12.6 would be normal for a battery that has been charged and allowed to rest for a few hours .
To check that the capacity is up to snuff you can have it load tested at a garage as suggested above .
But also check the battery terminals are clean AND the battery is clean ,an accumulation of muck and gunge on the outside of the battery in cold damp conditions can lead to a slow discharge as electricity can track across the battery between the terminals using the gunk as a conductor .
Blessed are the cracked, for they let in the light. - Spike Milligan
"When you come to a fork in the road ,take it ! When you come to a spoon in the road ,take that also ."