Voltage (V)
Potential difference between terminals. Automotive standard: 12 V (6 cells × 2.0 V). Trucks: 24 V (12 cells).
Voltage (V) is the electrical potential difference between the two terminals of the battery. In automotive use the standards are: 12 V for cars, motorcycles, caravans, marine and light machinery (6 lead-acid cells in series at 2.0 V each); 24 V for trucks, buses and heavy machinery (two 12 V batteries in series or 12 cells); 6 V for pre-war/classic vehicles and some veterans. A 12 V battery "in good condition" actually reads 12.6-12.8 V at open circuit (no load); 12.4 V indicates around 75% state of charge; 12.0 V is 50%; below 11.8 V it suffers permanent damage from sulphation.
With the engine running the voltmeter should read 13.8-14.4 V (the alternator charging it). If you read above 14.7 V or below 13.5 V with engine on, the problem is the alternator/regulator, NOT the battery.