Why Can’t You Run Multiple Dispensers from a Single PCS?

Joseph Gottlieb, CTO, Tuesday August 24, 2021

One question we hear a lot is “why can’t you run multiple dispensers simultaneously from a single PCS?” Or putting it another way, if I have a 125kW PCS, why can’t I hang five dispensers off of it, and have each one output 25kW? It is a fair question, but is an overly simplistic question – you also have to ask, “how much can the entire system cost?”, “how big can each of the components be?”, and “are the vehicles that connect to the dispensers all the same?” to fully answer the question. To adequately answer this question, you also have to define “the rules”. There are two “fundamentals” to a distributed DC vehicle charging system:

    • Each vehicle has to be isolated from the others to ensure that a fault on one vehicle doesn’t render the other vehicles unsafe – This is termed “galvanic isolation”.
    • If the PCS provides a single “bulk power” feed to all of the dispensers, it has to provide the bulk power at the voltage required by the car with the highest voltage requirements. If all of your fleet vehicles have the same power needs this is relatively easy; if they don’t, this gets hard quickly.

These “rules” only allow two potential architectures:

1. You can have five power output stages in the PCS, with each stage supplying a dispenser. The problem with this approach is that it requires almost as much circuitry as just using five separate PCSs, making it fairly expensive.

2. You can put an isolation stage in each dispenser. If all of the vehicles require the same voltage, this can be relatively easy, but with today’s IGBT technology, it does increase the size (and the cost) of the dispenser. With wideband technologies such as silicon carbide (SiC) and gallium nitride (GaN), this should be easier. If vehicles require different voltages, then you also need a DC-DC power conversions stage to lower the voltage.

The final question is perhaps the most important one: what is the benefit of charging five vehicles simultaneously versus sequentially, especially given that sequential charging generally results in the least expensive system architecture? In the end, it is the classical question of balancing features and capabilities vs cost, size, and complexity.

If you are looking for made in the USA high-power DC fast charging systems for those EVs, look to Rhombus Energy Solutions. Our market-leading bi-directional EV charging systems (which are designed from the start for the needs of fleet operators) are designed and built in the USA. Rhombus also excels in the design of high-power smart inverters for next-generation renewable energy and energy storage deployments. Our 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. We have built over a thousand 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.