That chart shows the top users of the battery and they will always add up to approximately 100%. If you leave the screen off for 8 hours and the phone uses 10% of the battery, the Android OS will show a very high percentage on that chart even though the battery only went down 10%. That's why the Android OS percentage drops when you use the screen. The total has to be near 100% on that chart. One thing goes up, something else is going to go down. Good luck.