Yes, you have to interpret the readings from the conductance testers. They don't give you a direct reading of AH.
I tend to develop baselines for each type of battery by testing those which are nearly new or just broken in with several charge-discharge-charge cycles, and interpret results against these.
Certainly agree that it would be nice to have a tester which could rapidly and easily give you a close approximation of remaining capacity.
Another thing with the Midtronics testers: you have to be careful to get the clamps very tight in order to get repeatable results.
Had a recent experience which may be of interest. I have four APC UPS650 units in my house for our computers
, cable modems/routers, etc. Just replaced all four batteries after the one on my own computer started giving evidence of failure.
The four batteries were all 12V 12AH AGMs of the same approximate age: 3 to 3.5 years old, and all were in 24/7 operation. The one in my computer was the worst, measuring only 10.5 volts after removal
from the UPS650.
The other 3 all measured 13 volts or so after resting for 24 hours after removal
, but had very different capacities according to the Midtronics tester, viz:
1. (mine) "bad cell"
2. 131 CA made in Taiwan
3. 101 CA made in China
4. 89 CA made in China
The last three were all PowerSonic batteries of the same model. The worst was a PowerWerk battery (China) from Batteries Plus.
Bottom line: four 12V 12AH AGM
batteries of the approximate same age, in identical
applications with identical use and charging
setup (in the APC UPS650), with very different outcomes after less than 4 years operation.
Go figure :-)