Building Highly Reliable EV Charging Systems

(Joseph Gottlieb, CTO, Thursday August 22, 2019)

In our last blog, we explored some of the impacts that occur when EV charging systems fail, andhow fleet EV charging systems differ quantitatively from personal automobile charging systems. While these differences would seem to be painfully obvious, a large number of fleet EV charging systems are still based on designs and technologies from automobile charging systems. Because these weren’t designed to deliver the power over the duty cycle typical of fleet EV charging systems, they often have very poor reliability. As an example of this, one fleet EV charging system deployment that we replaced which was derived from commercial automobile chargers had a 100% failure rate in its first month of deployment.

First of all, fleet EV charging systems typically separate the dispenser (which connects to the vehicle) from the power conditioning unit (PCU), which converts the AC power to DC, and provides the right power draw to minimize the impact to EV batteries. The PCU must also be able to provide nearly 10 times the power provided by automobile charging systems, and be able to do so at a duty cycle of 100% on a continuous basis. This means that the charging system needs to be designed to be both efficient (reduce heat loss, which impacts charging system life) and able to do so at high power ratings. And 60kW is just the starting point – it is often desirable to use a single PCU to drive multiple dispensers, which means the PCU could potentially be up to 500kW (enough to run eight or more dispensers).

Moreover, for PCUs that are in the hundreds of kilowatts (and can approach a megawatt), a “rip and replace” approach is not economical. Because of their cost and the impact of failure, fleet EV charging systems must be easily to diagnose, maintain, and upgrade (if necessary). Again, this requires designing the systems with these factors in mind from the start. It also requires system operation and maintenance software to provide the required diagnostics, and system management software that can coordinate those capabilities across multiple systems and deployment sites. These are the capabilities that are required of fleet EV charging systems for them to be able to provide a reliable and efficient fleet EV charging solution.

Rhombus has built high-power DC fast charging systems and smart 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, or by reading our Vehicle to Grid Solution Brief.