Megawatt Charging System Communication (MCS) Communication Topology | What changes on PLC, ISO 15118 and SLAC

Almost after 4 years of continuous research and testing, CharIN Subgroup for MCS has published several technical and non-technical aspects of the Megawatt Charging System (MCS). While the group’s main intention is to reuse as much as implementation of existing Combined Charging System (CCS), there are a few changes unavoidable due to the higher current/power requirements of megawatt systems.

This article is an excerpt of communication topology recommendations that may help in preparing design aspects of megawatt systems.

1.Charging Communication - Physical Layer (PLC)

Charging systems deployed throughout the world presently use physical layers with different technologies, each with their own pros and cons. CharIN members have successfully implemented improvements to the CCS architecture for many years, which uses power line communication (PLC) with the HomePlug GreenPHY communication protocol. This “single ended” PLC used for CCS supported a wide variety of use cases with the benefit of not needing dedicated connection pins for communication between EV and EVSE.

MCS is designed for a 6-fold higher current and up to 10-fold higher power compared to CCS. Therefore, the single-ended implementation of today’s PLC was considered not robust enough for the expected increase in electro-magnetic interference (EMI) emissions compared with CCS. Therefore, CharIN recommends adapting to differential PLC design, using the dedicated charging communication pins of the MCS connector.

Even by using unshielded twisted pair (UTP) wires and a matched impedance of 100 Ohm, the noise immunity is roughly 40 dB higher than single-ended PLC (which is the base of existing CCS1/2). In addition, since PLC natively supports the TCP/IP communication stack (just as Ethernet does), it can be easily adapted to a differential design while still using PLC transceivers already designed for single-ended PLC.

2. High-Level Communication (SLAC)

Another benefit of using this differential PLC communication and connection scheme, is that it also reduces the necessity for signal level attenuation characterization (SLAC) used in CCS implementations, because differential signals produce much less crosstalk between adjacent charging systems.

Therefore, the CharIN suggests eliminating the SLAC protocol, which will reduce complexity and accelerates startup times. The idea is to reduce the setup sequence of the charging communication to the exchange of the Network Management Key (NMK) only; after that, the data link is established, and the ISO 15118-20 application protocol is initiated.

3. Communication Application Protocol (ISO 15118)

ISO 15118-2 has been in use throughout the charging industry for many years. In addition, DIN and SAE protocols for communication have also been used in several legacy systems and applications, but these protocols have more limitations and loose interpretations – as a result there are many different implementations on the field. This needs to be regulated into one implementation.

In addition, as the market has progressed there is need for more complex use cases such as secure handling of payment systems with “plug and charge (PnC), encrypted kWh measurements transactions (E.g: Eichrecht in Germany), flexible charge management operations with fleets and large sites, vehicle to grid (V2G) export power needs, etc. 

Thus, CharIN recommends that MCS uses ISO 15118-20 exclusively, with no other (older) protocols supported, to ensure the absolute highest level of user experience and security to equipment using MCS.

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