Battery State of Charge using a Voltmeter

Voltage Measurements of State of Charge

Battery SOC Multimeter

Most of the lead acid batteries you will buy these days are sealed and are often described as being maintenance free.  As such, it is impossible to use a hydrometer to test for state of charge (SOC).  As a result, it is very common to determine the state of charge in these batteries by using a voltage measurement.

This is a very simple measurement, and though a bit inexact, can be a very useful way of identifying the obviously bad batteries from the likely good batteries.
Below is a table that shows you the state of charge based on voltage.  This chart is what I would consider to be accurate enough when the temperature is close to about 68 degree F (20 deg C).

Digital Voltmeter
Reading
State of
Charge
12.65 Vdc100 Percent
12.45 Vdc75 Percent
12.24 Vdc50 Percent
12.06 Vdc25 Percent
11.89 VdcDischarged

Limitations

Voltage measurements on to measure SOC on a lead acid battery are useful, but care should be taken to ensure that the results are interpreted correctly.  There are two key things to keep in mind about using voltage to measure state of charge.

First, the battery open circuit voltage is very temperature sensitive.  The smart thing to do when making this type of measurement is to use a chart that helps you to interpret the results based upon the temperature at the time of measurement.

Second, batteries are subject to something known as a surface charge. This surface charge is not representative of the true battery capacity.  Instead its representative of an uneven distribution of electrons on the plate material.   With a brief discharge, this surface charge will disappear.

Best Multimeter SOC Measurement Practices

  • Measure open circuit voltage.  In other words, make sure everything is disconnected.
  • Let the battery settle for at least four hours.  Doing so allows the battery chemistry to establish an equilibrium.
  • Don’t shake or disturb the battery.  Agitation of the electrolyte can give you erroneous readings.
  • Apply a brief discharge to avoid erroneous readings caused by what is known as a surface charge.