Watch battery

When using gps my Apple Watch battery will fall from day 50%. Down to 10%. I click low power mode but the watch reboots and when it comes back on the mileage is incorrect as it just works out a straight line from where the watch went off to when it came back on.

The battery on 10% will last a long time so it’s like an error causing this to occur. If I put the watch on charge after not long the % showed as 75% when it was 10% say 10 mins before. The watch does not charge this quick.

For some reason there is an error on the watch that’s causing the battery % to be incorrect and starting the 10% warning ?