Can alternator diodes "leak" and discharge the battery?

[Dale_K]

My battery went dead after the car (2000 S) sat for a couple of days. The previous owner had put in a Walmart battery and it was 5 years old. It didn't seem to respond to the charger the way I expected so I bought a new generic battery at O'Reily's (sp) and everything was good. Then after a month the battery was really low when I went to start the car. I charged it up, which took a really long time on a 6 amp charger and the car worked fine. The starter really cranked fast. After about a week the fast cranking starter was still starting fine but not cranking as fast.

I bought a voltmeter that plugs into the cigarette lighter and it said about 12.3 volts with the car shut off. That was after pulling the passing lamp switch for about 10 seconds and then releasing it. When I started the car the voltmeter showed almost exactly 14 volts. During a 30 minute drive it went down to about 13.8 and then 13.7 after I turned on the headlights. On other cars I'm used to seeing above 14 whenever the motor is running. Still it looks like it should be enough to keep it charged.

So I'm unsure about the next course of action. The guy at O'Reiley's said they have a tester and I guess I should do that. But I was reading another battery thread and there was talk about "diode response" and I don't know what that means.

Also, what should an amp meter read if I unhook the cable at the battery post and run it through the amp meter with the car off? Seems like there's a lot of stuff still running on these cars with the switch off, like the evaporative emissions check. I notice the hood & trunk lights will shut off after 2 hours so maybe I'd have to wait that long to see a true "shutdown" condition if the other car components follow the 2 hour rule.