Sorry I cannot help you with your key question. Re your analysis that you have a problem, what voltage do you expect to have at idle - is 12.5 indicative of a fault? I'd be more inclined to ask what the volts go up to at 2000 or 4000rpm.
I think you'd be expecting something around 14.8V (depends on temperature). Are you completely happy with the condition of the battery?
I think you'd be expecting something around 14.8V (depends on temperature). Are you completely happy with the condition of the battery?