State of charge (SOC) can be pretty accurately measured with a digital voltmeter, provided that there has been no charging
or discharging for several hours. I've done this on my boat for over 20 years, with no surprises. However, you do need to be able to "rest" the batteries for several hours -- best overnite -- in order for this method to be accurate.
However, and it's a BIG however, SOC is only one of the things you need to know. As important....or, arguably even more important....is the capacity
of your battery bank. It's perfectly possible to have a fully charged battery (100% SOC) which is totally incapable of, say, starting your engine
. We see this often in cars, where the battery has reached the end of its useful life.
Over time, battery capacity (i.e., the ability to accept and hold and deliver electrical
energy) deteriorates due to a number of factors, including: plate sulfation, corrosion
, contamination, stratification, physical damage, excess heat, overcharging, undercharging, repeated deep discharge to levels below 50%, etc., etc. Figuring out how much capacity remains in your batteries can be a tricky exercise.
Properly used, battery monitors (AH counters) can approximate battery capacity, but the only ways to really get a good estimate is either thru:
(1) a time-measured discharge with a known load calculated to the 20-hour rate; or (2) a relatively costly measurement device which measures internal battery conductivity/resistance (like the Midtronics testers).
The first method is by far the most accurate. The second method has become the "standard of the industry" since most mechanics/engineers don't have 20 hours to wait around for the battery to fully discharge.