Tesla NACS – There’s More to the Story

Tesla has taken a huge leap in accelerating EV adoption and bringing the vision of clean energy closer, by announcing the opening of its EV connector design to the world. The announcement invites charging network operators and vehicle manufacturers to install the Tesla charging connector and charge port, now called the North American Charging Standard (NACS), on their equipment and vehicles.

What got lost in the announcement is that while NACS is physically a radical departure from the CCS Combo connector, the digital compatibility between NACS and CCS paves a path for passive interoperability between the two standards. In a not too distant future, drivers with CCS-enabled vehicles may pull up to a Supercharger station, and Tesla drivers will be able to use CCS stations.

How will this accelerate EV adoption? What are its benefits? How can you access the NACS design and related data? Let’s deep dive a little to get answers to all these questions.

PREFACE

The North American Charging Standard (NACS) is the most proven and widely used charging standard in North America. The NACS connector (formerly the Tesla charging connector) will establish a plausible alternative to the CCS Combo connector. For years non-Tesla EV owners have complained about the relative inelegance and unreliability of CCS (and specifically the Combo connector) relative to the proprietary Tesla alternative, a notion Tesla alludes to in their announcement.

While detractors are quick to criticize Tesla for introducing yet another standard to the crowded charging standard arena, implying this move is intended as a shortcut for Tesla’s Supercharger network to participate in federal incentives, the potential impact of this announcement is much more subtle and could lead to a much simpler EV charging ecosystem.

HISTORY

The Combined Charging System (CCS)

The “Combined Charging System” (CCS) Combo connector was essentially born of compromise. When the standard was introduced in 2011, AC charging was — and remains — dominated by the IEC 62196 Type 1 connector (also known as the SAE J1772 connector) in North America and Japan and the IEC 62196 Type 2 connector in Europe.

DC charging, which allows for far higher charge rates, was a mix of CHAdeMO from Japan and GB/T 20234.3 from China. Both these connectors require a separate inlet for AC and DC.

Figure 1: The Nissan Leaf has separate inlets for CHAdeMO and J1772 | Image Courtesy: Wikipedia

Maintaining separate inlets is not only unsuitable for EV OEMs, who need to find additional packaging space and deal with complex charge port covers but It’s also a confusing situation for consumers, who need to remember which inlet to use in different situations.

The authors of CCS wanted to establish a single inlet envelope that would reduce confusion and overhead while remaining backward compatible with J1772 (Type 1) and Type 2 connectors, both of which had established regional dominance. The proposed solution was to extend the Type 1/2 envelopes with a two-pin DC extension. The new Combo 1 and Combo 2 connectors both remained backward compatible and allowed for the introduction of high-speed DC charging.

Figure 2: The Combo 2 inlet remains mechanically compatible with Type 2 connectors | Image Courtesy: Wikipedia

CCS Communication

Besides augmenting the existing Type 1/2 AC connectors to support DC fast charging, CCS also had to figure out how to get more data across the charging cable. AC charging leveraged a simple communications scheme defined in SAE J1772. A vehicle adjusts the voltage on the ‘control pilot’ (CP) to indicate ‘readiness’ to a charger, and the charger adjusts the duty cycle of a 1kHz square wave to indicate the maximum AC current the vehicle can draw.

This works well for AC because the onboard charger is responsible for communicating with the vehicle’s battery to adjust DC current and voltage levels. For external DC charging, where DC current is provided directly to the vehicle’s battery, it is critical that additional parameters and limitations are mutually agreed upon between the vehicle and charger.

To support these additional parameters, CCS specifies the use of Power-Line Communication (PLC) over the existing CP line. This technique, while complex, allows up to 10 Mbit/s bandwidth and does not require any additional physical pins on the inlet envelope.

The messaging protocol between vehicles and chargers over this new PLC line is specified in ISO 15118 and DIN 70121.

In addition to the additional charge parameters and limits required for base DC charging, ISO 15118 imagines additional use cases for the relatively large available bandwidth between vehicle and charger:

• Secure protocols to enable Tesla Supercharger-esque ‘Plug ‘n Charge’ capability, a key feature that massively improves user experience.
• Scheduled and reactive load balancing, critical as EV charging begins to account for larger fractions of the overall grid load.
• Bi-directional power transfer, enabling use cases such as V2H and V2G.

AMP’s proven CCS Software Stack (presently in hundreds to thousands of vehicles) implementing ISO 15118 and DIN 70121 which can help you jump-start your EVs CCS capabilities. Click here to learn more about AMP’s CCS-compliant charging solutions.

Though CCS works and has become the de facto charging standard in most parts of the world, legacy compatibility requirements yielded a standard that is bulkier than otherwise needed and is a compromise of various constraints.

TESLA NACS

Why NACS?

Tesla vehicles never had compatibility concerns. The Tesla charge connector cleverly reuses the AC pins for DC charging, and communication is based on a single-wire CAN spec for DC use cases. Since all vehicles at Supercharger stations have historically been Tesla vehicles, use cases like Plug N’ Charge were accomplished by far simpler means, enabled in part by the fact that Tesla has control over both the vehicle and the charger.

The result is a connector that is:

• Incredibly small
• Simple to use, and
• Part of an overall simpler ecosystem of chargers and compatibility concerns

Tesla’s NACS announcement signals the possibility that other vehicles and chargers can tap into this simplicity.

Compatibility

While recent media articles and the Tesla blog post itself present NACS in opposition to CCS, there’s more to the standard that meets the eye.

The mechanical attributes of the inlet and connector are only one part of the equation; communication compatibility and interoperability are critical. In a wise move, Tesla specifies the same communication protocol for NACS as CCS, ISO 15118, and DIN 70121, not the proprietary single-wire CAN signalling used by the Supercharger network up to this point.

With nearly every new EV and charger coming online with CCS compatibility, introducing an alternative communication standard would likely make NACS dead on arrival. With this decision, however, the path to adoption and even interoperability between CCS and NACS chargers and vehicles is much more straightforward.

On the charger side, NACS adoption or adding a cord set; all the digital communication logic and power generation components can likely remain untouched.

Because there is no active communication translation required between CCS and NACS, it should be relatively simple to develop a passive coupler that converts Combo 1/2 to NACS and vice versa. This appears to already be the case with the Combo 1 adapter Tesla has recently launched. Tesla owners are already very familiar with using the J1772 adapter that comes with every vehicle to use Type 1/2 AC chargers, this is a similar extension.

Megawatt Charging

The 1000V capability of the CCS Combo Charger has been considered an added advantage over the Tesla connector, which has been limited to 500V. While all Tesla vehicles today are ~450V designs, other OEMs have introduced ~900V vehicles that in some cases double the effective charge rate possible, with some vehicles capable of using 350kW CCS stations.

The NACS specification explicitly calls out 1000V-rated (mechanically compatible!) connectors and inlets that could work well for this use case. Tesla even indicates this connector would be capable of megawatt charge levels, a feature CCS Combo connectors are currently not able to deliver.

Functional Safety Implementation

An interesting technical challenge of NACS is the same detail that makes it so compact- shared AC and DC pins. As Tesla details in a corresponding appendix, there are specific safety and reliability hazards that must be thought through and accounted for to implement NACS correctly on the side of the vehicle.

Extreme safety measures must be taken to ensure that the AC grid voltage applied to the inlet is never connected to the battery and DC battery voltage shall never be connected to the grid. Fire hazards and equipment damage will certainly occur if the control electronics and software fail.

Figure 4: Implementations must hold K3 and K4 open during AC charging | Image Courtesy: Tesla

AMP ENERGY MANAGEMENT

At AMP our goal is to provide manufacturers with the safest, most reliable, and easiest-to-integrate solutions to enable the next generation of electric vehicles with AC and DC charging.

Our ‘off-the-shelf’ Energy Management Unit (EMU) is NACS-ready. It consolidates the components necessary to implement NACS, including a sophisticated 11kW onboard charger, PLC communication, UHF receiver, contactor control, and power distribution, all in one unit. The EMU also includes a 12V DC-DC further consolidating a complex powertrain architecture.

Click here to learn more about AMP’s Energy Management Unit.