go to Menu > Settings > Game Performance (i dunno if thats the right name, its the 2nd tab with all those sliders) > push that framerate/battery slider to the right (~25%)

Batteries decrease capacity over time, depending on usage. A 3000mAh battery won't stay 3000 forever, it may one day become 2700 max capacity

How are your charge cycles like? If you constantly charge that lith-ion battery from 0-100 every time you charge, it could harm the battery. (try charging between <20 to <90)