Originally Posted by Rockter
It is notoroiusly difficult to judge a battery condition by measuring the battery voltage, unless the battery is completely dead at (like) 12V or something similar.
Over the years, I have found that the only test is to charge it up, and work it to see if it meets a reasonable load, then re-charge it and see if it holds charge.
When your motor is charging, the battery voltage should be at about 14.1 V (or near to).
This is correct. A "rested" open circuit voltage is very accurate to determining SOC however. What it is NOT capable of is telling you what "capacity" you have left in the battery.
You can have a 100Ah battery reading 12.72V rested that will supply 100Ah. You can also have a 100% full 100Ah battery reading 12.72V that only has 60Ah of capacity left in it. Both batteries are at 100% state of charge but one now has less capacity due to use.
The only true measure of capacity is to do a controlled 20 hour capacity test at 80F... The Midtronics analyzers are also fairly accurate for looking at aging, if a when new baseline was taken, but they cost $600.00 to $2000.00... They still don't do a 20 hour "capacity" test or directly correlate over to capacity....