A fairly new battery is getting weak slowly. After charging the battrey seems good, shows 12.5 volts, and you place a load on it with a carbon pile tester, it drops to 11.5 volts. Good battery, right? Now, I start the engine, test the battery, and it jumps to 13 volts right away. Alternator is good, right? BUT, when I put a load on it with the carbon pile tester, voltage drops to 9 volts. Alt doesn't look so good now. So I take the alt to my old school family run auto electrical re-builder guy. The old timer is busy, so a younger guy helps me. I explain the situation, and he keeps saying he thinks the battery is bad. But the battery was replaced less than a year ago. The last one had a bad cell, under load it went right to 10 volts. This new one re-charges immediately, and only gets weak after being operated for a week or more. I think the alt is not keeping the battery charged properly. I think this alt is OEM and about to hit 100k so I don't mind getting it re-built. Side note- an engine oil leak has been quietly dousing the alt with crud. I can easily see that causing trouble. Am I right? Or am I nutz?