For the last month or so I've had problems if I leave my lights on for few minutes (maybe 5 minutes) or leave the radio playing while the engine is off etc. The battery is dead almost immediately which led me to believe it was going bad.... I took it off and had the parts store test it and it showed to be good and took a charge just fine. So I checked it's voltage sitting in the car (at this point thinking that the alternator isn't putting out enough), 12.55 with car off, the voltage at idle is 13.65 and at 2500 rpm is 13.7. Is it getting charged enough, bad battery, or is there something else wrong? (I'm out of ideas at this point!)

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k