Why does my battery show more charge before restarting than after? My battery shows that it's at 50% and after restarting my phone it shows 30%. Another time it was at 40% and after the restart it became 5%.

The device is a Samsung Galaxy Grand Neo, not rooted and with stock firmware.

Does anyone know the reason why it's doing this?
When you restart your Android phone, it uses more energy than when it's idle because it has to load up the JVM, the services, apps etc yet again.

I'm not sure why you saw an 80% battery decrease, but it might be due to the phone losing energy by heat when you're charging, and the phone somehow registering the real battery value on a clean restart.