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 crucial lever for TCO reduction. The State of Charge (SOC), the battery's level of charge, is the central metric in this regard. It not only determines the remaining range, but also influences the battery's service life and the efficiency of the charging process. A sound understanding of the SOC makes it possible to maximize the operational readiness of your fleet, lower operating costs, and meet the strict requirements of the Clean Vehicles Directive (CVD). This article shows how you can use the 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 the lifespan of a vehicle battery by several years. Permanently charging to 100% or regular deep discharging below 10% SOC stresses the cell chemistry and accelerates aging. Experts recommend keeping the battery within a charge corridor of 20% to 80% during daily operation. This approach can significantly increase cycle stability. Charging from 20% to 80% is also significantly faster than the last 20% to 100%. Observe these best practices for your fleet:
Optimize daily charge swing: Charge vehicles overnight via AC depot charging (22 kW) only up to 80% if the full range is not required the following day.
Avoid deep discharge: Plan routes so that vehicles return to the depot with at least 15-20% SOC.
Manage long idle times: Park vehicles with an SOC of 50-60% if they are not moved for more than a week.
Use full charge selectively: Only charge to 100% when a long-distance trip (such as the over 300 km of the HEERO Mittelniederflur Bus) is pending.
By adhering to these rules, you reduce the capacity loss of your batteries to less than 2% per year. Learn more about gentle charging processes in our blog. Intelligent charging is a central component 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.
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.
More useful links
NOW GmbH offers a factsheet that compares different drive types for passenger cars and highlights electromobility.
Fraunhofer ISI publishes a cost analysis comparing electric cars and combustion engine vehicles.
NOW GmbH provides a guide to action for introducing electromobility into vehicle fleets.
Bundesamt für Logistik und Mobilität (BALM) provides information on the Climate Protection and Sustainability (KSNI) funding program.
Umweltbundesamt offers an analysis of the environmental impact of motor vehicles.
Bundesministerium für Wirtschaft und Klimaschutz provides a dossier on the topic of electromobility.
Statistisches Bundesamt (Destatis) publishes a press release that could contain current figures on electromobility.
Verband der Automobilindustrie (VDA) offers comprehensive information on the topic of electromobility.
Fraunhofer ISI provides further information on the topic of electromobility.
