My 7.3L diesel requires two starter batteries: one under the frame rail and one under the hood.
When I bought this van a little over 4 years ago, it had terrible no-start and hard-start problems. The battery under the frame rail was fried, so I immediately replaced it with a new one. It was a Group 65 so that's what I replaced it with. The battery under the hood was dated from 2005, and it was a smaller 58R battery. That one tested out as still good, so I didn't replace it. I assumed it was the correct type, and perhaps there was just less room under the hood, but I should have smelled something wrong. The battery was held in with zip ties.
Well I eventually removed the zip ties and bought a proper correct bracket for the under-hood battery, but, since the battery was still good, I left it in place.
In the meantime, strange things happened. I fried several glow plug relays. Not sure why they fried. I have always had hard starting, even after fixing lots of stuff like the glow plugs and relays and filters and hoses and buying better fuel, but I attributed that to just something unavoidable when running biodiesel. Other than that, everything was fine.
Last year, I noticed that the resting voltage of the batteries was a bit lower than I remembered it being. Still, I didn't have good baseline data, and I got lazy, and didn't take the batteries out to check.
Last month, after parking from a long drive, I smelled smoke and ozone. Smelled to me like an electrical fire. I frantically looked around the interior of the van, and checked my solar house wiring and house batteries carefully, but the smell appeared to come from outside the van. So I figured maybe it was something not related to me.
This weekend, I wanted to get out of civilization so I went to a beach in a fairly distant area. I spent the day there. At nightfall, I tried to start the van. No dice. The battery meter level quickly identified the problem: dead battery or dead batteries. I checked with a meter: the under-hood battery seemed fine. I pulled off the case for the under-rail battery-- the one I'd bought in 2008--, and saw, the + terminal caked in corrosion and the whole top of the battery filthy with what appeared to be soot. And, a really bad smoky smell like an ashtray inside the battery box. The rail battery was showing low on the multimeter too.
I managed to get a jumpstart and cut my vacation short; heading back into civilization so as to buy a new battery the next morning. And I did just that-- after first cleaning off the corrosion (baking soda, warm water, toothbrush, worked great). And then... no start, still had low battery. Huh? Checked with multimeter, and the under-hood battery was showing 11.48v. Um, OK, that one is toast too, so off I go back to the parts store.
But the guy behind the counter said, you know, that van takes TWO group 65 batteries. This 58R is not the correct battery. It did seem like the battery shelf might be able to hold a bigger battery, so I took his word for it, and bought an additional group 65. It fit perfectly. The service manual doesn't really say what battery size to use, only that it must be 78 amp/hours or greater. But it does seem like the group 65 is correct in both locations.
So that'd explain a few things. First of all, why I kept frying glow plug relay contacts-- over current due to lower voltages from not having the correct battery. Also, that nasty burning smell I'd discovered a month ago was my under-rail battery dying. As was the low voltage I thought I'd seen a year ago. And the hard starting may have had to do with never really having the correct battery.
So now, several years later and several hundred dollars lighter, I have the correct batteries for this vehicle. It does seem to start faster, but I'll have to wait for more cold and/or wet weather to find out for sure.