system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do batteries mess up from excessive charging?
gpt
It depends on the chemistry. But it's probably Li-Ion that's relevant for you. That's what's being used in phones, tablets and laptops.

Li-Ion doesn't like extremes. They don't like to be charged or discharged too much. The more charged or discharged they are, and the longer they are that, the more worn or damaged they are.

There isn't really a point where they are "fully charged" or "fully discharged". It's a compromise.

* So a typical Li-Ion prefers to be at 3.7 V. That's about 50% full.
* You can charge it to 4.0 V and it's almost as happy, and about 80% full. 
* You can charge to 4.2 V. This is okay, if you discharge it soon-ish. This is maybe 98% full. (**EDIT:** This is what we usually consider 100% full. Just like a full glass of water isn't filled to the brim.)
* You can charge to 4.3 V. This is so bad that a bit of damage occurs immediately. This is probaly 99.9% full.
* You can charge to 4.4 V. This is very bad. Now you risk damage so bad, that the battery shorts internally, and releases all the energy very quickly. This means dangerous chemicals starts boiling, and large flames probably appears.

Normally they are charged to 4.2V. Lets say the battery will last for 1 000 charge cycles this way.

Some people choose to charge to 4.0 V. They get less energy in a charge, but now the battery lasts maybe 5 000 or 10 000 cycles.

Some people charge to 4.3 V to get as much energy as possible, but they get maybe 200 cycles from the same battery.