I lost a good pair of batteries over failing to understand this voltage measurement business, so I will add my $200 worth of learning to this matter. After charging to any extent the battery will measure well over the nominal 12.6 volts for hours. Maybe after no charging for 8 hours the surface charge will be sufficiently absorbed to get an estimate of the state of charge from the rule of thumb for lead acid batteries that 12.0 volts is half charge and as low as you can go before damage to the battery occurs. 12.6 or 12.7 is a full charge. So if you start charging in the morning with the engine, solar panel or generator, you cannot estimate the level of charge that day using a voltage measurement.
The only practical method of getting a good reading on charge level any time you want it is to install a battery monitor. These little computers measure the current flow in and out continuously so they can calculate the level of charge and display it in per cent or number of amp-hours of charge. Price range from $30 to $300. I have one of the $30 ones and I find it excellent.
Battery monitor thread
