Why did my battery indicator jump to 100% as soon as I plugged in the phone?

[mterrellg]

Moto X 2014
Lollipop 5.1

So, I went to plug in my phone before bed just now, and the instant I plugged in the charger, the battery indicator instantly jumped from about 20% to precisely 100%.

I unplugged and restarted the phone, hoping for the battery meter to reset, but no dice. It showed 96% after everything booted up. I plugged the phone back in, and now it's of course back to 100%.

If I just let it charge all night so that the battery is ACTUALLY at 100% by morning (or really in a few hours from now), will everything go back to normal? Or does my phone now think that 20% battery is 100% battery and I will forever have 1/5 normal battery life? Because that would be only slightly better than skinny-dipping in Burger King's grease trap.

Anyone had this happen before?

 

[Mooncatt]

It's possible that this is a software glitch, but erratic battery readings usually indicate the battery itself is going bad. This is one of the few times I'd suggest draining the battery until the phone shuts down, then fully charge before turning it on to see if that recalibrates the meter, but that may not be the actual problem.

Also get a monitoring app like Gsam that reports the battery voltage and see if that matches the percentage meter. At 20%, the voltage would be about 3.7V or so, and 100% would be about 4.2-4.3V depending on battery chemistry.

 

[Rukbat]

It's also possible that the battery grew a dendrite and plugging the charger in blew the dendrite. (It's a huge problem with lithium battery technology.) If it is, it's a sign that the battery has to be replaced, because it'll keep happening.