A friend's Nissan 100NX has been running fine up to now. Last night the battery light came on (briefly) while he was driving along and he thought nothing of it... Tonight he made a short trip after work, got back in the car and the engine wouldn't turn over. Just a click from the starter relay, no solenoid noise. The engine started with jump leads, but ran very lumpy and would not idle. He had to keep his foot on the pedal to keep it running. The battery connections are OK, the battery appears OK but low on charge. The connections I can see to the starter and alternator seem OK.
It's as if something is draining the battery when the key is turned but the starter isn't turning - at all. When it's connected to my car, it cranks over as normal.
Could a diode in the alternator be doing this, or the voltage regulator?
I looked under the distributor cap and it's in very poor condition as is the rotor arm. But the car must have been running OK until now.
Opinions please? I'm going to suggest a replacement battery to start with, but if that doesn't make a difference?