There is no fixed number of EVs a Mobile Battery Energy Storage System (Mobile BESS) can charge at once. It depends on the number of charging ports, available power, and usable energy.
A six-port system can connect six vehicles, but whether all six can charge at the required rate depends on the available power and each EV's charging demand.
This guide shows how to estimate simultaneous EV charging capacity and size a mobile BESS for EV fleet charging.
Ports, Power, and Energy: The Three Limits Behind Simultaneous Charging
These three factors limit EV charging in different ways. Ports determine how many vehicles can connect, power determines how much charging load can be supported at once, and energy determines how long that load can be sustained.
| Factor | What It Determines | Example |
| Charging Ports | How many EVs can connect at the same time | 6 ports - up to 6 vehicles connected |
| Power (kW) | How many EVs can charge at the target rate | 1,200 kW ÷ 200 kW/EV → up to 6 EVs |
| Energy (kWh) | How long the charging load can be sustained | 800 kW × 1.5 h → about 1,200 kWh |
Port count alone does not determine how many EVs can charge at full power simultaneously.
For example, six EVs requesting 300 kW each would require 1,800 kW in total. If only 1,200 kW is available for EV charging, the system must either share that power across all six vehicles or prioritize fewer vehicles at the target charging rate.
Energy capacity then determines how long that charging load can be sustained. Even when sufficient power is available, the mobile BESS still needs enough usable energy for the required charging window.
What Determines the Actual Charging Power per EV?
The rated power of a charger is not always the power an EV actually receives. In practice, the charging rate is limited by the weakest point in the charging chain.
1. Total BESS Power
The mobile BESS sets the total power available for all active charging sessions. If 1,200 kW is available for EV charging, that power has to be shared among the vehicles charging at the same time.
2. Charger and Charging Gun Rating
Each charging point has its own power and current limits.
For example, a 180 kW charger cannot deliver 300 kW, even if the mobile BESS has enough total system power. Charging-gun current, cable capacity, and thermal limits can also restrict the maximum output.
3. Vehicle Acceptance Power
The vehicle itself may become the limiting factor.
A charger may support 300 kW, but the EV may accept only 200 kW under current conditions. The actual charging rate can also change with SOC, battery temperature, and the vehicle's charging curve. Charger rating shows the maximum available power. It does not guarantee the power the vehicle will actually receive.
4. Site Power and Power Allocation
Available charging power may also depend on the wider site power setup.
Where the system architecture supports hybrid or parallel power input, the mobile BESS may operate alongside the grid, a generator, or another power source to support the charging load. The control system can then allocate available power based on vehicle demand, charging priority, SOC, and site power limits.
When combined charging demand exceeds the power available, the system must either reduce power across multiple vehicles or prioritize selected charging sessions.
How to Calculate How Many EVs a Mobile BESS Can Charge at Once
A simple way to estimate simultaneous EV charging is to compare the total available charging power with the target charging power per vehicle.
Approximate full-power EV count = Available EV charging power ÷ Target charging power per EV
The result must also stay within the number of available charging ports.
Example 1: 1,200 kW System, 200 kW per EV
Take Polinovel's MBS1300 as a practical example. The system provides up to 1.2 MW of power with six DC charging channels.
If the project target is 200 kW per EV: 1,200 kW ÷ 200 kW = 6 EVs
In theory, up to six EVs could charge simultaneously at around 200 kW each, provided each charging channel supports that power level. Actual charging power still depends on the charger configuration, vehicle acceptance limits, SOC, and operating conditions.
Example 2: What If Each EV Needs 300 kW?
Now keep the same 1,200 kW mobile BESS, but increase the target charging power to 300 kW per vehicle.
1,200 kW ÷ 300 kW = 4 EVs
Even with six charging ports, only about four vehicles could receive the full 300 kW at once. If all six vehicles remain connected, the available 1,200 kW would need to be shared between them instead. This is why charging-port count and simultaneous full-power charging capacity are not the same thing.
Do Not Forget the Energy Requirement
Power determines how many EVs can charge at once. Energy determines how long that charging level can be sustained.
For example: 4 EVs × 200 kW = 800 kW total charging load. If that load needs to run for 1.5 hours: 800 kW × 1.5 h = 1,200 kWh. This means the EV charging load requires approximately 1.2 MWh over the 1.5-hour charging window.
For a mobile BESS supplying that charging load from stored energy, capacity sizing should consider the full energy path: EV charging demand → system losses and auxiliary loads → usable BESS energy → nominal battery capacity
In practice, the required nominal battery capacity will be higher than the calculated EV charging energy. The operating SOC window and reserve SOC reduce the energy available for charging, while conversion losses and auxiliary loads add to the total system energy demand.
These figures are only an illustrative sizing example. Final capacity should be verified against the vehicle charging profile, system efficiency, usable SOC range, recharge conditions, and site operating requirements.
Common Mistakes When Sizing a Mobile BESS for Multi-EV Charging
A simple power calculation is useful for an initial estimate, but it is not enough for final system sizing. Several common assumptions can make the required mobile BESS power or energy capacity look very different from what the project actually needs.
Sizing by Charging Port Count Alone
Do not assume that the number of charging ports equals the number of EVs that can charge at full power simultaneously. Port count determines how many vehicles can connect, while available charging power determines how many can charge at the target rate.
Using Charger Rating as the Actual EV Charging Power
Charger nameplate power does not always reflect the power an EV will actually receive. Sizing should be based on the vehicle's realistic charging requirement and operating profile, rather than the charger rating alone, to avoid unnecessary oversizing.
Treating Nominal Battery Capacity as Usable Charging Energy
Not all of a mobile BESS's nominal battery capacity is available for EV charging. Operating SOC range, reserve energy, conversion losses, auxiliary loads, and thermal management all affect usable energy, so capacity sizing should be based on the energy actually available to support the charging load.
Ignoring the Charging Window and Available Site Power
The same energy demand can require very different power depending on the charging window. For example, 600 kWh delivered over 3 hours requires about 200 kW on average, while the same 600 kWh delivered in 1 hour requires about 600 kW. If grid or generator power can also support the charging load, the mobile BESS may only need to cover the remaining demand.
The right mobile BESS size therefore depends not only on how much energy the fleet needs, but also on when that energy is needed and how much power is already available on site.
What Information Is Needed to Size a Mobile BESS for EV Fleet Charging?
Accurate mobile BESS sizing starts with the fleet's real charging requirements-not just the charger rating or battery capacity. The following project inputs are usually the most important.
1. Vehicle Type and Quantity
Start with the number and type of EVs in the fleet.
Different vehicles can have very different battery sizes and charging capabilities. A fleet of electric trucks, for example, will have a different charging profile from light commercial vehicles.
2. Target Charging Power per Vehicle
Define how much charging power each vehicle actually needs.
The target may be 150 kW, 200 kW, 300 kW, or another value, depending on the vehicle and required turnaround time. This helps determine the total BESS power required for simultaneous charging.
3. Maximum Simultaneous Charging Count
Total fleet size is not the same as simultaneous charging demand.
A fleet may have 20 vehicles, but if only four need to charge at the same time, the mobile BESS should be sized around those four concurrent charging sessions, rather than all 20 vehicles at once.
4. Energy Required per Vehicle
Power tells only part of the story. The system also needs enough energy for each charging session. Useful inputs include:
- vehicle battery capacity
- starting SOC
- target SOC
- expected energy added per session
For example, charging from 20% to 80% SOC gives a much more useful sizing basis than simply saying the vehicle needs to be "fully charged."
5. Charging Window and Daily Duty Cycle
How quickly the vehicles need to charge can significantly change the system size.
A fleet that has 45 minutes between shifts may need much higher charging power than one that can charge for several hours. Daily operating patterns also matter. If the mobile BESS must support several charging cycles each day, recharge time and total daily energy demand need to be considered as part of the sizing process.
6. Available Site Power
Finally, determine how much power is already available from the site. This may include:
- utility grid power
- generator capacity
- renewable generation
If the site can already supply part of the charging load, the mobile BESS may only need to cover the remaining demand.
| Project Input | Mainly Determines |
| Vehicle type & quantity | Overall charging demand |
| Target power per EV | Required BESS power |
| Simultaneous charging count | Peak charging load |
| Energy per vehicle | Required usable energy |
| Charging window & duty cycle | Power, capacity and recharge needs |
| Available site power | Required BESS contribution |
With these inputs, a mobile BESS can be sized around the actual EV fleet charging profile-not a single charger rating or port count.
There is no single number of EVs that every mobile BESS can charge at once. The actual result depends on the number of charging ports, available charging power, vehicle charging limits, usable energy, and the required charging window.
A quick power calculation can provide an initial estimate, but accurate system sizing should reflect the fleet's real charging profile and available site power.
Planning a multi-vehicle charging project? Share your vehicle quantity, target charging power, simultaneous charging demand, charging window, and available site power with Polinovel. We can help determine the suitable Mobile BESS power, energy capacity, and charging configuration.