Measuring "state of charge" of a battery bank is much more complex than just using a voltmeter at any given time.
I have been doing a bunch of reading, since I suspect that my battery bank is not big enough for my needs (mostly house fridge).
To measure true SOC you need to make sure that the batteries have not been charged or discharged for several hours (2-3). Then you need a good table to tell you what the meter is telling you. For example, 12.1 to 12.2 volts after a 2 hour rest is about 50% SOC.
If you do a google on "state of charge" + battery (or + meter) you will get some good information. One site I found that seems to be understandable is:http://www.otherpower.com/otherpower_battery_metering.html
As Gary said, the best way to tell SOC is with a hydrometer. Even that is technique sensitive. The next best way is a special SOC meter that measures the wattage used and compares that to the watt capacity of your battery bank ($200-300). The last method is the voltmeter used properly.
The following is from my Trace owner’s manual (note: the voltages can be doubled for 24 volt systems):
BATTERY STATE OF CHARGE
A good estimate of a battery’s state of charge can be made by measuring the voltage across the battery terminals with the battery at rest (No energy input, no energy output) for at least three hours. These readings are best taken in the early morning, at or before sunrise, or in late evening. Take the reading while almost all loads are off and no charging sources are producing power. Connect a voltmeter across the positive and negative outputs of the battery or battery bank. Voltages are for a 12 volt battery system. For 24 volt systems multiply by 2, for 48 volt system, multiply by 4. Monitor your cell voltage, if you measure more than a .2 volt difference between each cell, you may need to equalize (Do not equalize Gel Cell Batteries). The following table will allow conversion of the readings obtained to an estimate of state of charge. The table is good for batteries at 77°F that have been at rest for 3 hours or more. If the batteries are at a lower temperature you can expect lower voltage readings.
Table 7, Battery State of Charge Voltage
PERCENT OF FULL CHARGE
12 VOLT DC SYSTEM CELL VOLTAGE
100% 12.7 2.12
90% 12.6 2.10
80% 12.5 2.08
70% 12.3 2.05
60% 12.2 2.03
50% 12.1 2.02
40% 12.0 2.00
30% 11.8 1.97
20% 11.7 1.95
10% 11.6 1.93