If that's a standard lead acid car battery you will see 12.8v when it's fully charged, 12.4v at 75% and 21.1v at 50% - don't use it if it's at 50%, you'll damage it. With the solar running in you may see up to 14.8v while charging, are you using a charge controller? That'll tick it over to top up at about 90% full, and you'll see the voltage go up and down while it's on float charge