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Integrating Energy Storage with EV Fleet Charging Infrastructure Requires More Than Just Hooking Up a Bunch Of Batteries to Your Chargers

 (Joseph Gottlieb, CTO, Thursday, October 3, 2019)

Our last blog dealt with some of the reasons that operators of electric vehicle (EV) fleets should be looking at integrating energy storage with the EV fleet charging infrastructure. Many in our industry (including EV charging system companies) believe that all you have to do to accomplish this is take a shipping container full of batteries (also known as “Just a Box of Batteries”, or “JBOB”), and connect it in front of your AC power feed from the utilities. While this can work (to some extent), it is far from an optimal solution for a variety of reasons. This blog will explore some of the design considerations that need to be considered when integrating energy storage into fleet EV charging solutions.

The first factor to consider when integrating energy storage with EV charging infrastructure is where you connect it into the system. Energy storage, whether a “JBOB” or an intelligent, managed solution, are inherently DC power-based systems. Not surprisingly, electric vehicles are also DC power-based systems, as are the “guts” of the charging systems that charge them. Simply putting a JBOB into the AC power feed directly in front of the charging system means that the incoming AC power is converted to DC power, which is then stored and converted back to AC power for consumption by the charging system. While AC-DC and DC-AC power conversion has become far more efficient than it previously was, this approach still wastes significant amounts of energy. The better approach is to integrate the energy storage system (and any renewable power sources such as photovoltaic, which are also generally DC sources) with the DC stage of the charging system. This avoid the need to convert power back and forth from one form to another.

The second issue is overall system management and intelligence. All battery-based power storage systems, whether on a bus or in a JBOB, require battery management software to ensure that the batteries are charged and discharged within their specifications and according to their correct charging profiles to maximize battery life. At a higher level, the software managing the charging and energy storage infrastructure also has to worry about issues such as:

  • Provisioning and Scheduling: How many EVs need to be charged tonight in order to meet tomorrow’s needs, and how much charge does each vehicle need? When will the vehicles be available for charging, and when do they have to leave to start their shift? How should the EVs be scheduled for charging to best meet these needs?
  • Peak Shaving and Load Balancing: Once a potential charging schedule has been established, the management software need to figure in the power that has been stored in the energy storage system, if renewable power will be available during the charging period, and the utility rate structure so that the overall energy costs can be minimized. Also, actual usage (versus expected usage) needs to be monitored to adjust things as required.

These are but a few of the intelligence requirements, and represent today’s “table stakes”. To be ready for tomorrow, the best EV fleet charging infrastructure and energy management system (EMS) software needs to have an open and expandable architecture to easily add new functions as EV charging infrastructure needs change and evolve.

At Rhombus, our mission is to help solve the world’s energy challenges. We are experts in building intelligent, extensible, and reliable EV fleet charging infrastructure for the most demanding environments. Our VectorStat EMS software provides the intelligence and open architecture required for fleet EV charging systems both today and tomorrow. Learn more at

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