ISO 15118-20 Released: The Future Standard for EV Charging Communication

The latest version of the ISO 15118 communication standard series—ISO 15118-20—has arrived!

ISO 15118-20 is the newest member of the ISO 15118 standard family, a set of future-proof communication standards for electric vehicle (EV) charging. The ISO/IEC Joint Working Group began developing this standard at the end of 2015, focusing on adding new functionalities that could not be included in ISO 15118-2. Furthermore, we have eliminated limitations identified by various vendors during the implementation of ISO 15118-2.

ISO 15118-20 is designed to be the standard serving all use cases, supporting the full range of electric mobility, whether cars, motorbikes, trucks, buses, or even ships and airplanes. Yes, you read that right, even airplanes.

ISO 15118-2 defines messages exchanged between an EV and a charging station to control AC (Alternating Current) and DC (Direct Current) charging sessions. Additionally, the standard supports smart charging features as well as the user-friendly and secure Plug & Charge functionality.

ISO 15118-20 extends ISO 15118-2 by adding support for Wireless Power Transfer (WPT). Each of these services can be delivered via Bi-directional Power Transfer (BPT) and Automated Connection Devices (ACD), which we will explain in detail below.

Bi-directional Power Transfer (BPT, i.e., V2G)

The idea behind Vehicle-to-Grid (V2G), as the name suggests, is that when an EV is connected to a charging station, it can feed part of the energy from its traction battery back into the grid as needed. This concept is likely as old as Willet Kempton's first paper on V2G published in 2001, "Vehicle-to-Grid Power: Battery, Hybrid, and Fuel Cell Vehicles as Distributed Energy Resources for California." Mr. Kempton is considered the father of V2G.

As you can see, V2G is not a new concept; research on this topic spans nearly 20 years. With more EV models hitting the market, growing global EV sales, and significant increases in solar and wind energy worldwide, the industry seems to finally be embracing the idea—these are excellent prerequisites for successfully bringing V2G EVs to market.

Recently, both Tesla and General Motors announced the introduction of "million-mile batteries" to the market. Another recent article reported that Tesla has secretly introduced V2G capabilities in its EVs. This growing momentum is all the more reason why V2G applications should come to market sooner rather than later, as battery aging concerns will not be as critical as discussed today.

In ISO 15118-20, the technical characteristics of reverse power transfer (i.e., BPT) systems are defined by introducing two new parameters: BPTChannel and GeneratorMode:

• BPTChannel: This parameter distinguishes between single-channel and dual-channel architectures. The former uses a single meter for energy flow metering; the latter uses two independent meters to measure energy flow in two opposite directions separately. Consequently, the system uses two switches that open/close depending on the current direction.
• GeneratorMode: This parameter indicates whether the system (EV + Supply Equipment) operates as a "Grid-Following Generator" (injecting only active and reactive power) or a "Grid-Forming Generator" (capable of controlling the network's voltage and frequency).

Combining AC, DC, and Wireless (WPT) transfer modes with Bi-directional Power Transfer and the use of Automated Connection Devices, ISO 15118-20 currently offers a total of 12 possible services:

AC, DC, WPT, AC_ACD, DC_ACD, WPT_ACD, AC_BPT, DC_BPT, WPT_BPT, AC_ACD_BPT, DC_ACD_BPT, WPT_ACD_BPT.

Automated Connection Device (ACD)

ISO 15118-20 defines an ACD as "an assembly capable of supporting an automated connection and disconnection process for conductive energy transfer between an EV and an EVSE (Electric Vehicle Supply Equipment)."

A typical use case for ACDs is charging electric buses using a Pantograph, a device mounted on the roof of the bus that collects power through contact with overhead lines. In fact, this specific use case was the first one specified in ISO 15118-20, with Siemens leading the drafting of the relevant technical requirements.

Wireless Power Transfer (WPT)

Wireless charging increases the convenience of charging cars. Imagine all you have to do is drive your EV over a ground charging pad, and your car automatically starts communicating with the ground equipment, guiding you to park in the optimal position for maximum transfer efficiency, and initiates the charging process. You don't even need to plug in a cable. This has the potential to take the user convenience of "Plug & Charge" to a whole new level.

For WPT systems, the IEC 61980 standard series is responsible for defining "General requirements" (IEC 61980-1) and "Specific requirements for communication between electric road vehicle and infrastructure" (IEC 61980-2). However, IEC 61980-2 references ISO 15118-20 for details on how to transmit all necessary data between the EV and the infrastructure.

Dynamic Mode (For Ancillary Services)

In addition to the existing "Scheduled" mode in ISO 15118-2, ISO 15118-20 introduces a new "Dynamic" control mode. This mainly distinguishes between two scenarios:

• Scheduled Mode: The EV is responsible for ensuring mobility needs. The EV and the charging station negotiate a power profile based on exchanged information. In this mode, the EV is responsible for calculating a charging profile that meets the user's mobility needs (e.g., how much energy is needed for a full charge, when to leave) and adhering to the charging station's power limits.
• Dynamic Mode: Control is fully delegated to the off-board system (i.e., the charging station), with no negotiation required. The EV sends the same parameters as in Scheduled mode, but in Dynamic mode, the charging station only sends single power setpoints to the EV without any price information or forecast schedules, and the EV must adhere to these setpoints. The controlling off-board system is responsible for ensuring the user's mobility needs are met. This mode is particularly useful when fast response mechanisms are needed to provide grid services (like frequency control).

Multiplexed Communication

Besides the regular message flow (where each message is identified by a specific payload type), multiplexed communication utilizes newly introduced payload types to allow parallel exchange of certain messages outside the predefined message flow control (also known as the state machine).

These messages involve service renegotiation (e.g., changing from charging to discharging), system status messages required for ACD use cases, exchanging metering information, or parking assistance messages.

For example, while exchanging DC_ChargeLoop messages during a DC charging session, if the EV or charging station wants to change the charging profile (power over time), they can trigger a schedule renegotiation without interrupting the existing exchange of charging messages. This is a significant advantage for DC charging, as in the original ISO 15118-2, renegotiation required opening and closing contactors, which caused charging interruptions.

Enhanced Data Security

In ISO 15118, data security is enabled at both the transport and application layers. Transport Layer Security (TLS) 1.2 and above is used to encrypt the communication channel at the transport layer. At the application layer, XML-based digital signatures and X.509v3 certificates are used to verify the sender's authenticity and the integrity of exchanged messages.

However, in ISO 15118-2, TLS was only mandatory when using the "Plug & Charge" identification mechanism. For EIM (External Identification Means, such as RFID cards or mobile apps), TLS was optional. In my opinion, making TLS optional was not a wise move, but it was likely pushed by some standardization members to lower the initial implementation difficulty.

Fortunately, in ISO 15118-20, TLS is mandatory for all use cases and all identification mechanisms, meaning there are no more security loopholes.

Simpler Multi-Contract Handling

The original design philosophy of ISO 15118-2 was for the EV to authenticate itself to the charging station on behalf of the user using a single contract certificate. Over time, standardization body members realized that supporting only one contract certificate as an identification token at any given time had its limitations.

ISO 15118-20 now defines requirements for the smooth installation and use of multiple contract certificates. For example, a user can easily use a dedicated contract certificate for (free) charging at company premises, while using another contract certificate for private charging at home.

ISO 15118-20 Release Timeline

(Note: The following reflects the expected timeline at the time of original writing, for reference on the ISO standard evolution process)

ISO 15118-20 was originally scheduled for release in 2020. However, given the introduction of many new features, members of the ISO 15118-20 standardization body are currently preparing a document close to the Final Draft International Standard (FDIS). Once ready, we will have a few months to thoroughly review the document. The FDIS marks a technical freeze, after which only editorial changes are allowed.

If all goes well, we will have the FDIS in early April and submit it to ISO by the end of June. It is expected that ISO will take two to three months to finally publish the document.

Incompatibility Between ISO 15118-2 and ISO 15118-20

Although ISO 15118-20 is based on ISO 15118-2, the similarities stop there. They are not compatible with each other. That is, an ISO 15118-20 compliant charging station cannot establish a communication session with an ISO 15118-2 compliant EV, and vice versa.

There are too many differences between the two versions, including different message names, new data types and fields, a range of new messages, and changes to the state machines defining message sequences. Consequently, the XML Schema Definition (XSD) files used to encode and decode messages have also changed.

However, you will still see many shadows of ISO 15118-2 in ISO 15118-20, so you don't have to throw away your code base entirely. On the bright side, hardware requirements remain the same. You still need a HomePlug Green PHY compatible modem to enable Power Line Communication.