How can a standard automotive or marine battery's state of charge be accurately measured?

How can a standard automotive or marine battery's state of charge be accurately measured?

The state of charge of a lead acid battery is most accurately determined by measuring the

specific gravity of the electrolyte. This is done with a hydrometer. Battery voltage also indicates

the level of charge when measured in an open circuit condition. This should be done with a

voltmeter. For an accurate voltage reading, the battery should also be allowed to rest for a

period sufficient to let the voltage stabilize