The Charging Station Is the Grid Interface
Every vehicle-to-grid design starts with one location question: where does the bidirectional inverter live? Put it on the vehicle and you have V2G-AC. Put it in the EVSE and you have V2G-DC, where the charging station does all the grid-facing work and the car just hands over stored energy.
That single choice puts the entire grid-facing job in one box, so bringing an interoperable V2G-DC charger to market comes down to proving two separate things about it: that its inverter meets the local grid code, and that it speaks the bidirectional charging protocol the car expects.
Those two answer to different standards and test in different ways, and getting them straight before you scope a program separates a defined target from an open-ended one.
The charging station is the DER — and its grid code is the key
A grid code defines the functions any DER must perform to support the grid: voltage and frequency ride-through, active and reactive power control, anti-islanding, and the response curves that govern how a device reacts as grid conditions drift out of range. In V2G-DC, those functions live in the EVSE inverter, because that is where the AC/DC conversion and grid interconnection happen. So the EVSE is the DER, and only the EVSE has to be certified for grid-code compliance — the vehicle does not carry grid-code certification in the DC topology. That is the clean part: the device under test is one box, not a vehicle-and-charger pair.
But grid codes are regional — a product certified for one market is not compliant in another, and for a V2G-DC EVSE the decision is whether to certify for one or more markets:
- North America. DER interconnection and conformance testing are governed by IEEE 1547-2018 / IEEE 1547.1-2020, adopted state by state; a V2G-DC EVSE certifies against the UL 1741 SB supplement. California’s Rule 21 requires UL 1741 SB certification for any DER that exports to the grid, plus IEEE 2030.5 CSIP certification for devices that talk directly to a utility’s DERMS.
- European Union. DER interconnection is standardizing on EN 50549, applied together with individual country grid codes. (Other regions have their own codes — Australia, for example, requires AS/NZS 4777.)
Common to all regions for V2G-DC: the EV–EVSE communication link is not specified in the interconnection rules[1]. The grid code governs how the inverter behaves electrically; how the car and charger talk is a separate standard — and that is where ISO 15118-20 comes in.
ISO 15118-20 enables the car and the charger to conduct bidirectional power transfers — but it is not the grid code.
ISO 15118 is the open IEC/ISO standard that manages EV-to-EVSE interactions. Today’s deployed version handles DC charging — rate of charge, start and stop, payments, etc. — but not bidirectional power flows. Most ISO 15118 implementations today only cover EVSE-EV functions for DC charging and is not useful by itself for V2G.
ISO 15118-20 is the version that changes that. It introduces the bidirectional-power-transfer functions that let a car and a DC charging station negotiate an export session — the handshake V2G-DC needs at the connector, standardizing an interface that until now has largely been custom on the older ISO 15118-2. The critical point for anyone scoping a program: it carries the communication functions, not the grid-code functions. An interoperable V2G-DC charger has to satisfy two independent standards at once and certifying one does not get the other certified.
Why interoperable V2G-DC is harder to test than it looks
The one-box grid-code story is simpler than V2G-AC; the interoperability story, while less complex, is not simple. Getting a charger to work in the field means clearing several hurdles at once:
- Three V2G related certifications on one device. Grid-code conformance and charging-protocol interoperability are separate proofs on separate standards[2]; a charger can pass one and fail the other. The third certification is for the EVSE to CPO or other aggregator of EVSEs and DERs.
- The bidirectional protocol is still maturing. ISO 15118-20 is new and its formal certification is not yet in place — the CharIN Alliance is still standing up CCS programs, so products shipping now aim at a settling target.
- The EVSE-CPO protocol in widest use is OCPP. It too is a maturing standard, especially, the newest version that supports grid code communications, OCPP 2.1.
- Interoperability is multi-OEM, not pass/fail. A charger that conforms on the bench can still fail against a specific vehicle; real interoperability is proven across many EV/EVSE pairings — the reason CharIN “Testivals” exist — and re-proven per region. And the same is true for upstream OCPP communications to a CPO. Fortunately, unlike EV-EVSE interoperability, once installed and interoperable, this aspect of V2G does not change frequently.
Residential V2G-DC?
Residential V2G today is largely DC primarily due to lack of EVs with grid compliant inverters. The early home pilots, like California’s BRIDGE program (2025–2027, with Nuvve, Fermata, and Kia), run on bidirectional DC chargers. What is unsettled is cost and rules. Because the inverter sits in the charging station rather than the car, a residential DC unit is inherently more expensive than an AC equivalent; QL’s own analysis puts a DC system at roughly twice the installed cost of the AC version. Our view is that V2G-AC makes the most sense for residential V2G while V2G-DC makes more sense for higher-power applications such as fleets, school buses and some other non-residential applications. The landscape will be messy for a time as different combinations of OEMs bring V2G-DC and V2G-AC systems to market.
How QualityLogic tests and certifies interoperable V2G-DC
Covering the conformance testing of the grid code compliance is exactly where QualityLogic works. It built the first automated certification test product for UL 1741 SB testing. It supports the DER protocols IEEE 1547 requires (IEEE 2030.5, SunSpec Modbus, IEEE 1815.2) and drives the full grid-code sequence end to end, including for V2G-DC EVSEs. QualityLogic’s automation cuts the Type and Interoperability a grid-code certification test process from 4-5 weeks of manual effort to as little as 32 hours.
A major aspect of the UL 1741 SB certification is interoperability testing to validate that one o the required communications protocols to the EVSE has been implemented correctly. That is a key feature of the QualityLogic test tools.
For the EU, QualityLogic has enhanced it’s grid code test automation tools to support EN 50549. The enhanced EU version includes adjustable parameters to accommodate individual country grid codes along with analysis of the test results to assess compliance. The EU does not yet require specific communications protocols for DERs so the 1547 interoperability testing is not applicable to the EU unless the EVSE already supports a standard communication protocol for US applications – IEEE 2030.5, SunSpec Modbus or DNP3.
On the interoperability side, QualityLogic’s CCS Analyzer takes on the car–charger conversation. It is a software-only tool that analyzes DIN 70121, ISO 15118-2, and ISO 15118-20 charging sessions from packet-capture files, with no added hardware, isolating interoperability problems and flagging timing and conformance anomalies. It is used in multi-OEM interoperability work, including at the American Center for Mobility for their interoperability testing.
That is the whole V2G-DC picture in one place — the proven foundation QualityLogic is extending into V2G-AC certification testing, a role it is proud to hold as part of the efforts shaping these standards as they’re developed.
If you are scoping an interoperable V2G-DC program — a bidirectional charger, a test lab, or an inverter line moving into V2G — talk to us about our test tools, training, and consulting.
********
- [1] However, the EU is mandating ISO 15118-20 support in new and refurbished EVSEs starting January 1, 2027.
- [2] A major issue with ISO 15118 is that, while a certification program is offered by CharIN, it is not mandated anywhere to date and therefore not used. Grid code certifications, however, are mandated and heavily used.
********
