RG40XXV Battery draining very fast

Hi, ive recently got an anerbic 40xxv to emulate old roms and ive noticed the battery life is quite low, the battery drains unexpectedly fast ( in 25 minutes it drained by 22% when playing pokemon emerald).

I have updated the OS to the latest version and fully charged the device. i have noticed the battery drains slower when it reaces about 60% battery life, around 4% an hour. is this normal behaviour or is my hardware playing up?