you would find batteries in batteries, but that would be unusual.

I agree with you in principle but I've come to accept the fact that
definitions evolve and words mean what people want them to mean. Thus (and
unfortunately) it has become acceptable to refer to a single power cell as
a battery.

One would hope, however, that when the apparently indivisible (to a
layperson) unit is indeed composed of several sub-units (and by this I mean
car batteries, 9V batteries, etc.) we might at least in this instance
continue to refer to the sub-units as cells and not batteries within
batteries!

But it's an unwinnable war. Someday I fully expect the term "it's" to
become fully acceptable in standard English in the possessive sense and
then the barbarians will have won. In the meantime I wish web forum
software would come with a plug-in that would, on displaying posted text,
randomly convert "it's" into "it is". Correct users will still be correct
if a bit stilted. Incorrect users will be made to look moronic.