Choosing the Right Charger for Your EV Fleet (part 2)

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Camber Team

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While selecting a charger for a single vehicle can be tricky, designing an entire charging infrastructure for a commercial fleet can feel like a huge challenge. As commercial fleet electrification moves from pilot projects to full scale conversion of fleets, facilities and fleet operators need to charge hundreds of vehicles instead of just one or two. To meet this rising need, charger manufacturers are beginning to offer various types of chargers with different power levels and numbers of charge points. Commercial vehicles typically have large batteries that need to be charged in a shorter amount of time than your average passenger vehicle. Therefore, most commercial vehicles are designed to use Direct-Current Fast Charging (DCFC or DC), which can charge a vehicle much faster than a residential AC-Level 1 or AC-Level 2 charger. 

In our last article on charger selection, we covered Vehicle Specifications and Fleet Operations. In this post, we will go over Charge Times and how it all comes together.

  1. Charge Times

Estimating the amount of time it takes to recharge an electric vehicle can be complicated. Each vehicle and charger will have its own “charge curve” which illustrates how quickly a vehicle will charge with a given charger over a voltage range.  Typically, the first 10% and last 10% of the vehicle’s state-of-charge (SOC) charges at a slower rate than the middle 80%. These are guardrails to protect the health of the battery. 

Charge time is determined by the interaction between vehicle and charger. During a charge session, the rate at which the battery is charged will be limited either by the charger or the vehicle. There are several factors that determine whether the vehicle or the charger is the limiting factor:

  • Maximum current (Amperage) the vehicle can accept
  • Maximum power (kW) the vehicle can accept
  • Maximum current the charger can provide
  • Maximum power charger can provide

The charger will reduce its power output based on the limiting factor at any given point in the charging session. It is possible that the limiting factor may change over a vehicle’s State of Charge, meaning that it is possible that each unique combination of vehicle and charger will have its own unique charge curve, and therefore charge time. 

Working with vehicle OEMs will get you rough estimates on approximate charge times, while accurate charge times can only be obtained through empirical data or simulations. Detailed charging simulations can be performed to get estimate fleet energy usage and charge times. 

It should be noted that charge times often assume the vehicle is returning empty. However, in real world applications, vehicles will return with varying remaining amounts of onboard energy. Remaining energy is often not considered in initial sizing calculations to make conservative sizing selections utilizing worst-case scenarios. 

Tying it All Together

Let us look at a couple examples to illustrate the charger selection process.

Example 1:

A school district is looking to purchase two electric school buses to add to its existing fleet. The fleet operates two routes, twice a day to bring students to and from the local school. 

Vehicle Specifications:

Battery Voltage: 540V

Battery Capacity: 220kWh

Maximum Current Acceptance: 200A

Fleet Operation:

Route 1: 6:00AM – 8:30AM, 2:30PM- 5:00PM

Route 2: 6:30AM – 9:00AM, 3:00PM- 5:30PM

Charge Times:

60kW Charger: 4.5 Hours

120kW Charger: 3 Hours

Based on the information provided by the manufacturer, we know that the calculated maximum power the vehicle can accept is 108kW (P = 540V x 200A / 1000). This means the maximum charger power will be limited to 108kW and any amount of power specified above that limit will not be utilized. 

Fleet operation is determined by the route schedule established by the operator. Based on the route hours, both Route 1 and Route 2 have 6 hours available to charge during the day and 13 hours to charge at night. To make a conservative estimate, we can reduce each window by 1 hour to account for vehicle maintenance and cleaning. This means that our midday charge window is 5 hours, and the evening charge window is 12 hours.

Combining the operating schedule with the estimated charge times for the vehicle we can make a reasonable estimate for the charger selection. Assuming both vehicles arrive in the evening completely empty, we know it would take a total of 9-hours to charge the fleet. 

In general, the design goal for charger selections is to install the lowest power charger that will still meet the charging requirements of the fleet. In this case, since the charge window is relatively long, Camber would recommend a single 60kW charger with two remote dispensers. Installing only one charger reduces the overall infrastructure cost while still having the ability to charge multiple vehicles. 

Please note that electrical infrastructure can be a significant factor in charger selection. Infrastructure considerations while installing fleeting charging systems will be covered in a later article. 

Example 2:

A transit agency is looking to purchase eight electric transit buses to add to its existing fleet. The facility has various routes with multiple buses serving each route.

Vehicle Specifications:

Battery Voltage: 700V

Battery Capacity: 600kWh

Maximum Current Acceptance: 300A

Fleet Operation:

Route A: 5:00AM – 11:30PM

Route B: 5:30AM – 9:00PM

Charge Times:

150kW Charger: 5.5 Hours

180kW Charger: 4.5 Hours

Based on the information provided by the manufacturer, we know that the calculated maximum power the vehicle can accept is 210kW (P = 700V x 300A / 1000). This means the maximum charger power will be limited to 218kW and any amount of power specified above that limit will not be utilized.

The fleet operation is dictated by the route schedule established by the transit agency. Based on the route hours, Route A has 5.5 hours and Route 2 has 8.5 hours to charge at night. To make a conservative estimate, we can reduce each window by 1 hour to account for vehicle maintenance and cleaning. This means that our midday charge windows are 4.5 hours and 7.5 hours respectively.

Combining the operating schedule with the estimated charge times for the vehicle we can make a reasonable estimate for the charger selection. In this case, since the charge window is relatively short, we would recommend a 180kW charger with a single dispenser for every vehicle operating on Route A, and a 150kW charger with a single dispenser for every vehicle operating on Route B. 

Depending on the facility’s layout and available infrastructure, we would alternatively propose a single 1440kW charging station with 8 dispensers. This station would be able to charge 8 vehicles at 180kW each. This would increase the site’s electrical demand, but the reduction of conduit and space required by the charging equipment could make this a viable option. 

Additional Considerations

As mentioned in the previous sections, infrastructure can be a significant factor in charger selection. Site limitations including available power, space allocation for charging equipment and additional infrastructure requirements can further complicate charger selections. Infrastructure considerations while installing fleeting charging systems will be covered in a separate article. 

The above examples also assume that the vehicles can complete their routes with the available onboard energy. In some cases, there is not enough onboard energy to meet route requirements. In this case, on-route or opportunity charging may be needed. (Opportunity charging was not a consideration in this guide and will be covered in a future article.)

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