What does it mean when it says invalid battery?

It may be because the battery is discharged beyond its threshold and does not take a charge. Once that happens, your phone does not get the right information (proper voltage is part of the signature information) from the (smart battery)it declares it as “invalid” and will not charge it.

What does invalid battery mean on Android phone?

The phone won’t charge or use an “invalid battery” which may destroy it, so you get no battery symbol. If this doesn’t work, replace the bad battery with a new one.