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Batteries: Measuring State of Charge

Accurately knowing the state of charge of your batteries and keeping them properly charged can extend their life, keep your system running properly, and save you money.

Please note that voltage is not the most accurate measure of state-of-charge. Voltage readings fluctuate based on various prevailing conditions at the time of measurement. Use change of voltage as an indicator of changing conditions or relative state-of-charge under similar conditions over time. For more accurate state-of-charge readings, a shunt based monitoring system is recommended.

Measuring State of Charge in Flooded Lead-Acid Batteries

Measuring the specific gravity (SG) of the battery electrolyte can provide the best measure of the battery’s state of charge (SOC). Measuring voltage is one way to estimate the state of charge of a battery; however voltage readings may vary depending upon whether the battery is being charged, discharged or is at rest (open cell). Whether the battery is under load at the time of the reading has little effect on the specific gravity of a tested cell. Additionally, specific gravity is good for measuring overall battery bank health. By measuring the SG of each cell, you can catch cell-to-cell or battery to battery differences that can result in untimely battery death.

Trojan Battery Tips for Specific Gravity Testing of Flooded Batteries (Source: Trojan Battery)

  1. Do not add water prior to testing.
  2. Fill and drain the hydrometer 2-4 times before drawing a sample from the battery.
  3. Have enough sample electrolyte in the hydrometer to completely support the float.
  4. Take a reading, record it and return the electrolyte to the cell.
  5. Check all cells in the battery, repeating the steps above.
  6. Replace vent caps and wipe off any electrolyte that might have been spilled.
  7. Correct the readings to 80°F:
    • Add 0.004 to readings for every 10° above 80°F
    • Subtract 0.004 for every 10° below 80°F.
  8. Check the state of charge using the table provided by the manufacturer.
    • The readings should be within the factory specification of 1.277 +/- 0.007.

Examples of SG Readings as Compared with SOC (Source: Rolls Battery)

% Charged Specific 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

Measuring State of Charge in Sealed Batteries

The state-of-charge of sealed batteries cannot be determined by looking at the specific gravity of the electrolyte, since the batteries are, well, sealed! SOC can be determined by measuring the voltage; this is most accurate when the battery has been removed from a load for at least 6 hours, preferably 24 hours. Otherwise voltage values can fluctuate based on whether the battery is being charged or discharged. Here is a chart from MK Battery showing different levels of charge for flooded lead-acid and sealed batteries – open circuit voltage.

Comparison of Battery State-of-Chart to Open Circuit Voltage for 12-Volt Batteries

% Charge FLA Gel AGM
100 12.70-12.60 12.95-12.85 12.90-12.80
75 12.40 12.65 12.60
50 12.20 12.35 12.30
25 12.00 12.00 12.00
0 11.80 11.80 11.80
Note: Divide values in half for 6-volt batteries.

Source: MK Battery Manual

The time it takes to fully recharge a battery depends upon:

  • Depth of discharge
  • Temperature
  • Size and efficiency of charger
  • Age and condition of battery

Measuring State of Charge in Lithium Batteries

As difficult as it is to measure State of Charge (SoC) with lead acid batteries, it is even more so with lithium. This is because lithium batteries have a very narrow voltage window. They are charged until they suddenly aren’t. A rough SoC chart shows the voltage window. Actual voltage varies by manufacturer.

A more accurate, and recommended method for SoC of lithium batteries is with an amp hour (Ah) meter counting amps in and out of the battery. Some lithium batteries have a counter built into their Battery Management System (BMS). If you have remote access to the BMS, either through Bluetooth or Wifi, that is a better way of measuring SoC over voltage. Affordable external battery monitors with a shunt are also available for accurate SoC measurements, like the Victron Energy Smart Battery Monitor.