State of Charge

The truest measure of a battery's state of charge is the specific gravity of the battery acid. The following shows the approximate state of charge at various specific gravities at 77ºF / 25ºC.
 

ChargedSpecific Gravity
100%1.255-1.275
75%1.215-1.235
50%1.180-1.200
25%1.155-1.165
0%1.110-1.130

 

Hydrometers can be difficult to use and at best accurate to +/-0.005 points. Voltage can be used to estimate state of charge but caution must be taken when interpreting voltage readings.

 
All Ampere Hour capacities based on specific gravities of 1.280. Reduce capacities 5% for 1.265 specific gravity and 10% for specific gravities of 1.250