I have a battery that is going down. And have been told that if you have to replace one that you must replace both. The truck was parked for a week or so and when I tried to start it, it was dead. Rolled it off and it was fine the rest of the day. Worked fine the next morning. If it sits for more than a day it will go down. Roll it off, then you can turn off the truck and it will start right back up. So, if I replace one do both have to be replaced. I was also told that if you drive one with a low battery that it can screw up something else in the truck. Dont remember exactly what it was called.

 

I suggest first, make sure all of you battery connections are clean and tight. Then charge both batteries completely. Just driving around town for a day won't fully recharge a drained battery. Also, Autozone, Batteries Plus, and other places can do a load test on your batteries to check to see if they are indeed bad.

 

I agree with the above.

If you replace one, you should do both.

 

ive owned diesels for years and have never changed both batterys at the same time,i dont believe its needed,unless you have lots of money to spend,if one tests good why change it?its all a matter of what one preffers.

 

Up here, we've always replaced both at the same time. -40 puts a huge strain on the system and any battery short comings are quickly noticed.

 

Replace them both as a pair.. When you put only one new battery on, the difference in available C/A and recovery rate causes the a excesssive wear on your new battery.

 

Thanks guys, I appreciate it. BTW.... -40 WTH, thats crazy! haha

 

As batteries age they charge and discharge differently. A new battery will show 12.9 volts fully charged were a older battery will be around 12.6 volts. A battery @ 12v will have about 50% of it's capacity. It's probably because the fluids & metal plates wear out in time.

If there is two different age batteries, in parallel, they will charge and discharge differently and actually be robbing from each other, trying to equalize. This makes for a inefficient system that will not produce it's full potential. I will stick with changing batteries in pairs (like I do on my boat). FYI

 

I agree with doing them in pairs. The batteries are produced and installed at the same time, roughly, and will last about the same time. Why get dirty twice to save buying the inevitable for a few weeks, not to mention the added strain on the new battery.