The Electric Power Research Institute (EPRI) demonstrated the first prototype 2.4-kV, 45-kVA-based, solid-state, direct current (DC) fast-charging technology for electric vehicles (EVs) at its Knoxville, Tenn., laboratory in February. The Utility Direct Fast Charger technology enables the rapid charging of a plug-in electric vehicle (PEV) and can serve as a utility–owned distribution asset that can be used for other electricity delivery purposes.
The demonstration was conducted using a Nissan Leaf and a Mitsubishi iMiEV. EPRI demonstrated the communication capability of this fast-charging technology with (EV) battery-management systems. A user interface and Web-based mobile data collection system also were included in the demonstration.
EPRI testing confirmed the 2.4-kV, 45-kVA fast-charger prototype’s ability to provide a full vehicle charge to commercially available PEVs. The PEVs were charged using the EPRI Utility Direct Fast Charger. An important part of the demonstration was to verify the communications compatibility of the fast-charging technology with the battery-management systems using the industry-standard CHAdeMO communications protocol.
With an entry-level medium voltage of 2.4 kV, the input-side current is only 16 A. For three-phase inputs of 480 V and 208 V, this current could have been more than 50 A and 110 A.
Within this medium-voltage range are voltage classes. Systems with voltage levels up to 4 kV are designated as 4-V class distribution systems, while systems with levels up to 15 kV are designated as 15-kV class distribution systems.
EPRI’s next step will be to build a fast-charger prototype with 8-kV input, which would represent compatibility with a 15-kV class distribution system. This second prototype will demonstrate the viability of EPRI’s Utility Direct Fast Charger design for 15-kV class systems that use a medium-voltage level such as 12.47 kV, 13.8 kV, etc.
The current is controlled by the PEV’s battery-management system (BMS), which ensures that the PEV’s li-ion batteries receive the appropriate charging current in accordance with their required constant-current then constant-voltage charging profile. The BMS in the Leaf commands a current with linear reduction rate to maintain a slow linear voltage increasing rate when the battery voltage exceeds 384 V. The iMiEV’s BMS maintains the constant-current charging for a longer period until the voltage reaches 364 V. Afterward, it maintains constant-voltage charging with a nonlinear current reduction rate. The shutdown command in both cases was pre-programmed at 80 percent state of charge (SOC).