I need some information as to how the battery works in conjunction with
the cameras inbuilt meter. Or how the battery is measured by the camera,

I have Nikon EN-EL15 battery one of which shows 0 (new) and the other
one shows 3 (getting old) both fully charge. both are about 8 months old
, used and recharged randomly with the Nikon charger.

Accordingly my thinking is that one battery has started to fail and
there maybe a warranty claim.

thanks