I had to jump my duramax once. It was last winter in -20 weather, left an assorsory on. It took a truck off the same size, 2 sets of #2 gauge wire jumper cables hooked to each battery and needed to charge about 1/2 an hour before the truck would roll over. I still have those batterys in the truck and they havent gone dead yet.
I am told that if they are not very good jumper cables it won't work either.
Bingo. You need the fattest jumper cables you can find to start a dual battery truck.
No you don't. Just hook up to the posts of the main battery and not the sister battery. This way you dont have to go through the extra cable in the parallel circuit. The Fords are always harder on batteries beacuse of the glow plugs and having to have a minimum rpm requirement before the injectors will pop.
Just because we have diesel engines doesn't really mean they take a whole lot more to turn over an engine. A V8 gas engine starter could require more juice from a battery than the one in my Cummins due to the way the starters are built.
My truck was built in Oct of 03. I am still on the original batteries and they spin it over just fine even after the grid heater has cycled in single digit temps.
Just went through this yesterday with Dad on one of the tractors... Cummins 5.9L powered, by the way
He turns the key, it cranks over a few times, then nothing. Puts his best 2ga jumper cables on it. Nothing but clicks.
He's got a carbon-pile load tester. Puts that on, and the battery's got nothing. Meter drops to zero when he hits the load button.
Dang battery is only 5 years old. Back in the 1990's we were replacing batteries that were 15-20 years old. Now none of them are more than 3-5 years old.
Those carbon-pile load testers are a great investment if you've got several batteries to maintain. It takes the guesswork out of whether the battery is good or not.
2002 Chevy 3500 DRW 8.1L/Allison
2000 Palomino B1500
...and the reason why I need a DRW to haul a Palomino:
2004 United 7x14 tandem axle enclosed toy trailer
2011 PJ 8x20 7-ton deckover equipment trailer
Fyi, The batteries went dead in my 99 PSD a few yrs ago,( left the dome lite on for a couple of days (dummy here)), but my jumper box/compressor from Costco was able to jump it. My mechanic says he uses his all the time to jump diesels.