Battery State of Charge Explained

‍Battery State of Charge Explained and How It Is Measured

Battery state of charge is the estimated amount of charge remaining in a battery, expressed as a percentage of a defined full-charge capacity. Usually shortened to SOC, it is the battery equivalent of a fuel gauge.

A reading of 80% means the battery is relatively full. A reading of 20% means much less charge remains. However, neither figure tells you exactly how many hours the battery can run equipment.

Runtime also depends on battery capacity, electrical demand, operating limits and losses. Two batteries showing 80% can therefore provide very different amounts of useful energy.

For businesses using battery storage, understanding this percentage helps explain what the system can do now and what it may need to recharge before doing later.

What does battery state of charge actually tell you?

SOC answers a specific question: how much charge remains compared with the capacity used as the reference?

A simple expression is:

SOC (%) = remaining charge (Ah) ÷ reference full-charge capacity (Ah) × 100

For example, a battery with 60 ampere-hours remaining against a 100 ampere-hour reference has an SOC of 60%.

The reference matters. Some systems use a rated capacity, while others estimate a changing full-charge capacity as the battery ages. Check the monitoring system's documentation before comparing readings from different products.

Analog Devices' guide to SOC estimation explains that charge estimation depends on battery characteristics and operating conditions. The percentage is a calculated estimate, rather than a direct measurement of a visible fuel level.

State of charge vs state of health vs depth of discharge

These terms describe different aspects of a battery. Using them interchangeably can lead to incorrect assumptions about performance.

Term	What it describes	Example
State of charge (SOC)	How full the battery is against its reference capacity	70% SOC indicates a partially charged battery
State of health (SOH)	How the battery's condition compares with a defined new-battery reference	Capacity-based SOH may show how much capacity remains after ageing
Depth of discharge (DOD)	How much charge has been removed relative to a defined capacity	Discharging from full to 30% SOC gives 70% DOD on the same basis

‍

For a discharge measured from full, DOD = 100% − SOC, provided both percentages use the same capacity reference.

An older battery can show 100% SOC while holding less charge than it did when new. It is full relative to its present estimated capacity, but ageing may have reduced that capacity.

SOH is also not always a capacity-only measure. Depending on the system, it can reflect changes in resistance or the ability to deliver power. Read the manufacturer's definition alongside the percentage.

How is battery state of charge measured?

Battery monitors estimate SOC from measurements such as current, voltage, and temperature. Three common approaches are coulomb counting, voltage-based estimation, and battery models.

Coulomb counting

Coulomb counting tracks charge flowing into and out of the battery over time. Think of it as keeping a running balance.

If a battery supplies 10 amps for one hour, it delivers 10 ampere-hours. Against a fixed 100 ampere-hour reference, that represents a ten-percentage-point SOC reduction in a simplified calculation.

The method needs a reliable starting point and accurate current measurements. Small errors can accumulate, so the estimate may need correction.

Research on errors in coulomb counting identifies current measurement, capacity uncertainty, numerical integration and timing as potential error sources.

Voltage-based estimation

A monitor can compare battery voltage with a known relationship between voltage and SOC. Open-circuit voltage means the battery voltage when no external current is flowing, after suitable settling.

A reading taken while equipment is drawing power is different. Load and recent charging can affect terminal voltage, so a quick meter reading may not reliably indicate charge level.

Texas Instruments' SOC estimation guidance discusses the limits of voltage-based and coulomb-counting approaches.

Model-based estimation

More advanced systems combine measurements with a mathematical battery model. The model helps interpret behaviour during charging, discharging and changing operating conditions.

Methods such as Kalman filtering update the estimate as new measurements arrive. Their accuracy still depends on suitable battery data, sensors and configuration.

Why lithium battery voltage charts can be misleading

A battery voltage chart is only useful when it matches the battery chemistry, cell configuration and measurement conditions.

Lithium iron phosphate, or LFP, is a lithium-ion chemistry with a relatively flat voltage profile across much of its operating range. A small voltage difference can correspond to a substantial SOC difference.

Texas Instruments' LFP design guide illustrates how the voltage-to-SOC relationship differs between chemistries. That is why a generic “48 V battery chart” cannot reliably describe every 48 V battery system.

Voltage remains useful information. It simply needs context. A monitoring system that combines current tracking, voltage and battery-specific data can provide a more useful estimate than a generic chart alone.

How SOC relates to battery energy and runtime

SOC is a percentage. Energy capacity, commonly expressed in kilowatt-hours, tells you the scale of the storage system. Power, measured in kilowatts, tells you how quickly energy is being delivered.

Consider a hypothetical battery with:

A 200 kWh energy reference corresponding to its displayed 0–100% range.
A starting SOC of 80%.
An operating reserve of 20% SOC.
A steady 50 kW load.

For this simplified example, assume stored energy changes in direct proportion to SOC and ignore losses.

Calculation	Result
Available SOC window	80% − 20% = 60 percentage points
Energy above the reserve	200 kWh × 0.60 = 120 kWh
Ideal runtime at 50 kW	120 kWh ÷ 50 kW = 2.4 hours

‍

Actual runtime would also depend on conversion losses, auxiliary equipment, temperature, battery condition and changing demand. SOC is usually charge-based, so treating it as a perfectly linear energy gauge is an approximation.

The battery and inverter must also be able to deliver 50 kW. Having enough stored energy does not automatically mean having enough power capacity.

These figures explain the calculation only. They are not CleanDesign product specifications or a recommended reserve setting.

Why can a battery SOC reading change unexpectedly?

Temperature and electrical load

Cold conditions and high discharge current can reduce the charge a battery can deliver before reaching its operating voltage limit. That can change available runtime even when the starting SOC looks familiar.

Analog Devices' battery fuel-gauge explanation describes how temperature, current and voltage behaviour affect charge assessment.

The practical point is to read SOC alongside temperature and load, rather than assuming the same percentage always means the same operating time.

Capacity settings and measurement errors

An incorrect capacity setting changes the basis of the calculation. Missing current measurements can also distort the running balance.

Victron's battery-monitor troubleshooting guide identifies incorrect settings, synchronisation issues and unmeasured current as causes of inaccurate SOC readings.

Recalibration or synchronisation

A monitor may correct its estimate when the battery reaches a recognised reference condition. The displayed percentage can then change more quickly than expected.

That does not necessarily mean the battery suddenly gained or lost physical charge. The estimate may have been corrected. Follow the equipment's documented procedure rather than manually forcing a reassuring percentage onto the display.

What is a good state of charge for a battery?

There is no single best SOC range for every battery installation.

The appropriate limits depend on chemistry, manufacturer instructions, warranty conditions and the job the battery performs. A system holding energy for an outage may use a different operating plan from one that charges and discharges throughout the day.

Battery ageing also depends on more than cycle count. National laboratory research on lithium-ion battery lifetime examines the effects of temperature, SOC and depth of discharge on degradation.

This means a general rule such as “always stay between 20% and 80%” should not replace the operating instructions for a specific system. Some products also include internal buffers that differ from the percentages shown to users.

A useful operating plan defines the normal charge window, reserve requirements and conditions that change charging or discharge limits.

How SOC supports microgrid energy management

In a microgrid, the controller needs to know how much battery capacity is available before deciding how to use it.

SOC information can help coordinate charging from available generation, preserve a reserve and determine when other resources may be needed. However, the controller must also account for battery power limits and system conditions.

The U.S. Department of Energy's battery energy storage report identifies SOC monitoring as part of storage-system control.

CleanDesign's microgrid support page explains the wider role of coordinating batteries, generators and renewable sources. SOC is one input to those decisions, alongside the demand the system must serve.

The battery management system and site energy management system also have different roles. Battery-level monitoring and protection support operation within battery limits. The site controller coordinates the wider energy resources. CleanDesign's hybrid energy management systems provide context for that broader coordination.