Did you idle for an extended period before the battery was low? My alt doesn't put out enough at idle to keep up with the ac, radio, and aux cooling fan and it drains the battery in about 5 minutes.

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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