So, you can measure your battery Voltage to determine how charged they are. And you can measure the Current the see the rate of charge or discharge (how fast the batteries are gaining or losing power). Use the chart below to interpret your battery voltage readings.
This chart will allow you to get a pretty good idea of how charged the batteries are. They must not be charging or discharging for these voltages to be correct. Also, it should be at least 1 or 2 hours since they were. A good time to check is early in the morning before charging starts or any appliances are turned on. This will tell you the SOC (state of charge) or simply put, how full they are.
Battery Voltage Chart
% of Full Charge__________ Voltage
100 % charged_____________12.7 volts
90 % charged______________12.6 volts
80 % charged______________12.5 volts
70 % charged______________12.3 volts
60 % charged______________12.2 volts
50 % charged______________12.1 volts
40 % charged______________12.0 volts
30 % charged______________11.9 volts
20 % charged______________11.8 volts
10 % charged______________11.7 volts
completely discharged_____11.6 volts or less
Remember, this measurement is most accurate after the batteries have been at rest at least 1 hour, and neither charging nor discharging.