DC-Meters for Fair and Smart EV Fast Charge Billing

The innovative technology of electric vehicles (EVs) is leading to a reinvention of mobility habits, while their deployment on a mass scale is bringing up new technological obstacles. Charging stations are a key example. The challenge for the next five years is to build global networks to ensure drivers do not have ‘range anxiety’ and can perform a quick “refill” at any charging station along their route. Major charging networks are spreading across Europe, supported by and integrated into all economic recovery plans.

 

A DC Meter for Harmonization of Costs for Fast Charging

Today’s public charging infrastructure mostly supplies alternating current (AC). This leads to a long charging time of eight hours on average, due to a low power capability of around 44 kW. To reduce this charging time to only a few minutes, charger manufacturers convert the AC into direct current (DC) outside the car, bypassing the OnBoard-Charger (OBC) and supplying the battery pack of the electric vehicle directly. Although this method boosts charging power up to 400 kW, this conversion stage generates power losses, which should not be billed to the EV owner. Thus, an electric meter is placed after the conversion stage to monitor and bill the exact energy transferred to the electric vehicle, rather than the charge duration.

To meet this need, LEM is launching today its DC Billing Meter (DCBM). This new DC meter solution is smart, extremely compact, and fully compliant with the German “Eichrecht” regulations, which enforces billing based on precise energy measurement. Standards are being established not only at the European level but also internationally.

 

Figure 1: LEM DCBM

 

The LEM DCBM has been developed to meet market demands for inter-operability and data security, for easy and fast retrofitting of charging stations already deployed, and for high power measurements, up to 600A/1000V.

 

A Gateway from Electrotechnical Systems to the Cloud

The LEM DCBM is integrated into a global charging station architecture. When a charge is requested by the EV owner, a chain of requests is triggered, going through the charge point operator, the charge controller, and up to the DCBM. The latter starts measuring energy over the whole duration of the charging session. When charging is complete, the measurement data are gathered and stored in long-term memory in the DCBM, together with several identifiers carried by the successive requests, and secured by a signature. During the charging session, the DCBM can provide the controller with data on the currently transferred energy, as well as live current, voltage and temperature measurements.

 

Figure 2: Global charging station architecture

 

Future-oriented communication

Offering Ethernet communication, the DCBM is Plug&Play. Instant network integration is assured through the global TCP/IP standard, with optional DHCP for automatic addressing. The product offers HTTPS/REST interfacing for easy, secure integration with data systems. The measurement data are also signed, providing proof of authenticity. Following the S.A.F.E initiative, the DCBM supports the OCMF format, offering the best inter-operability for cloud service operators.

 

HTTP/REST

HTTP is the data exchange protocol used by every web browser, making it compatible with any software. HTTP/REST is a subset of the protocol that allows simple data formatting. The format chosen for the DCBM is JSON, giving the device human-readable, IoT-ready data sets. Using the main HTTP methods (GET / POST / PUT), the DCBM can easily be requested to start, stop and retrieve charging sessions. Moreover, in-session readings provide a live display to the driver, offering a gas-station-like experience and ensuring an easy evolution to the mass use of electric vehicles.

 

OCMF

The OCMF (Open Charge Metering Format) is an open-source standard developed by the S.A.F.E initiative. Designed to achieve system independence, it addresses a wide variety of existing EV charging standards that govern the final billing of the meter. Moreover, the standard was designed to support extra fields to address the rapidly changing billing processes of EV charging sessions. The S.A.F.E alliance provides a certified transparency software for OCMF to verify the authenticity of the charging session and results.

 

Signed Billing Data

The DCBM supports its own format of data along with the OCMF format. Both formats are digitally signed with standard ECDSA methods, allowing verification of authenticity and integrity of the billing data. Each DCBM has its own public key that shall be registered onto a PKI (Public Key Infrastructure) institution, thus protecting the end user.

 

Figure 3: Example data set for a DCBM charging session

 

With its new DCBM, LEM opens up the field of electricity billing, which can include applications such as data centers, photovoltaic applications and DC grids, allowing a sustainable, ecological transition

 

About LEM

LEM is a mid-size, global company with production plants in Geneva (Switzerland), Sofia (Bulgaria), Beijing (China) and Machida (Japan). With its regional sales offices close to its clients’ locations, the company offers seamless service around the globe.

 

This article originally appeared in the Bodo’s Power Systems magazine.