Electric vehicles present a great opportunity to provide backup (V2H or Vehicle-to-Home) and other off-grid power (such as at a construction or camping site: V2L or Vehicle-to-Load), as they store a substantial amount of energy and already have AC-DC conversion equipment on board. Although many of today’s EVs do not have the ability to do the conversion in reverse, this does not represent a substantial extra cost, and as Vehicle-to-grid technology becomes more prevalent, many will already have this capability. This presents a problem, however, in North America: here, the most common AC charging couplers have only two power carrying contacts, and our residential electrical systems are so-called split-phase and, therefore, require three power terminals to provide power to many loads (2P+N). In an ironic twist, in most of the rest of the world, split-phase is not used, and the most common AC charging coupler includes four AC contacts to allow for three-phase charging (3P+N). The three-phase coupler used in most of the world (outside of China) is standardized in North America under SAE J3068 for medium and heavy-duty use, and was considered by at least some OEMs for adoption in North America. However, the transition to the Tesla/NACS/SAE J3400 connector has closed the door on any such ambitions for passenger cars.
There is an alternative solution in a “neutral-forming transformer” (NFT), a type of auto-transformer, but this adds weight, cost, and complexity to the system that would be best avoided if possible. All of this leads to the question: can an additional, optional contact be added to the existing coupler systems in a backwards-compatible way? For SAE J1772/Type 1, we believe the answer to be no, and that system has been largely deprecated for new designs anyway. For the SAE J3400 coupler system, the answer is likely yes.
By what Sorcery?
Before we get into the details of how this might be accomplished, let us consider what the requirements of such a system might be, and some nomenclature to ensure clarity. We will refer to the standard, unmodified SAE J3400 coupler as sNACS (standard-NACS), and the coupler system with the additional contact as NACS+. In regards to the additional contact, it must: not interfere with, or cause excessive wear to sNACS equipment mated to it; have a similar cycle life to the rest of the coupler; have a point of contact such that when unmated it disconnects no sooner than the other power-carrying contacts and no later than the ground contact; and have no less creepage and clearance distance from the DC contacts (same as the AC power carrying contacts in the NACS coupler) than the ground contact does. Note that it does NOT need significant creepage or clearance, or indeed even insulation, from the ground contact as it is. By definition, neutral is a grounded conductor (as used in this EVSE application under UL 2594/2231), and can be verified by the supply-side equipment to be properly bonded at the service entrance (or in the breakout box in the case of vehicle-to-load).
In this context, we see two reasonably practical solutions: The contact can be on the bottom of the connector, with a spring-loaded mating contact at the bottom of the inlet, or the contact can be on the shell above the ground contact of the connector, and the mating contact protruding from the star-shaped baffle of the inlet. The latter of these possibilities is preferred, as it is both more protected and does not have to share limited space with the latching mechanism.
Outside Contact Proposal
Inside Contact Proposal (Preferred)
Let us further consider the latter solution. It may be desirable for the inlet contact to have the spring pressure removed or be retracted entirely when power export is disabled on the vehicle (e.g., the customer did not pay for the option) to minimize wear on sNACS equipment. It may also be prudent for SNACS connectors produced after the introduction of NACS+ to have metal or some other hard-wearing material where the NACS+ connector contact is located to eliminate the concern of excessive wear from the inlet contact, although it may be addressed by requirements on the design of the new inlet contact.
Closing Thoughts
It is worth noting that sNACS and NACS+ must be treated differently by the charging system, but also must seamlessly interoperate with equipment that may have been produced before the introduction of NACS+. The required changes in control pilot communications between the EVSE and EV are outside of the scope of this piece, but safety with existing systems must be guaranteed.