Batteries on a ford 7.3

With the two batteries. Does one battery heat the glow plugs ? and one battery start it or both ?

I was wondering if that might be my hard start in the winter problem . Starts right up in the summer. but last winter it started getting hard to start when it was mid 30's or below. The glow plugs were all replaced about 3yrs ago. How often should they be changed ?
I was gonna go get my batteries tested next week to make sure their good or maybe going bad.

Both batteries do all tasks together if they are wired in parallel.

I know GM's are. I would assume the same for Ford.

Yep, batteries are in parallel, you need to replace them in pairs. Your local auto parts store (big chains) will usually do a stress test for free.

Lucky for me I have a friend that owns a battery store.
Just wasn't sure if the batts were in parallel.

Check the cables between the batteries especially the ground for signs of corrosion.

Will do jet

Yes, they are in parallel and one battery won't cut it. I usually replace both of them at the same time.

scottz wrote:Yes, they are in parallel and one battery won't cut it. I usually replace both of them at the same time.

X2 same for me I replaced both of them twice now had a bad new one.

jetskier wrote:Check the cables between the batteries especially the ground for signs of corrosion.

Right on jetskier. It's absolutely amazing how much a little bit of corrosion will mess up the power going out of the batts to the truck.

Next check glow plugs.