Napa AGM CCA dropped 10% after 2 months

[Jetronic]

It seems to me that a CCA tester only infers the CCA measurement because the test isn't being done at the stated CCA temperature. The tester must therefore assume that a battery follows a particular performance curve at different temperatures and who's to say every battery matches exactly the curve built into the tester. It wouldn't be a surprise to me to find a battery tested a little differently at the ambient temperature inside a house as compared to a test done with the battery at winter temperatures outside the house.

they basically measure the resistance in the circuit, then apply a voltage correction for temperature and voltage (corrected)/resistance gives CCA. So surface charges can seriously impact the results in a positive way, and poor connections in a bad way. They also say nothing about the capacity of the battery which drops with age regardless of voltage

If the tester doesn't know the battery temperature, some assumptions have to be made, and the user should use common sense with the result he's getting, as usual.

 

[slomo]

Drop an old wrench across the terminals. Take it back a week later for a replacement. Give it time to expire. Do this yearly so you always have a fresh battery.