What is the Best Charger-Dispenser Architecture for V2G?

(Joseph Gottlieb, CTO, Thursday December 3, 2020)

As a manufacturer of EV charging systems with remote dispensers, one of the questions I hear quite often is “can I share one power conversion system (PCS) across several dispensers?” After all, a PCS (depending on the manufacturer, power output, and dispenser capabilities) can be up to an order of magnitude more expensive than the dispenser. If you have an electric vehicle (EV) with 150 kWh of battery capacity (pretty common for school buses and many delivery vehicles), a 60kW charger could fully charge the EV in 2-1/2 hours. If your EVs have a 13-hour charging window (say from 5pm to 6am), five EVs could be charged in this window with a single 60kW charger. Sounds great, right – unless you want to use your vehicles in a vehicle to grid (V2G) solution. The reason that sharing a PCS across multiple dispensers for V2G is that the vehicle-to-grid energy transfer window is significantly shorter than the charging window – in most cities it is 4pm to 9am, or 5 hours. If the vehicles are not available until 5pm, you are down to four hours for V2G.

So what makes the most sense? A big part of the answer is “how much power do your vehicles return to the yard with?” The less power a vehicle comes back with, the less time that is required to discharge the power to the grid. Using the same vehicles, a 60kW PCS, a four-hour V2G window (5pm-9pm), and a 9-hour recharge window (9pm-6am), three vehicles could be supported by the one PCS. Each vehicle would get 2-1/2 hours to charge, and 80 minutes for V2G – each vehicle could theoretically discharge 80kWh of energy (over 50% of its battery capacity) onto the grid. However, there are two problems with this scenario:

  • The vehicles have more than 50% of their power remaining: While this sound non-sensical, it is not uncommon for some classes of vehicles such as school buses.
  • The possible effect of V2G on vehicle battery life: Rapid battery discharging during V2G can increase battery temperature and reduce battery life. Slower discharge reduces the impact of heating on battery life.

For instance, the EV in the shared PCS scenario above is discharge its 80kWh of energy at a rate of 1 kWh per minute. If the EV had a dedicated PCS but the rest of the scenario stayed the same, it would discharge the 80kWh of energy over four hours, at a rate of 1/3 of a kWh per minute, reducing the impact of V2G on battery life. Obviously, the right answer depends on the vehicles and use case, so work with your vehicle and charger providers to find the right answer, or you can connect with Rhombus’ application engineers, and we can help you build the right solution.

Rhombus has built high-power inverters for renewable energy such as “behind the meter” grid energy storage systems for several years. We have applied that expertise to the design of our latest-generation bi-directional EV charging systems (which are designed from the start for the needs of fleet operators) and bi-directional smart inverter systems for microgrids. Rhombus expertise in energy management system (EMS) software is also embedded in our VectorStat EMS controller and software which is embedded in our EV charging systems and smart inverters. Rhombus also excels in the design and testing of high-power electrical equipment to meet the requirements of UL and other certification organizations. We have built hundreds of V2G-capable high-power, high-reliability chargers and bi-directional smart inverters for a variety of different sizes and classes of EVs. Find out how we can help you by contacting us at news@rhombusenergy.com, or by reading our Vehicle to Grid Solution Brief.