system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is happening to a battery when it's charging?

What's going on to recharge a battery?
gpt
Batteries don't produce any electricity.   Instead, batteries are electricity pumps.  The path for electric current is *through* the battery, then back out again through the other terminal.

As with any pump, in order to pump a flow of charges through the wires, energy is required.   This energy is stored in a sort of "chemical fuel" in one or both battery plates.  When a battery runs your devices, its stored chemical fuel gets converted into waste products.  It's a bit like combustion.  When all the fuel is gone, the battery is "dead," and the plates contain only the waste.

But unlike combustion, we can convert the waste products back into fuel: just force the electricity backwards through the battery!   Weird, eh?  It's like making gasoline by pushing your car backwards up a hill while pumping exhaust into the tail pipe.   Or more sensible: a battery is like a molecular wind-up motor: it can run down, but it can be wound back up by forcing it to spin backwards.

In theory, all batteries can be recharged.   But in practice, "non-rechargable" batteries have their plates destroyed at the end, when they're almost dead.   We can't restore the plates to their "fully charged" state unless they still exist.   Rechargables are designed so this doesn't happen; the plates never fully dissolve.   Even so, the plates don't perfectly rebuild themselves during recharge.   After many discharge/recharge cycles, errors build up, so perhaps the rechargable battery develops an internal layer of insulator, or perhaps the plates will electrically connect together (internal short circuit.)