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state of charge (SOC)

Contributor(s): Kate Brush

The state of charge (SOC) is a measurement of the amount of energy available in a battery at a specific point in time expressed as a percentage. For example, the SOC reading for a computer might read 95% full or 10% full. The SOC provides the user with information of how much longer the battery can perform before it needs to be charged or replaced. Understanding the state of charge is important because understanding the remaining capacity of a batter can help make a control strategy.

Essentially, the SOC acts like a fuel gauge in a car. It informs the users of how much longer they can operate the device or machine before it runs out of energy and can no longer perform. In fact, SOC readers have replaced the fuel gauge in electric cars. Electronics with batteries utilize various methods to measure the SOC, such as measuring voltage, specific gravity, internal impedance and counting coulombs. Advancements made with artificial intelligence (AI) have also created new ways to estimate the state of charge.

Depth of discharge (DOD) is the opposite of SOC. This closely related term refers to measuring the amount charge that has been used up in the battery.

Methods for determining state of charge

As mentioned above, the primary methods for calculating state of charge are: measuring voltage, specific gravity or internal impedance and counting coulombs. These methods depend on measuring levels that change as the battery is charged or discharged.

In the voltage method, the higher the voltage, the fuller the battery is. A high voltage indicates that the energy in the battery is under a lot of pressure and, therefore, that the battery is full. A lower voltage indicates lower pressure due to excess space within the battery.

Measuring SOC with voltage is simple, but it can produce inaccurate results as temperature and cell materials affect the voltage. In order to produce an accurate reading with the voltage method, batteries must first rest in an open circuit state for four hours at the very least but most battery manufacturers recommend letting it rest for 24 hours. Therefore, the voltage method is unsuitable for measuring batteries that are actively and continuously being used.

Using specific gravity to determine SOC involves using a hydrometer to measure the relative density of liquids based on buoyancy. The hydrometer tracks changes in the weight of active chemicals within the battery as it discharges. As a battery is used, the amount of sulfuric acid -- an active electrolyte -- decreases, thus proportionately lessening the specific gravity of the battery.

Some modern lead acid batteries incorporate electronic sensors, instead of using hydrometers, which produce real time readings of specific gravity measurements and provide a fairly accurate state of charge. A major drawback to using specific gravity measurements to determine SOC is that the method can only be used with lead acid batteries as it will not be effective with other battery chemistries.

Since the active chemicals within the battery change composition when converting from one form to another during the charging and discharging processes, then internal impedance can be used to measure state of charge. Measuring internal impedance means measuring the amount of opposition presented by a circuit to a current whenever voltage is applied. This method is not a common choice for determining SOC because impedance is temperature dependent and it is difficult to measure with active cells.

Finally, coulomb counting can be used to determine state of charge by measuring the current flowing in and out of the battery. Ampere-second (As) is the unit of measurement used for both charging and discharging. Some electronic devices incorporate a tiny coulomb counter that measures the current used by the host device, adds it up over time and then compares this metric to the programmed battery capacity in order to determine the SOC. However, this method does not consider the efficiency of the battery and it is expensive and hard to find accurate current measurements.

Examples of state of charge determining methods

While the voltage method produces inaccuracies, it is the most commonly used method for producing state of charge measurements due to its simplicity. It is frequently used in golf cars, wheelchairs and scooters.

Professional portable devices, such as laptops and medical equipment, frequently rely on the coulomb counting method to produce SOC measurements.

The internal impedance method finds applications within automotive manufacturing. During testing and debugging, some batteries are discharged for longer times than others and require charging before transit. Measuring internal impedance is also utilized by load leveling systems that require a battery to continuously be under charge or discharge.

This was last updated in August 2019

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