Battery running down, engine stopping while driving

2004 Ford E350 V8 6.0L Diesel

My battery has been running down while I am driving. I can watch as the charge needle drops lower and lower until the dashboard lights turn off and the engine stalls.

Problem first occurred in June just after changing the brake master cylinder.

It has been to the shop twice for the issue - first time the alternator was rebuilt, second time they checked the alternator (OK) & battery (weak but adequate) and found a dome light in the back of the van that had been left on.

Less than 30 miles and 18 hours after leaving the shop it repeated the performance.

Batteries are about 18 months old.

For whatever reason, the alternator is not working…Even with the volt gauge, you should have a “alt” or “batt” light warning you something is wrong. I would have the alternator bench tested.

You need to get an inexpensive digital multimeter.
Periodically check the battery voltage with the engine idling and accessories off.
Check it before (cold engine) and after a drive.
Should be 13.5-14.5V.
I think it’s an intermittent problem you’ll have to track down as you use the vehicle.

The batteries are 18 months old and that can equate to plenty of time for failure.
Many batteries fail within a week or even on the shelf before they’re sold or installed.
I’ve bought new batteries that were junk the day after and the new battery I just put in my lawn tractor failed within 2 weeks. Since it was pro-rated, no big deal but the replacement battery has also gone south.

You really need to investigate this “weak but adequate” diagnosis, whatever that means.
Adequate is acceptable on old cars with mechanical fuel pumps, contact point distributors, and not much in the line of electrical gee-whiz stuff. On modern vehicles, adequate is a coin flip.