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Found my truck dead after it sat for 2 days this past weekend. Load tested the batteries found that the passenger side battery had a dead cel and needed replacement, driver side tested ok.
So I had a set of Motor Craft 850 CCA's in it, this will be the 3rd set of batteries in 10 years, and I've kept track and it seems, for my truck anyway, its always the passenger side battery that poops first. It makes sense I suppose as its more exposed to the heat and such. This time I'm trying a set of A/C Delco Professional AGM's. I'm hoping that maybe they will deal with the heat and vibration better than a regular battery.
I almost thought about Optima Batteries but I've never had good luck with them. I'll keep everyone posted.
Hey Sarge, same scenario here when I had my 6.0. I would usually go back to AZ at app. 30 months and have them do a seperate load test. Usually got a free set. No ryme or reason just the way it was.
I rotate the two batteries once or twice a year, and keep the trickle charger on the driver's side. My Motorcraft 750cc (original OEM spec) died every 3 years with the stock alt, but they died equally for the most part on the load tester. IDK if the DC Power alt and Ed's cables will help since my GP-cycle voltage is in the 12s and not the 10s now.
the big draw is the glow plugs.
Switch to AGM batteries.
Stay away from the Optima crap... buddy of mine has them in his 250, every couple years there eat themselves.
Passenger side drives the glow plugs, why it eats that one first.
And don't forget the starter. The two highest amp draw items pulling primarily from the passenger battery, with the drivers side battery only helping through the long wire in front of the radiator that naturally has resistance supplying less available amps. This is why when you jump start you use the passenger side battery.
Rotating them left to right is a good practice.
The batteries are tied together, if you are losing one over the other you need to check the cables and connections, including grounds. Any voltage loss to the drivers side battery will have to be made up by the pass side. Same goes for charging, alternator is charging the driver side battery through the same cable.