SOC State of Charge: Optimise fleet range and reduce TCO
State of Charge (SOC) is the most important key figure for operating electric commercial vehicles. An inaccurate SOC display leads to range anxiety and inefficient dispatch planning.
The topic briefly and concisely
State of Charge (SOC) is the most important key metric for planning range and charging operations in e-fleets.
A charging window between 20% and 80% SOC maximises battery life and reduces total cost of ownership (TCO).
The accuracy of the SOC depends on the temperature, the age of the battery and an intelligent battery management system (BMS).
For fleet managers, the precise planning of routes and charging times is a decisive lever for reducing TCO. State of Charge (SOC), the battery’s charge level, is the key metric here. It determines not only the remaining range, but also influences battery service life and the efficiency of the charging process. A solid understanding of SOC makes it possible to maximise the operational readiness of your fleet, reduce operating costs and meet the stringent requirements of the Clean Vehicles Directive (CVD). This article shows how you can use SOC as a strategic tool for your fleet management.
State of Charge (SOC): The key metric for e-fleets
The State of Charge (SOC) indicates the current charge level of a vehicle battery in per cent. A value of 100% means a fully charged battery, 0% an empty one. For fleet operators, SOC is the basis for daily deployment planning and route optimisation. A precise SOC display, as ensured in HEERO vehicles by a modern battery management system (BMS), is essential. It prevents unplanned vehicle downtime and significantly increases efficiency. A reliable SOC value is the basis for meeting delivery time windows. The display takes into account only the usable net capacity of the battery approved by the manufacturer in order to protect the cells. This buffer can account for part of the gross capacity. This helps ensure a long service life for the 137 kWh battery in the HEERO eDrive system. A precise understanding of SOC is therefore the first step towards optimising your operations.
Measurement methods and accuracy: What influences your SOC value
The accuracy of the SOC value depends on complex measurement methods and external factors. Modern battery management systems (BMS) usually use a combination of current measurement (coulomb counting) and voltage measurement. Coulomb counting is precise, but requires regular calibration, for example through a full charge to 100%. External conditions have a significant influence on accuracy. Even a 10 °C deviation in outside temperature can have a considerable impact on range. An inaccurate SOC display of just 5% can already lead to considerable range anxiety. The following factors are crucial for precision:
Temperature: Low temperatures below 0 °C reduce the available energy and distort voltage measurement.
Battery ageing (State of Health): As age increases, total capacity declines, which the BMS must take into account. (More on State of Health here.)
Load profile: Strong acceleration and high speeds lead to voltage drops and make SOC calculation more difficult.
Calibration: Without regular charging cycles from 0% to 100%, small measurement errors can accumulate over weeks.
A high-quality BMS, as developed in-house by HEERO and installed in all electrified Sprinter models, minimises these deviations through intelligent algorithms. This ensures a reliable basis for planning your logistics processes.
SOC management to maximise battery service life
Strategic management of the state of charge can extend a vehicle battery's service life by several years. Constant charging to 100% or regularly deep-discharging below 10% SOC stresses the cell chemistry and accelerates ageing. Experts recommend keeping the battery in daily operation within a charging window of 20% to 80%. This approach can significantly increase cycle life. Charging from 20% to 80% is also considerably faster than the final 20% to 100%. Please observe these best practices for your fleet:
Optimise daily charging volume: Charge vehicles overnight via AC depot charging (22 kW) only up to 80% if the full range is not needed the following day.
Avoid deep discharge: Plan routes so that vehicles return to the depot with at least 15-20% SOC.
Manage long idle periods: Park vehicles with a SOC of 50-60% if they will not be moved for more than a week.
Use full charging selectively: Only charge to 100% when a long-distance journey (such as the over 300 km range of the HEERO Mid-Door Low-Entry Bus) is due.
By adhering to these rules, you can reduce the capacity loss of your batteries to below 2% per year. Find out more about gentle charging processes on our blog. Intelligent charging is a key building block in reducing total cost of ownership (TCO).
Impact of SOC on charging performance and efficiency
The State of Charge directly influences the speed and efficiency of the charging process. The charging curve of an electric vehicle is not linear. At low SOC (below 50%), the battery can absorb the highest charging power. The HEERO eDrive system achieves up to 165 kW at a DC charging station. From a SOC of around 80%, the HEERO-BMS gradually reduces the charging power to protect the battery cells and ensure their service life. Charging from 80% to 100% can take just as long as charging from 20% to 80%. This knowledge is crucial for planning charging stops on long routes. For fleet operators, this means that short, fast charging sessions in the optimal SOC window are often more efficient than a full charge. A HEERO electric bus based on the Sprinter charges under ideal conditions in just 30-40 minutes from 20% to 80% SOC. The distinction between AC and DC charging plays a key role in route planning. Intelligent charging management that takes SOC into account can significantly reduce downtime.
Practical application: SOC as the basis for range estimation
The range indicator in your vehicle is a forecast that is based primarily on the current State of Charge. However, it is not a static calculation. Modern systems incorporate real-time consumption data, outside temperature and even topographical data into the calculation, in addition to the SOC. A SOC of 50% therefore does not necessarily mean that exactly 50% of the maximum range remains. In cold temperatures below 5 °C, the actual range can be 20-30% lower than at 20 °C. For fleet managers, it is crucial to understand this dynamic calculation and to factor in safety reserves. HEERO's telematics systems provide precise SOC data that enable realistic route planning. This allows you to ensure that your HEERO makes full use of its range, without your drivers having to make an unplanned stop at a charging station. A precise understanding of a battery's charge cycles helps to further refine the forecasts. Reliable SOC data are therefore the basis for maximum vehicle utilisation and compliance with your service level agreements.
More useful links
NOW GmbH provides a factsheet that compares various powertrain types for passenger cars and highlights electromobility.
Fraunhofer ISI publishes a cost analysis comparing electric cars and internal combustion engine vehicles.
NOW GmbH provides a practical guide for introducing electromobility into vehicle fleets.
The Federal Office for Logistics and Mobility (BALM) provides information on the funding programme Climate Protection and Sustainability in Inland Transport (KSNI).
The German Environment Agency offers an analysis of the environmental impact of motor vehicles.
The Federal Ministry for Economic Affairs and Climate Action provides a dossier on electromobility.
The Federal Statistical Office (Destatis) publishes a press release that could contain the latest figures on electromobility.
The German Association of the Automotive Industry (VDA) offers comprehensive information on electromobility.
Fraunhofer ISI provides further information on electromobility.
FAQ
What is the difference between SOC (State of Charge) and SOH (State of Health)?
The SOC (State of Charge) describes the current charge level in per cent, comparable to a fuel gauge. The SOH (State of Health), on the other hand, indicates the battery’s ageing condition and describes how much of its original maximum capacity is still usable. A new battery has 100% SOH.
Why does the SOC display sometimes fluctuate, especially in cold weather?
The SOC calculation is based on measuring voltage and current. In cold conditions, the battery voltage drops more sharply under load, which can suggest a lower state of charge to the battery management system (BMS). After a rest period or as the battery warms up, the value may correct itself slightly upwards again.
How often does the SOC value need to be calibrated?
Modern BMSs calibrate themselves to a large extent. However, it is recommended to fully charge the battery from a low state (approx. 10%) to 100% once every 1–2 months. This process helps the BMS maintain measurement accuracy and correctly assess the battery’s actual condition.
What happens if the SOC falls to 0%?
When the display reaches 0%, the energy available for driving has been used up. The vehicle switches off the drive system. However, the BMS maintains a small energy reserve (deep-discharge protection) to continue supplying the 12-volt systems for a short time and prevent harmful deep discharge of the main battery.
How does HEERO ensure an accurate SOC for its eDrive systems?
HEERO integrates a specially configured battery management system (BMS) into every electrified Sprinter in the 907 series. This system continuously monitors over 200 data points in order to calculate the SOC precisely and transmit it to the vehicle display as well as to fleet management tools.
Does the SOC affect regenerative braking performance?
Yes, significantly. At a high SOC, typically above 95%, the recuperation power is greatly reduced or completely deactivated. The battery can no longer accept any additional energy. Full recuperation is only available at a lower SOC, which also allows the battery to be charged. You cannot keep filling a full glass. This becomes particularly relevant when planning journeys involving climbs or descents.
