Actually, a fully charged flooded cell battery, like a car battery, will read 12.65 Volts. Each cell will produce between 2 and 2.1 Volts, depending on age, plate wear sediment and electrolyte balance. A "hot" battery, one that has just come off charging will sit at very close to the charge voltage applied, say 13.8Volts in a car. 13.8 Volts is halfway between minimum charging voltage 13.2 and 14.4, where gassing will occur, and you boil the battery. This is why 13.8 was chosen for cars. Once the battery has settled, and cooled down, it will read 12.65 again. Cool down and normalization depends on the mass of the battery and its ability to cool.
Gel batteries tend to run a little higher, but not much. They should be charged differently also, constant current to get them up to nominal voltage, then constant voltage float for maintaining them. Mostly in alarm sytems a fixed voltage regulator keeps them alive, since tehy go in new, and rarely get flattened.