Cells are ~2.1V each. 6 * ~2.1V = ~12.6V If one cell was dead and the rest where good the voltage drop would be 5 * ~2.1V = ~10.5V. 12.2V indicates the battery is ~50% discharged. Which is a real deep discharge for a starter battery. A good bet is: the battery is heavily Sulfated, a buildup of Lead ...