Originally Posted by ripcord
Alternators (functional ones anyway) have a diode that prevents current from flowing from the battery back to the alternator. Think of it as a one way valve. There should be no problem charging a battery with the cables attached. That said, you should check your owners manual to see what it says.
Yes, but as a corollary to that an alternator with an open diode can wreck a battery. In a moment of weakness, I once bought a rebuilt alternator from AZ. My battery would discharge quickly. Took it back, got another, did the same thing, PITA to R&R. Bad diodes in both cases. Warrantied. Insisted on a NEW Denso unit for my time and trouble, they actually agreed. No problems since. 1987 Accord. Great car but soon for sale.