Why is my battery draining fast?

[Buttercup]

My tablet's battery just suddenly drain fast. Before, after I charged it fully and let it do nothing, after few hours when i check it the battery percentage is still full. But then when my tablet turns to a year old the battery just drain faster. Though I was just looking at some old photos in my gallery for 5 minutes, the battery from 89% turns down to 82%. My tab's brightness is in the lowest tho. And the data connection, wifi, gps are off. But I often open an ebook app and sometimes read until the battery turns to 15%. Does this greatly affect the battery draining fast? Please I need your help.

 

[Almeuit]

What tablet? Would you be able to get a screenshot of the battery stats?