What is SoC in electric vehicles? All about state of charge

SoC stands for “State of Charge” and it refers to the charge level of the battery pack in an electric vehicle. Specifically:

State of Charge is expressed as a percentage that indicates how much capacity is left in the battery relative to its maximum capacity when fully charged.
For example, when a EV battery has a 50% SoC, it means there is still 50% of the total battery capacity available to use. When the SoC reaches 0%, the battery is fully discharged.
The SoC changes during driving as the battery provides electric energy to power the vehicle. It increases when the battery is charging and decreases when discharging to propel the vehicle.
Knowing the SoC is important for EV drivers to understand how much driving range they have left before needing to recharge, as well as for ensuring they don’t fully discharge the battery which can damage battery health over time.
The vehicle’s battery management system monitors SoC by integrating the inflow and outflow of current over time while factoring in other parameters like temperature. It uses this data to give an accurate SoC estimate to the driver.

SoC in a nutshell

SoC stands for State of Charge, which is the amount of stored energy in the battery. For example, if you have an 80% charge, you have an 80% SoC.

SoC is not the total potential capacity of the battery; it merely signifies the energy in the battery relative to its capacity. A 20% SoC means the battery is 20% charged and a 0% SoC means the vehicle is fully discharged.

We measure SoC in all batteries as a percentage. If you have an iPhone or Android phone, you can set it to display the percentage. Your electric vehicle will also display your remaining battery as a percentage and calculate your range.

Battery health, charge times and SoC

When charging your electric vehicle, your charger – whether it is a public charger or home charger – will refer to the SoC with estimated charge times.

Most EV manufacturers recommended charging an electric car to 80% SoC and recharging when it drops to around 20%.

In our article about how long EV batteries last, we revealed that a 20-80% charge demonstrates the best capacity maintenance over a battery’s lifespan. This is based on a study of 6,000 EV batteries by GEOTAB .

The reason a 20-80% charge is better for the battery versus a 5-80% or 0-100% charge is that it produces less heat and reduces the charge cycle time.

When charging in public, you will also notice charge times drop off a cliff after 80% – this is because DC rapid chargers cut the power output at 80% to a much slower rate to protect the charger from heat.

For example, Car Magazine found that the Polestar 2 needs 36 minutes to charge from 10 to 80% at 150kW, but the last 20% takes another 87 minutes.

Why SoC is important

The SoC level is critical information for electric vehicle drivers when on the road and planning travel. Here are some key implications of SoC that EV drivers should keep in mind:

Range – The SoC indicates the approximate driving range left available before needing to recharge. A higher SoC means more miles of range. Monitoring SoC helps drivers understand if they can make it to their destination or next charger.
Route Planning – Planning travel routes and charging stops relies heavily on knowing the SoC and range. Drivers need to locate charging stations within reach along their route based on the SoC. Running out of charge mid-trip is a risk.
Charging Time – At charging stops, the SoC informs drivers how long they may need to charge. Low SoC may require 30-60 minutes at fast chargers to replenish capacity and range. High SoC may only need 15-20 minutes for a quick boost.
Battery Health – Fully draining the battery too often can degrade battery health over time. Keeping some buffer SoC like 20% while driving helps prolong battery life. Occasional full discharges are ok.
Drive Mode/Power – Some EVs may limit acceleration power at very low SoC levels to preserve range. Performance returns to normal after charging to higher SoC.