95 nissan pathfinder 170000 mi sev6 manual trans. About 1 1/2 yrs ago my battery light went on visiting out of town. Checked alternator charging voltage with a multimeter I had on hand (from helping the folks I was visiting with home electric work = how ironic) and was just reading battery voltage (12ish). Repair shop replaced alternator for 400ish. Out of town upcharge I guess. Just before the 90 day warranty, light lit again then went out. I bought a voltmeter I could leave in the accessory outlet and read while driving. Alternator would charge in the beginning but stop charging after some driving, maybe 30 minutes or so. Voltage would drop to 12, 11ish etc. Went back & got alternator replaced under warranty but he said I should get a new battery as even though my 4 year old die hard tested fine, he said it might be the problem so bought a new battery from him same day. Now less than 2 yrs later, the battery light went on. Plugged in my volt meter. When I start out, I get a 14.1 volt readout but after 10 or 15 minutes of driving it drops to 13ish, then quickly to the 12's & eventually below. Sometimes it even jumps back up to the 13's depending on rpm for a while - other times not. It's varied but usually eventually just goes down to no charge til I cool down a couple of hours and start again with 14.1. Luckily at this point i'm not driving far so haven't gotten stuck. That repair shop is 80 miles away. Don't want to go back as there is no more warranty but reluctant to just buy a alternator & replace myself as this is too strange. Can it be something else? Belt is not slipping (I also had him replace all belts (except timing) when I got the first alternator. Only about 15,000 miles on it in the 2 yrs.