Join Date: Aug 2006
Location: Arlington, VA
Thanked 13 Times in 11 Posts
Rep Power: 12
I believe the original post was questioning battery capacity, not state-of-charge. They are different.
For example, a new 6-volt T-105 golf cart battery is rated at 225AH at the 20-hour discharge rate. If you take two of these, hook them in series, you get a 12-volt battery with a (design) capacity of 225AH. If you then apply an approximately 11A load, then 20 hours later the battery should be depleted (10.5 volts, resting).
Now, let's say the batteries are now 4 years old and have been treated only moderately well. It's likely that they will have lost much of their capacity, i.e., their ability to store and deliver electrical energy. Measuring actual capacity cannot be done with a multimeter, and it can only be estimated with load testers, hydrometers, and the like. A sophisticated internal resistance tester, like the Micronics series, can estimate battery capacity, and this has become the standard way of doing it these days. It's not a precise measure, but seems to be a pretty good indication. Unfortunately, these devices are costly...upwards of $500-600 for a decent one.
Note that resting battery voltage is NOT an indicator of capacity. You can have a full-charged battery showing a resting voltage of 12.6 or above, but which has lost much of its capacity (thru sulfation, stratification, contamination, plate erosion, etc.).
Aging batteries can be like aging runners...their sprint capacity may be little diminished, but they can't really hang in there for the long run :-)