Vehicle Electrical Charging System.

That little GP thing cost me a really good looking truck when I was buying at an auction back in 2008. The batteries were dead in most of the trucks, they'd boost them, then pull the cables off after a few seconds and the trucks would die. I was slobbering all over a really nice wine colored truck, figured something was wrong with it when it stalled, and watched it sell for $18,000. After I figured out that the trucks weren't charging for a minute or so, I started bidding, and ended up paying $21,000 for a white 2006 one because the sale was almost over and prices were climbing.
I was told the PCM doesn't let the alternator charge while the GP's are on because it's too hard on it. Never knew they drew 200 amps...   

I've worked on a lot of the 7.3 Powerstrokes over the years being a diesel mechanic. We had a 2001 F550 at work that had fits of not starting even though it cranked really well. Tach signal was good, GP's were good and the HUEI pressure was good but still no go. The dealer messed with it for 2 days and said they fixed it. The next morning after we got it she wouldn't start...LOL....My boss brought it back to our shop and put me on it. I hooked up the scanner and watched a few things while I cranked it. I found the battery voltage would drop below 11V in no time while cranking even though it sounded like it was cranking over just fine. If I had a charger hooked to the truck it flashed right up every time. I did some reading and found out that if the PCM dropped below 10.6V it won't fire the injectors....2 new batteries fixed it.  ....The dealer wouldn't believe my boss when he told them what I did. That's all I changed. I can't say all the 7.3 PSD's would be that sensitive to voltage but that truck is.  It's still starting good to this day.