Usually when it's a new phone, when you turn it on for the first time, the battery juice is halved. It's always advised to use up the remaining battery juice until jt's drained and then you charge it all the way up to 6 -8 hours. It's so that the charging and battery won't be messed up. When you use charge it without using up the first halved juice, the battery quickly deteriorates (e.g. slow charging, quickly drained). That's usually what I do.

Hope I helped! :)

Normally if you buy a brand new phone , you usually need to charge it for 6-8 hours. But i don't really know why.

You didn't ask your sister?