Mediaserver is used to access media files - music, videos, etc. If you do that a lot (play music, watch videos), mediaserver will be using a larger percentage of your total battery usage.
But be aware of what that percentage in the battery app is showing. 56% usage by one service means that 56% of the total amount of battery being used is being used by that service. If 5% of your battery has been used (IOW, you have 95% charge left), about 2.5% of your battery charge was used by that service. If you add up all the percentages (and some of them are so small that they're not shown), they'll always equal 100%. IOW, 100% of the amount of battery charge being used is being used by something. That could be 1% of the total charge or it could be 90% of the total charge, but it's 100% of whatever amount is being used. So the fact that mediaserver is using 56% of the amount of charge being used doesn't mean that 56% of your battery charge is being used, it means that 56% of what's being used is being used by mediaserver. Don't just look at what's using what percentage of used charge, also look at the total amount of charge remaining.
(About the only way to reduce the amount of charge used by mediaserver is to play less music and watch fewer videos.)
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