Fully charged = 13.2v
fully depleted = 10.8v
50% charged = 12v
Battery consists of 6 cells, each individual cell is 2.2v when fully charged, 1.8v when fully depleted....
Temperature is merely one variable which can affect a batteries reading or charge state, in general a warmer battery will display higher readings for both voltage & capacity....
EDIT; More info added....
Cold starting is the heaviest use your battery is likely to see, (with exception to Audio loads), so will display issues here first....
Colder temperatures means;
1, Lower battery performance, (irrespective of battery age)
2, Thicker engine oil, (increases power required to spin motor)
3, Thicker gearbox oil, (to avoid this issue start with dipped clutch)