You should be reading about 13.5 volts on a good battery. 13.5 with engine running. 12.4 or . 6 is fine but you could have a dead cell.
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The 12-volt rating of a battery is the nominal voltage and it may be slightly higher or lower depending on the state of charge and loads. We sometimes use 24V battery systems in larger trucks and busses due to the vehicle's higher power needs and long cable runs.
Although all battery electric bus batteries will store electricity in the form of DC, or direct current, power, the electric grid transmits and provides electricity in the form of AC, or alternating current, power. Therefore, a critical process of charging a BEV is the transformation of AC power into DC power.
dc bus voltage is ~1.414 x the rms line voltage (e.g., for a 480V ac drive, the dc bus should be ~678V dc. A dc voltage value that is too low can cause the drive to trip. At the cause, the mains input voltage is probably too low, or the input sine wave might be distorted by flat topping.
LFP (Lithium-iron-phosphate) batteries used in almost all electric school buses have better thermal stability compared to NMC (nickel-manganese-cobalt) batteries commonly used in electric cars.