I went to buy a couple batteries the other day and the helpfull fella said that most diesel owners only buy one battery. Since the primary wears out first, they put the secondary in the primary spot, and the new battery in the secondary spot. True this?

 

Always replace BOTH batteries at the same time.Many owners are trying to save money but don't know better. Make sure their good quality batteries too.
Nut

 

never replace one at a time. it will drain the strongest one and balance to the weakest

 

I third the above comments! Always replace both batteries at the same time. Just think of it as a good investment every 5 years or so. Some of the motorhomes I have seen have 6 or 8 deep cycle batteries and the same would apply as one would be a weak link in the chain.

 

Powerstrokes don't have a primary and secondary battery. They're both used, especially to start the truck. Like stated, the weak one will kill the new one.

 

Well that's what I thought. Thanks.
They are the best battery Sam's club has. And only $63.00/ each.