Turtle...the wife wants to know where to fine the formulas you referenced from so she can stash for later when new batteries are needed.
Thanks a bunch!
k1 = 0.61 + 0.0082*temp - 0.0000417*temp[SUP]2[/SUP]
k2 = 0.61 + 0.0082*ccatemp - 0.0000417*ccatemp[SUP]2[/SUP]
newcca = k1 * (cca/k2)
You need the CCA rated temperature (which is zero for the base), and the current battery temperature, and the battery CCA as stated on the battery label.
The easiest way for most people is to just read the CCA on the battery label and know that it'll be more than that if it's warmer than zero and less than that if it's below zero. And that it'll be less than that if the battery is not new and fully charged.
When buying new batteries, you need to have as many as the starting motor requires to start the engine, but generally speaking, get a battery (or batteries) with the most CCA available. Mostly you're limited by the footprint of the battery (Group 31 or Group 29, etc.) but sometimes you will find two Group 31 batteries with very different CCA ratings. Get the higher one. That battery contains more lead and therefor can put out more amps.
One thing to keep in mind for those with multiple battery setups, like in a truck with 3, 4 or 5 truck batteries, or with a house bank of batteries, you add up all of the CCA (or CA), so three batteries each with 800 CCA gives you 2400 CCA to start the truck. It's also the number of amps that will be dumped toot-sweet into a short circuit.