Rudy, For flooded cell batteries use the following table: 100% 12.6V 75% 12.4V 50% 12.2V 25% 12.0V 0% 11.6V These voltages are not precise but can be used as a general rule-of-thumb and are easy to remember. They are rounded to the nearest tenth volt and are for the "average" battery. Differences will exist according to manufacturer and specific gravity of the acid used to initially charge the battery. For more exact values check the battery manufacturers data/website for the specific battery in question. For instance Trojan's fully charged voltage is 12.73V. Perhaps the most important thing to consider in relating voltage to a battery's state-of-charge is that the battery must be at-rest. That is, it is neither being charged nor discharged and has had a reasonable time for the voltage to settle. Additional voltage from what is called surface charge can take a long time (6+ hours) to dissipate from a battery just removed from charge. Applying a small load for an hour will help to bleed off the surface charge. Likewise it can take an hour or so for a battery that has just been under load to stabilize. (This is due to the time it takes for the chemical reaction to catch up with the chemical-to-electrical energy conversion process.) One other thing to be considered is the accuracy of the voltmeter being used. As can be deduced from the table above, a ten percent change in charge is represented by an 80 millivolt change in voltage. A typical voltmeter might have an accuracy of plus or minus one percent which is 120 millivolts. So for a typical meter the uncertainty due to the inherent accuracy of the meter relative to the state-of-charge of the battery is plus or minus 15%. Obviously it is important that the meter used to make this measurement be very accurate and that the uncertainty of the reading due to meter accuracy be taken into consideration when interpreting the reading. The preferred method for determining SOC of a flooded battery is through the use of an accurate, temperature-compensated hydrometer used to measure the specific gravity of the electrolyte. (Note that approximately the same precautions relative to resting the battery apply.) A table correlating specific gravity to SOC should be available on the battery manufacturer's website. Typical values are: fully charged = 1.260 and fully discharged = 1.050. However, because a hydrometer can not be used with sealed batteries (AGM and Gel), the voltage method is a reasonable alternative to determining SOC. Good Luck, Tom Collins - Misty Sea