When the truck sits for a night in 0*F, it doesn't have enough juice to crank over in the AM. If I hookup my 200amp jump pack, then that is enough of a boost to get it to crank, barely. If the temp is closer to 20*F, it starts fine.

The batteries are Autocraft Gold rated for 850cca each, and are just under 3yrs old. When each is tested individually, I get 752 and 790cca (the EXP-1200 tester says this = good). But measured in parallel I get 1099cca.

Both ground cables and positive cable are same age as the batteries and motorcraft brand and battery terminals are all clean.

Why do I have low cca's in parallel - maybe the block grounds need to be cleaned? Maybe the cable went bad? Thanks!

 

Are you measuring them in parallel IN the truck or just hooked together with jumper cables?

 

In the truck.

 

Then you have something very well could be a dirty ground affecting your CCA. I'd start there first.