Wife's 6S has started losing charge rapidly. She charges it, uses it not a lot, then its at 1%

Ive gone through the usual things to see if an app/setting is hogging it, updated all apps, etc all seems ok

I charged it, did not much on it, charge % is normal and stable. 

Checked it this morning its at 1%. Battery Health is 83% so not horrid. 

Charged it at 8-10pm last night. Activity meter shows almost zero activity since then. The battery level green bars are high, as expected as it was just charged, very slowly decline till midnight which is normal, then bang, (at midnight oddly),it plummets to 1%. Its as though the battery health is more like 20%. Works fine (although it does seem to lose charge faster than my iPhone X (84% Battery Health) then it just loses all charge instantly. That it did that at midnight last night is probably a red herring.

Is the battery done and dusted? Its never been a high use phone, apart from her walking in the back yard for a half hour with it playing music most days. Or has its calibration screwed up?

TIA