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Battery State of Charge—What is it, and How is it Measured


January, 20 2009

Knowing Battery State of Charge is like knowing the amount of fuel in your fuel tank: it is relevant information for both peace of mind and safety. To avoid getting stranded with a dead battery, accurate battery bank monitoring is needed.

  • The Hydrometer Method—Using a hydrometer allows measurement of the specific gravity of your battery, but this can only be done with flooded batteries and not with Low Maintenance, Absorbed Glass Mat, or Gel batteries. Using a hydrometer is a potentially dangerous and dirty job involving contact with a solution of battery acid and lead sulphate.

  • The Voltmeter Method—Monitoring voltage allows measurement of the potential charge of your battery. The difference from a fully charged battery to a fully discharged one is only 1.0V in a 12V system, so the meter must have good resolution and accuracy. This method is generally sufficient to monitor batteries which experience intermittent use, such as starter or thruster batteries. However, a battery must not have been charged or discharged for over 12 hours for this measurement to be trustworthy. This makes this method unsuitable for monitoring house batteries, which are in a charge or discharge pattern much of the time.

  • Amp-Hours Remaining Method—The best way to accurately measure Battery State of Charge is to continuously monitor voltage, amperage, and ampere hours remaining. This is a complex calculation of the energy available, energy consumed, and the energy returned to the battery in charging. It also adds the important element of time to the equation. Blue Sea Systems is pleased to present a battery monitoring system in the Vessel Systems Monitor VSM 422 that combines the best attributes of voltage and current monitoring while using its internal computer to calculate State of Charge under conditions of both high and low current flow. By offering accurate and timely Battery State of Charge monitoring, the VSM 422 provides boaters with vital information to ensure that they never suffer a dead battery at sea—the number one cause of a boat needing towing assistance.

    Three methods of monitoring your battery banks
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