Actually, if your 12v car battery is reading 12v at rest, it is dead. It should read 12.65 if it is properly charged. And not all systems charge at 14.4. My diesel motorhome charges at 13.6. It has to do with the converter for the 12v house system. The convertor is charging 6, six volt house batteries, and two, 12v chassis batteries, extra heavy duty to turn over an 8.3L CAT Turbo Diesel.