How Much Electricity is Needed for a Roadside Assistance Rush? How Do Roadside Assistance Companies Calculate the Capacity and Power of the Door Energy Mobile EV Charger?

When an electric vehicle runs out of power and stops on a highway, suburban road, mining area, or industrial park, the first thing a roadside assistance company needs to solve is not "how to fully charge the vehicle," but rather "how much power to replenish so the vehicle can safely leave the scene."

There is a significant difference between the two.

Traditional fixed charging stations rely on a continuous grid power supply and are mainly used for daily power replenishment. Door Energy's product, however, is a mobile integrated energy storage and charging device: it first stores electrical energy in its internal energy storage system, and then outputs it via DC to provide emergency power to EVs with low power upon arrival at the scene; if necessary, it can also output AC power to loads such as electric excavators, water pumps, and lighting.

Therefore, the Mobile EV Charger searched by overseas customers, in Door Energy's product logic, is not an ordinary charging station, nor a small charger carried in the vehicle, but a mobile energy storage and power output system suitable for roadside assistance, large vehicles, and outdoor industrial scenarios.

According to data from the International Energy Agency, global electric vehicle sales exceeded 17 million units in 2024, representing a year-on-year increase of over 25% and accounting for more than 20% of global new car sales. With the increasing number of EVs on the road, issues such as power shortages on the roads, outages of fixed charging facilities, and lack of grid access in remote areas will drive the growth in demand for mobile charging and roadside assistance.

So, how much electricity is needed for a roadside assistance operation? And how should roadside assistance companies calculate the energy storage capacity and output power of a mobile EV charger?

I. Why doesn't roadside assistance require charging the EV to 100%?

1. The goal of roadside assistance is to restore safe mobility

In roadside assistance scenarios, the primary goal of on-site charging is usually to get the vehicle:

* Move off the highway or dangerous lane;

* Reach the nearest public charging station;

* Return to the fleet base or repair center;

* Enter a safe parking area to await further processing.

Assuming the disabled vehicle is only 20 kilometers from the nearest available charging station, adding 30 to 40 kilometers of safe range is usually sufficient to complete the assistance. Continuing to recharge the battery to 80% or 100% on-site will actually prolong equipment downtime and reduce the number of tasks the Mobile EV Charger can complete that day.

The U.S. Department of Energy's Alternative Fuels Data Center indicates that, under suitable conditions, DC Fast Charging can add approximately 100 to 200 miles of range in 30 minutes. However, actual charging results are affected by vehicle battery capacity, current state of charge (SOC), vehicle charging capability, temperature, and equipment output power.

Therefore, roadside assistance companies should first define a "minimum safe range" before calculating the amount of electricity needed to be delivered.

The above data is within the operational planning range and is not fixed parameter for specific vehicle models. The rescue company still needs to adjust according to the actual energy consumption of the vehicle and road conditions.

2. More Charge Doesn't Guarantee Higher Rescue Efficiency

If a Door Energy mobile charging station can provide 160kWh of effective power during the current mission cycle, replenishing an average of 40kWh per vehicle, theoretically only 4 rescues can be completed. If each vehicle receives an average replenishment of 16kWh, it can cover approximately 10 missions.

Furthermore, electric vehicles typically accept higher charging power at low SOC levels. As SOC increases, the vehicle's BMS may actively reduce the charging current. Therefore, the last 20% charging time can significantly increase.

Therefore, a more suitable principle for roadside assistance is:

> Calculate enough power for safe vehicle transfer, rather than assuming every vehicle is fully charged.

II. What Data Needs to be Collected to Calculate Rescue Power?

1. Five Essential Variables

The amount of kWh needed for a rescue cannot be determined solely by the battery pack capacity of the disabled vehicle. Dispatchers need to collect at least the following five types of information.

If the customer cannot provide an accurate SOC, the assistance company can use the "target distance method"; if the vehicle's battery capacity and current SOC can be confirmed, the "SOC Variation method" can be used.

2. A vehicle's inability to start doesn't necessarily mean the battery is depleted.

Dispatchers also need to differentiate between the following:

* Depleted high-voltage battery;

* Faulty 12V low-voltage battery;

* Faulty vehicle charging system;

* Battery temperature too high or too low;

* Collision, water ingress, or damage to the high-voltage system;

* Software or vehicle control system malfunction.

If it's just a 12V low-voltage battery fault, directly using a large mobile charging device to replenish the battery may not solve the problem. Conversely, if the vehicle has experienced a severe collision, has damage to the bottom battery pack, is smoking, has an unusual odor, or is abnormally hot, it should not be directly connected to a charging device.

EV rescue data released by the US fire department indicates that electric vehicles have both high-voltage and low-voltage electrical systems, and damaged lithium-ion batteries may also experience thermal runaway. Therefore, vehicle identification, traffic isolation, and high-voltage safety checks must be completed before on-site charging.

III. How to calculate the amount of electricity the vehicle needs to replenish?

1. Method 1: Calculation Based on Target Travel Distance

Suitable for scenarios where the State of Charge (SOC) cannot be accurately read, but vehicle energy consumption and target location can be confirmed.

The calculation formula is:

Vehicle-Required Electricity (kWh) = Target Distance (km) × Actual Vehicle Energy Consumption (kWh/km) × Operating Condition Factor × Safety Factor

The following ranges can be used for preliminary estimation for different vehicles.

These data are applicable to initial equipment planning, not to guarantee range for a specific vehicle model. Large vehicles vary significantly in load, air resistance, gradient, and speed; therefore, actual operational data should be prioritized.

2. Method Two: Calculation Based on Target SOC

If the vehicle's available battery capacity is 75kWh, the current SOC is 5%, and the rescue goal is to replenish the vehicle to 25%, then the theoretical demand on the vehicle side is:

75kWh × (25% – 5%) = 15kWh

This 15kWh represents the energy that needs to enter the vehicle's battery, not the actual energy consumed by the Door Energy storage system. This is because there are power conversion, cabling, temperature control, and auxiliary system losses between the device's internal battery and the vehicle's power battery.

3. Examples of Four Typical Roadside Assistance Missions

Therefore, a roadside assistance mission for a passenger vehicle may only require replenishing 6 to 20kWh, while a large electric vehicle may require more than 70kWh. If equipment is configured solely based on the "average demand of passenger vehicles," it may not be sufficient to cover large vehicle operations.

IV. How to convert vehicle-side power consumption into energy storage capacity?

1. If a vehicle receives 20kWh, the equipment consumption will be higher than 20kWh.

The Door Energy Mobile EV Charger performs the energy storage system discharge process on-site. Energy needs to pass through the internal battery, DC/DC module, charging control system, cables, and vehicle battery; therefore, the equipment-side consumption is typically higher than the final power received by the vehicle.

The following formula can be used:

Equipment battery consumption = Vehicle-side power delivery ÷ DC output overall efficiency + Auxiliary system power consumption

Assumptions:

* The vehicle needs to receive 20kWh;

* The DC output overall efficiency is estimated at 92%;

* Communication, cooling, and control system power consumption is estimated at 0.5kWh.

Therefore:

20 ÷ 0.92 + 0.5 ≈ 22.2kWh

That is, when the vehicle receives 20kWh, the equipment's internal energy storage system will consume approximately 22.2kWh.

The 92% and 0.5kWh figures here are just calculation examples. Actual efficiency and auxiliary power consumption vary depending on the power, temperature, and operating time of different devices. Formal calculations should be based on Door Energy's specific configuration and test data.

2. Rated capacity is not equal to total dispatchable power

Mobile energy storage devices typically do not operate within the absolute 0% to 100% battery range for extended periods. The BMS reserves certain upper and lower limits to protect the battery, maintain safe operation, and reserve necessary power for the control system.

Dispatchable power can be expressed by the following formula:

Dispatchable power = Rated energy storage capacity × Available SOC ratio

Assuming a device has a rated energy storage capacity of 200kWh and an operating SOC window of 5% to 95%, then the available SOC ratio is 90%:

200kWh × 90% = 180kWh

After deducting DC output losses and auxiliary power consumption, the final amount of power delivered to the vehicle will further decrease.

This table does not account for auxiliary costs such as temperature control, standby, and communication; it is only used to illustrate the difference between "rated capacity - dispatchable capacity - vehicle-side delivery".

3. Temperature Changes Demand on Both Sides of Equipment and Vehicle

A study by the U.S. Department of Energy shows that compared to an environment of approximately 22°C, test vehicles experienced an average range reduction of 41% at approximately −7°C; at approximately −18°C, the average range reduction was close to 50%. Results will vary depending on vehicle model, thermal management system, driving conditions, and in-vehicle temperature settings, but the impact of extreme cold on rescue power calculations should not be ignored.

If the "range reduction percentage" is known, the correction formula should be: Low Temperature Energy Consumption Coefficient = 1 ÷ (1 - Range Reduction Percentage)

For example, when the range decreases by 20%, the energy consumption correction coefficient is approximately 1.25, not simply an increase of 20%.

Therefore, roadside assistance companies in cold regions should set up separate capacity models based on winter data, rather than using the same set of parameters year-round.

V. How should output power be calculated instead of blindly pursuing 420kW?

1. Capacity and Power Address Two Different Issues

For mobile energy storage and charging equipment:

* kWh determines how much electricity can be stored and how many missions can be completed;

* kW determines how quickly the energy can be delivered to vehicles or other loads.

A device with a large energy storage capacity will still have a long single rescue time if its DC output power is low. Conversely, a device with high output power but insufficient energy storage capacity may only be able to complete one or two missions quickly.

The theoretical charging time is:

Charging Time = Vehicle-Required Electricity ÷ Actual Average Output Power

These are ideal mathematical results. Actual tasks will require additional time for vehicle identification, on-site inspection, equipment deployment, communication handshake, cable connection and disconnection.

2. The maximum 420kW refers to the device's output capacity, not the fixed power level of the charging station.

Door Energy mobile charging devices can achieve a maximum DC output power of 420kW, suitable for roadside assistance, large vehicle, and fleet tasks requiring high charging speeds.

However, 420kW represents the device's maximum output capacity and does not mean that all vehicles will be charged at 420kW. The actual charging power depends on the lowest of the following:

* The power currently available from the charging device;

* The vehicle's maximum DC power capacity;

* The vehicle's current State of Charge (SOC);

* The vehicle's battery temperature;

* The allowable power of the CCS interface, cables, and charging gun;

* The BMS's real-time power limit;

* The device's remaining charge and thermal management status.

For example, if the device has a maximum output capacity of 420kW, but the vehicle can only currently accept 100kW, then the actual power will not exceed 100kW.

Similarly, a vehicle may briefly approach peak power at low SOC levels, and then gradually reduce power as the SOC increases. Therefore, rescue time should be calculated based on average power, not simply by dividing by peak power.

3. Large Vehicles Require Simultaneous Attention to Capacity and Continuous Power

Passenger cars have relatively limited power requirements for short-distance outages, while large electric trucks, construction vehicles, and commercial vehicles have higher energy consumption per unit mile.

If a large vehicle needs to replenish 100kWh:

* Based on an average power of 100kW, the theoretical time is approximately 60 minutes;

* Based on an average power of 200kW, the theoretical time is approximately 30 minutes;

* Even if the equipment can output up to 420kW, if the vehicle can only receive an average of 150kW, the theoretical time is still approximately 40 minutes.

Therefore, large vehicle rescue cannot only consider the equipment's peak power. Roadside assistance companies also need to confirm continuous output capacity, thermal management capabilities, deliverable capacity per charge, and remaining SOC after the mission.

VI. How to Calculate Daily Mission Capacity and Recharge Turnover?

1. Plan Capacity Using "Peak Days" Instead of "Average Days"

Roadside assistance companies should consider the following when selecting equipment:

* How many missions are expected to be performed per day;

* How much kWh is delivered per mission on average;

* The ratio of passenger cars to large vehicles;

* Whether the equipment can be recharged between missions;

* Whether a minimum amount of power needs to be reserved for the next emergency mission.

Daily capacity can be calculated using the following formula:

Rated Capacity ≥ [Daily Vehicle Deliveries ÷ Output Efficiency + Daily Auxiliary Power Consumption] × Reserve Factor ÷ Available SOC Ratio

Assuming a roadside assistance company performs 6 missions per day, delivering an average of 20 kWh to vehicles per mission:

* Daily Vehicle Deliveries: 120 kWh;

* Overall DC Output Efficiency: 92%;

* Auxiliary Power Consumption per Mission: 0.5 kWh;

* Peak Reserve Factor: 1.15;

* Available SOC Ratio: 90%.

The calculated result is approximately: [120 ÷ 0.92 + 3] × 1.15 ÷ 0.90 ≈ 170.5 kWh

Under the condition that mid-mission recharging is not possible, this mission structure requires a minimum rated energy storage capacity of approximately 171 kWh. Actual selection should also consider product configuration, ambient temperature, and capacity degradation margin.

The capacities in the table are planned ranges calculated based on uniform assumptions and do not represent Door Energy's fixed product models or rated parameters.

2. Self-Recharge of the Device Must Be Calculated Separately

Door Energy's energy storage and charging products have two main recharge paths:

* Connecting to a compatible DC charging station: Under matched input conditions, the device can be recharged from 0% to 100% in approximately 1 hour;

* Connecting to a compatible AC distribution box: Under matched input conditions, the device can be fully recharged in approximately 2 hours.

The "approximately 1 hour" and "approximately 2 hours" here refer to recharging the mobile energy storage device itself, not fully charging the vehicle being rescued.

Actual recharge time depends on:

* Device energy storage capacity;

* Rated power at the input terminal;

* Current State of Charge (SOC);

* Battery temperature;

* Input conversion efficiency;

* Whether the power supply can continuously provide rated power;

* Power control by the BMS during high SOC phases.

The following is the theoretical average input power calculated based on 95% input efficiency.

Therefore, when planning equipment, customers should also check whether the base or service station has the corresponding input power conditions. The larger the energy storage capacity, the longer the time required for the equipment to recover to full power will be if the input power does not increase accordingly.

3. Establish a Minimum Dispatch SOC

If the dispatch center still dispatches a large electric truck when the equipment has only 20kWh remaining, the rescue mission is likely to fail.

It is recommended to set a minimum dispatch SOC based on the mission level.

Through OCPP, operators can view equipment status, charging process, and task data. Based on this, the dispatch system can decide whether to dispatch a vehicle based on remaining SOC, task distance, and vehicle type.

VII. FAQ: Common Issues with Mobile Storage and Charging Roadside Assistance

Q1: How much battery power is typically needed for one EV roadside assistance?

A1: For short-distance passenger vehicle assistance, a starting plan of 6 to 20kWh is usually sufficient; SUVs and light commercial vehicles may require 20 to 50kWh; large electric vehicles may require 50 to 150kWh or more. The final result should be calculated based on the target distance, vehicle energy consumption, weather, and safety margin.

Q2: Is the Door Energy Mobile EV Charger a regular charging station?

A2: The Door Energy product mentioned in this article is not a fixed charging station, but a mobile integrated energy storage and charging device. The device first stores electrical energy internally, then replenishes the EV's power on-site via DC output. It can also power some engineering and emergency loads via AC output.

Q3: Is it necessary to charge the vehicle to 100% during roadside assistance?

A3: Usually not. The main goal of roadside assistance is to get the vehicle to the nearest fixed charging station, service area, or fleet base. Replenishing only the power needed for safe transfer can shorten on-site time and increase daily workload.

Q4: Does a maximum of 420kW mean the vehicle can be charged at 420kW continuously?

A4: No. 420kW is the maximum DC output capability of the Door Energy device. The actual power is determined by the vehicle's charging capacity, SOC, battery temperature, interface, cables, and BMS, and the peak power usually cannot be maintained throughout the entire charging process.

Q5: Why can't the rated capacity of the equipment be directly equated to the rescue power?

A5: The equipment needs to maintain a safe SOC window, and there are also power consumption issues related to DC conversion, temperature control, communication, and control systems. Therefore, a rated capacity of 100kWh usually does not mean that 100kWh can be fully delivered to a vehicle.

Q6: How should I choose between CCS1 and CCS2?

A6: CCS1 is mainly aimed at the North American market, while CCS2 is widely used in Europe and other markets that adopt this standard. Roadside assistance companies should determine the configuration based on the service country, local vehicle population, and interface standards.

Q7: Can the equipment recharge multiple vehicles simultaneously?

A7: This depends on the specific output interface and system configuration. Even with multiple interfaces, the total output power, power per interface, remaining equipment capacity, and power distribution strategy among multiple vehicles must be calculated.

Q8: How much additional capacity is needed in low-temperature environments?

A8: Generally, a 15% to 35% increase in planned capacity is recommended for low temperatures; even higher capacity may be needed in extremely cold conditions. However, the thermal management capabilities of different vehicles vary significantly, and rescue companies are advised to establish their own correction factors using local winter mission data.

Q9: How long does it take for the equipment to fully charge?

A9: With the input power supply and equipment configuration matched, it takes approximately 1 hour to charge from 0% to 100% using a DC charging station; using a compatible AC distribution box, the estimated time is approximately 2 hours. Actual time depends on capacity, input power, temperature, and SOC.

Q10: Besides EV roadside assistance, what other scenarios can the equipment be used for?

A10: With the appropriate configuration, the Door Energy charging and storage device can also power loads such as electric excavators, water pumps, and lighting via AC output, suitable for construction sites, outdoor industry, emergency response, and temporary areas without power grid.

Conclusion

When roadside assistance companies choose a Mobile EV Charger, the real calculation they need to make is not "whether it can fully charge a car," but whether the energy storage device can deliver enough safe driving power to the target vehicle within the specified time and continuously complete the expected task throughout the day.

Capacity determines how many rescues can be completed, and power determines how long each rescue requires. In addition, available SOC window, DC output efficiency, auxiliary power consumption, vehicle charging capability, low temperature effects, and the device's own charging conditions will also change the final selection result.

For short-distance rescue of passenger vehicles, a single mission may only require 10 to 20 kWh; for large electric vehicles, remote roads, or high-load scenarios, the demand for a single mission may reach 50 to 150 kWh or more. Therefore, roadside assistance companies should establish capacity models based on actual vehicle types, peak daily workload, and charging windows.

Door Energy's mobile integrated energy storage and charging equipment supports up to 420kW DC output and can be configured with OCPP, CCS1, or CCS2. Meanwhile, its modular design facilitates fault location, module replacement, and long-term maintenance. For operators needing to cover roadside assistance, large vehicles, and outdoor industrial power needs, the value of this type of equipment lies not only in providing charging interfaces but, more importantly, in flexibly bringing stored energy to areas where the fixed power grid cannot provide timely coverage.