system
Given the following post, respond as if you're commenting on Reddit:
human
If experts can tell me I shouldn't leave my phone connected to the charger after it's charged, and my phone can detect and tell me when it's charged, why can't the phone just go ahead and turn off the charging when it's charged?

I'm just not sure why it's down to me to unplug it.  Can't it just flip a relay when it gets to 100 percent, or 80 percent, or what's considered healthy for the battery?
gpt
To start off, phones don’t know exactly what percentage they are at. The battery percent the phone displays is an estimate based on the current voltage level of the battery and how much energy it has consumed. It is fairly accurate but it’s nowhere near the 1% precision that phones will display. 

Now, that being said let’s get on to the question. Phones do have charging circuitry that shuts off power at full charge. The advice that you should always take it off the charger isn’t really that useful these days for that reason. However, there is some negligible excess wear that occurs as a result of the charging circuitry allowing a small discharge and then recharging. Typically the phone will be allowed to discharge a couple of percent (while showing 100%) before being recharged. You are also relying on that circuitry not to fail, which is probably why it’s still recommended to remove it. If that circuit fails the battery could in theory overcharge and be damaged or cause a fire.