I'm fighting a find the weakest link battle on my '01 F250. Turns out my glow plug relay wasn't working. So I replaced it. But, when my glow plugs finally kicked in, my batteries can't take it. Time to see if either us worth keeping.

 

If your speaking of the batteries, nope replace them both in a set. I don't know where your from but winter is coming and you will thank yourself later for having two good batteries.

 

I went to Harbor Freight and bought a battery analyzer.
Digital Automotive Battery Analyzer
This instrument will give you the CCA value for your batteries.
It will help to determine when they are ready to be replaced.
As I understand, it takes 850 CCA between both batteries to crank a 7.3.
That would mean that when the batteries are at about half strength you replace them.
However I am not sure what waiting that long does to your starter.

On a side note, I have 4 batteries in my boat, one pair for each engine.
One pair got to where they would not start the engine.
Normally I would have replaced all 4, but this time I tested them and found one of each pair was 95%, the other two were bad. I kept the good ones and replaced the bad ones.
Now I check all of the batteries periodically and will replace when failing.

LA

 

Very worthwhile tool. I will add it to the collection. Thanks guys.