MY ANDROID PHONE BATTERY DRAINS SO FAST......... plz HELP

I have a rooted micromax a90s with ics 4.0.4 and a 1600mah battery, and when I play or use FULL SCREEN apps like Clash of clans, clash royale, adobe reader and some memory puzzle games, MY BATTERY DRAINS SOOO FAST, for example if I play Clash Royale for 15 min, battery drains from 100% to 70%!!!! but it does not drain that fast if I dont play or use FULL Screen apps.....PLZ CAN ANYONE EXPLAIN THE PROBLEM WITH A SOLUTION, (battery cant be changed)????

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