Dual Battery Question

I have a 1990 F250 with a 7.3 dsl engine. I'm having starting problems and I believe that there is something wrong with the batterys. So my step father gave me a good battery. The cold crank amps are somewere in the range of 550-600 and it is only a year old. He olny had it in his TR6 for a few months. I was wondering because it is a dual battery setup. Can I just replace the weaker of the two batterys with the newer one or will this just cause problems. I would just replace both at once, but I plan on getting rid of it in March.



Thanks

I would see no problem with it as long as they're in the same group. If they are there should be no problem.

You shouldn't have a problem as long as you hook the Batteries in Parallel instead of Series. If you hook up in Series you'll end up with 24 volts instead of 12 volts, and that would fry all you electronics.