Originally Posted by bob-o
The dealer put his tester on the battery and it reported 720 CCA. This test is supposed to be done at 12.0V for 30 seconds. How could it possibly have done that when it drops to 11.5 for just the headlight load? It is supposed to have 125 minutes of reserve capacity at 20A and it is half dead after a minute at less current.
That's a very good question . One answer is that the CCA rating is defined as the current a lead-acid battery at 0° can deliver for 30 seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12-volt battery).
This test indicated your battery provided 6AH (720A * .008H) during the test, but it's not really a measure of battery capacity, but rather a measurement of internal resistance amongst other things. The battery was not depleted during the test.
On the other hand you say that your battery is rated at 20A for 125 mins, which indicates it's capacity to be 41.7AH.
So your argument that your battery is suffering from reduced capacity was not proven incorrect by the CCA test.
After this thread has gone on for so long aren't you curious to find out what the real capacity of the battery is. I for one am really interested in how your resting voltage theory compares with a real capacity measurement. You could also try De-sulphating the battery to see if it changes anything.
Of course to do the test properly you may have to bring the battery indoors to run the test at the standard temp of 80°.
FYI the official definition of RCM (reserve capacity minutes) is the time it takes for the battery to reach 10.5V with a 25A load at 80°F. On my battery the RCM is 120 mins resulting in a 50AH battery.
EDIT: Battery capacity tests are normally performed with a load of C/10 or C/20, not 25A. This site
says that battery capacity in AH is roughly eaqual to the RCM in minutes divided by 2 , or C(AH)= RCM(mins)/2. So my battery would have a capacity of 60AH