"So I get he cold cranking amps, and divide by the amps drawn and thats tell me how long it'll run."
NO!
"Cold cranking amps" is just a salesman term! It does not have really relate to anything, other than how much current the battery will put out at a certain temperature, for a brief period of time.
The amphour rating of a battery is the rating that tells you directly the capacity of a battery.
For example, a 60 amp hour battery will put out 60 amps for an hour, 10 amps for 6 hours, 1 amp for 60 hours, you get the idea.
A 60 amp hour lead acid battery is capable of putting out a lot more than 60 amps for a shorter period of time. This just a WAG number, but I would not be surprised if a 60 amp hour battery could have a CCA rating of 300 to 400 amps.
So when they are trying to sell you a battery, it sounds more impressive to try to sell you a battery with 600 to 800 CCA, even though another battery may actually have a higher amp hour rating.
