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I would condition your battery by doing 10 full discharges to self switch off point, and then getting in the habit of fully discharging it at least once a week as I do with Samsung batteries in 3 Samsung smartphones,...though all my Samsungs up to Note 4 edge have removable batteries which I also rotate with spares once a month.
Whilst Lithiium batters are allegedly memory free, that is not always the case, and even Apple suggest fully discharging
I would expect better battery performance after that process.
You may consider doing a reliable data back up and re-setting to factory settings and a fresh re-install, in case you have any software corruption, which can surruptitionally build up over time.
A "Like" is always appreciated. If my advice is a solution, please mark it. I'm not a Telstra employee. 50 + years exp. in radio comms., eg mobile phones since 1963, two-way radios, base stations, antennas & pagers. One of my mottos: "Believe in yourself, and you can accomplish anything".