12.3v is somewhere around 50% charged Peter - cant be precise because voltage depends on load. However a battery that is left 50% charged isnt going to be happy for long so you might like to attach a trickle charger.
The common problem is that it takes quite a long run for the alternator to fully charge a 50% discharged battery. On my boat it takes 6 hours because the current flow into the battery falls significantly as the charge state of the battery rises.And thats with the alternator running at 14.5 v
