Apple Watch 5 sudden battery warning and crashes

I’ve had a series 5 watch since it was released. In the last couple of weeks it will suddenly tell me the battery is at 10% (when it’s not) then forces a shutdown. When placed on charger, it’s fully charged again in 10 mins or so.

Rhis started happening mid-workout and was temporarily resolved by the recent big IOS update (Sept 2021) , but did it again whilst I was doing nothing.

its lunchtime and usually battery would be at around 60%. It suddenly says it’s going into low power mode and then shuts down completely.

really annoying. Particularly when it happens during workouts.