Disappearing battery percentage

If you mean the system menu the stats below the chart are not the absolute power usage but the fraction of the power usage due to that app, so if you had used 20% of the battery and half of that was the screen you would expect display usage to show as 50% rather than 10%.

So if those stats only add to 30 that means 30% of the 70% you've used, i.e. 2/3 of your usage is not being displayed.

So, what items is it showing? If it's just apps then the rest will be display, cell standby and/or Android OS. But normally these things are shown. If it's a long list have you tried scrolling, in case there are other items off the bottom of the screen?

If you can't find anything, try installing GSam Battery Monitor from the Play Store - that will give you better battery stats than the built in one anyway.

 

Might i suggest the batteries collaboration might be off or might i know how old is the phone and it's battery it might just be giving out,but you could try letting the battery go completely dead and keep pressing power till the LED light doesn't blink no more then set the phone on the charger without switching it on and let it charge to a completely full then when full turn it on and use it till its completely dead without charging it in between if it was a collaboration problem it should set it back to its original if it was the collaboration problem it should said it back 2 its original percentage and if it wasn't a collaboration problem and if wasn't the collaboration im sorry for wasting your time hope you find the solution your looking for.