Typically it should be about 1% However if you have ton of apps ,games, & nearly you've been charging your phone daily ( once or twice) Heavy usage , Over night charging , Almost 4 years of usage passed of average usage , Having much more apps staring/launch in a restart. Charging phone without a specified charger, Regular fast charging, Regular toast of battery through Heavy work load( GAMES) May be reason If you have a removable battery and If you remove and reinserted it ,A clear battery drain could be observed!
Discharge your phone fully until it turns itself off.
Turn it on again and let it turn itself off.
Plug your phone into a charger and, without turning it on, let it charge until the on-screen or LED indicator says 100 percent.
Unplug your charger.
Turn your phone on. It's likely that the battery indicator won't say 100 percent, so plug the charger back in (leave your phone on) and continue charging until it says 100 percent on-screen as well.
Unplug your phone and restart it. If it doesn't say 100 percent, plug the charger back in until it says 100 percent on screen.
Repeat this cycle until it says 100 percent (or as close as you think it's going to get) when you start it up without it being plugged in.
Now, let your battery discharge all the way down to 0 percent and let your phone turn off again.
Fully charge the battery one more time without interruption and you should have reset the Android system's battery percentage.
