Purely a guess based on a Wikipedia article about Alternators... But if your machine has an alternator for charging you might consider checking the charging lamp...
"Once the engine is running and the alternator is generating, a diode feeds the field current from the alternator main output, thus equalizing the voltage across the warning light which goes out. The wire supplying the field current is often referred to as the "exciter" wire. The drawback of this arrangement is that if the warning light fails or the "exciter" wire is disconnected, no excitation current reaches the alternator field windings and so the alternator, due to low residual magnetism in the rotor will not generate any power."
I am not qualified to volunteer an answer...
Peccavi
July 2009