3 Ways To Measure Battery State-of-Charge

In this guide, we teach you how to measure the battery’s state of charge for your home solar system, and it’s just like checking to see the amount of fuel left in the tank for both peace of mind and safety.

How Do I Check The Battery State Of Charge? [3 Ways]

Checking the state of the battery charge doesn’t not give you information on the capacity or the condition of the battery.

  • Use hydrometer
  • Voltmeter
  • Smart state of charge monitor

The simplest way to check the battery status is to use a multimeter.

It’s the most reliable way to determine the state of charge and tells you the SOC voltage in each cell of the battery.

Use Voltmeter

Connect + and – clamps to appropriate terminals on the battery, and the measured voltage tells you the state of charge.

For a 12V car battery:

  • 12.6V = 100 % SOC charge
  • 12.4V = 75 %
  • 12.2V = 50 %
  • 12.0V = 25 %
  • 11.9V = Discharged

For a 24V battery

  • 25.5V = 100 % SOC charge
  • 25.0V = 80 %
  • 24.2V = 50 %
  • 23.3V = 20 %
  • 22.9V = Discharged

Use Hydrometer

A hydrometer can measure the specific gravity (density of the electrolyte) of the battery for flooded batteries, absorbed glass mat or gel batteries.

The higher the density, the higher the state of charge.

Safety measures

Always wear eye protection when performing battery checks with a hydrometer. Battery acid can burn through the fabric, and it can cause severe skin burns if not careful.

Allow the battery to be discharged and charged back up at least once before measuring the electrolyte fluid, and always make sure the water is uniformly mixed with the battery’s solution before an accurate number can be measured.

  1. Carefully insert the hydrometer into the cell and monitor fluid filling (Brown or gray color indicates the battery is near the end of its lifespan)
  2. Hold the hydrometer in a vertical position to see where it meets the scale on the float, and do it for all cells
  3. Account for temperature changes (For every 10-degree difference, add or subtract 0.004)
  4. Get final reading to estimate the state of charge

For example

  • Between 1.274 and 1.239 is about 75 – 100 % charged
  • Between 1.238 to 1.200 is 25 – 75 % charged
  • Under 1.2 means a low & discharged cell

Smart State Of Charge Monitor

A smart state of charge monitor is the best way to accurate measure the battery’s charge state by continuously monitor voltage, amperage and amp hours remaining.

It’s basically a simple to read device that can calculate battery energy consumed and available to the user. Anyone can use this without messing with PPE (personal protective equipment) or looking up a battery SOC charge chart.

Ying Xu

Graduated from the University of Kansas with an Aerospace Engineering degree & served 4 years as an US Air Force officer. Experienced with basic survival skills, weapon system security,  technical disaster recovery, and system troubleshooting

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