I want to diagnose a faulty charging system in a European coach bus ('99 Neoplan). It has 2x 12V batteries in series and also two alternators. With the engine running I measure ~21V across the end poles (AFAIK should be ~28.8V). One (or both) of the alternators might be faulty. After taking the batteries out to charge them, I've measured the voltages one by one, and they were different. 11.8V and 10.5V.

I'm not sure if it's an alternator, a battery or a short somewhere else (starter motor maybe?). How would I start diagnosing a 2-battery, 2-alternator 24V system? How are the alternators usually wired?

Thanks, cheers!

  • "how are alternators usually wired" ? I really don't know but I wouldn't be surprised if one alt charges the starting battery and you have another auxilary circuit for lighting and such served by the other alt. Anyway I would put the batteries one at a time on a "12v" charger and if they dont readily come up to near 14v they are probably bad. Always replace dual batteries in pairs.
    – agentp
    May 30, 2017 at 20:40

2 Answers 2


After taking out both alternators, they turned out to be working fine. Tracing the wires inside the chassis, I've found two 80A fuses (one for each alternator) - both of them blown. Not sure what caused it, but replaced those and things are back to normal. About wiring the two alternators - As far as I understand based on inspection, they are simply wired parallel. Two needed because of the amount of load.


A fully charged battery should be about 12.8V approx roughly about 2.1V per cell. I would change both batteries based on the low voltage values 1 dead cell on one and 2 dead cells on the other.

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