Dead batteries / Off year, not off brand

[71 LONG BED]

My buddies work truck is a 200? F550 Powerstroke Diesel with a bucket boom.
This mourning the truck started OK but only got him 5 miles down the road before the battery light came on and the truck died. He got a jumpstart and got to the auto parts store. After idling for over a half hour it still wouldn't start without a jump. He replaced both batteries and continued. Another few miles down the road the battery light came back on. Needs new alternator, right? Any other thoughts?

[71 LONG BED]

[Ares68F100]

Even with the alternator being bad I don't see it sucking up ALL the juice from the batteries that quickly and leaving him stranded. Most batteries have a 70 - 120 minute reserve. Make sure everything is hooked up right get it jumped and have a free electrical test done at a Kragen's or Autozone. They will be able to tell you if the alternator is bad.

[Brett6269]

Yea thats awful quick to drain both batteries down, you may have a combo of 2 problems.

[heep70]

Whats a few miles down the road. Get the alt checked. They are known for going bad all of a sudden, not slowly.

[71 LONG BED]

Prolly is the alternator. The dealer can't take the truck until Wednesday. He's going away for a long weekend anyway so it should be fixed when he gets back. I should have told him to pull the battery cable while it was running. If it shut off, then it is definitely the alternator, but I didn't think of it.

[QC]

You probably shouldn't pull the cable off any late model vehicle when it is running. You could cause a voltage spike that could do some damage. The electrical systems are supposed to be protected from spikes but it doesn't always work.