The figures that follow are for lead acid wet cell batteries - pop top and maintenance free and not for AGM batteries. The typical car battery consists of 6 separate cells When fully charged each cell will produce 2.1 volts dc. Six cells at 2.1 volts equals 12.6 volts. So fully charged battery will read 12.6v or higher if just taken off a charger or a running engine. 12.6 = Full Charge 12.4 = 75% charge 12.2 = 50% charge 12.0 = 25% charge If your 12v battery reads 12v, it really isn't "good", it is only at 25% of charge. Charging voltage will run between 13.5 - 14.5 volts dc @1500+ rpm with an electrical load on the system (high beams and blower motor on high). This is measured at the battery post, not the terminal clamps. Charging voltage that is constantly higher than 15 volts is a concern since it will cook the battery in a short time. This is evident with the top of the regular wet cell battery being wet. Maintenance free batteries will vent a sulfur rotten egg smell.