- The Physical Layer: PLC on the Control Pilot Pin
- The Actors: SECC and EVCC
- The Handshake Sequence: Step by Step
- EIM vs Plug and Charge: How Authentication Works
- The IEC 61851-1 Fallback: What Happens When ISO 15118 Fails
- V2G: Why the Protocol Exists But the Infrastructure Does Not
- ISO 15118-20 and India's Trajectory
- Key Takeaways
Most EV owners experience a CCS2 fast charge session as: plug in, tap the app or RFID card, wait for charging to start, unplug. What actually happens between "plug in" and "charging starts" is a precisely sequenced digital negotiation between two complex systems — the car's EV Communication Controller (EVCC) and the charger's Supply Equipment Communication Controller (SECC) — running over a Power Line Communication (PLC) channel that did not exist in charging equipment a decade ago. ISO 15118 is the protocol standard that defines every message in this exchange. Understanding it resolves the mystery behind failed sessions, 'communication error' messages, the missing reality of Plug and Charge in India, and the technical wall blocking V2G deployment.
- ISO 15118-2 defines the application-layer protocol for CCS2 DC fast charging communication. It runs on a HomePlug Green PHY PLC network established on the control pilot (CP) wire of the charging cable.
- The EVCC (car) and SECC (charger) go through a defined sequence: SLAC network discovery → IPv6 address assignment → TLS session → service discovery → charging parameter negotiation → cable check → precharge → current demand loop.
- Plug and Charge (PnC) — automatic authentication via X.509 certificates — is defined in ISO 15118-2 but unavailable in India because Indian charging networks have not deployed the backend PKI infrastructure.
- When the ISO 15118 PLC handshake fails, both devices fall back to IEC 61851-1 PWM control pilot — simpler, no smart features, but sufficient for basic DC charging.
- ISO 15118-20 (2022) adds V2G (bidirectional DC), V2H, and wireless charging communication — none of which is currently deployed or standardised in India.
The Physical Layer: PLC on the Control Pilot Pin
The control pilot (CP) pin is a signal wire that has existed in EV charging since IEC 61851-1 was published. In its original function, it carries a PWM signal whose duty cycle communicates the charger's maximum available current. This signal operates at 1 kHz and requires nothing more sophisticated than a comparator circuit on each end.
ISO 15118 adds a second, orthogonal communication layer on the same CP wire: a Power Line Communication (PLC) channel using the HomePlug Green PHY specification. HomePlug Green PHY was specifically developed for EV charging applications — it is a power-efficient subset of the HomePlug AV specification that operates on the physical cable at up to 500 kbps using Orthogonal Frequency Division Multiplexing (OFDM) across frequencies from 1.8 to 30 MHz.
The two signals coexist on the same wire because they occupy different frequency bands: the IEC 61851-1 PWM operates at 1 kHz (well below the HomePlug Green PHY band), while the PLC data operates at 1.8–30 MHz. Band-pass filters separate them on each device.
The CP pin in a CCS2 connector carries three things simultaneously: the IEC 61851-1 PWM duty cycle signal (1 kHz, communicating maximum available current), the HomePlug Green PHY PLC data channel (1.8–30 MHz, carrying ISO 15118 messages), and the DC offset voltage whose level indicates connector connection state (±12V open, 9V connected, 6V charging). All three are decoded independently by each device. A damaged or contaminated CP pin contact is the most common cause of CCS2 session failures in Indian field deployments.
The Actors: SECC and EVCC
SECC (Supply Equipment Communication Controller) is the hardware and software stack inside the charging station that manages ISO 15118 communication. It is connected to the charger's power electronics and to the charging network backend (via OCPP or proprietary protocol). The SECC is the ISO 15118 server — it listens for and responds to EVCC requests.
EVCC (EV Communication Controller) is the hardware and software stack inside the vehicle. It is integrated with the BMS and vehicle control unit. The EVCC initiates the ISO 15118 session and drives the charging parameter requests. It is the ISO 15118 client.
The Handshake Sequence: Step by Step
The complete ISO 15118-2 session from plug insertion to current flow follows a defined sequence. Each step has a timeout; if a step fails or times out, the session either retries or falls back to IEC 61851-1.
Immediately on plug insertion, the EVCC begins broadcasting SLAC ATTEN_CHAR_IND messages on the CP PLC channel to discover if an SECC is present. The SECC responds with attenuation profile data. Both sides select the optimal QAM modulation scheme for the cable path. This establishes the HomePlug network between EVCC and SECC. Duration: 600–1200 ms.
Both devices assign themselves IPv6 link-local addresses using their PLC MAC addresses. The SECC acts as the DHCP server providing the EVCC with a full IPv6 address and gateway. Duration: ~300 ms.
The EVCC initiates a TLS 1.2 session to the SECC using the V2G Root CA certificate chain. For External Identification Means (EIM) sessions — the standard in India — this is a lightweight TLS handshake without vehicle certificate exchange. For Plug and Charge sessions, this is where the EVCC presents its ISO 15118 vehicle contract certificate. Duration: 1–3 seconds.
The EVCC sends a ServiceDiscoveryReq. The SECC responds with a list of supported ISO 15118 services (DC charging, internet access, optional V2G services). The EVCC selects "DC charging" and requests the service parameters. Duration: ~500 ms.
The EVCC sends its charging requirements: maximum DC voltage it can accept, maximum current, energy amount requested, departure time (for smart charging scheduling). The SECC responds with what it can supply. Both sides agree on the session parameters. Duration: ~500 ms.
The EVCC commands its charging relay to close and requests the SECC to apply a low-voltage test to the DC pins to verify cable insulation integrity. The SECC measures the insulation resistance; values below a threshold cause the session to abort. This is a safety check against damaged cables. Duration: 2–10 seconds.
The SECC ramps up its output voltage to match the battery pack voltage before the vehicle's main contactor closes. This prevents the large current surge that would occur if the charger voltage and battery voltage were significantly mismatched at contactor closure. The EVCC monitors the DC voltage convergence and closes the main contactor when the precharge voltage is within tolerance. Duration: 2–5 seconds.
The main charging phase. The EVCC sends periodic CurrentDemandReq messages specifying its desired charging voltage and current. The SECC responds with CurrentDemandRes confirming the actual delivered voltage and current. This request-response loop runs every 250 ms throughout the charging session — the EVCC can adjust its request dynamically (increasing current as BMS allows, reducing current for thermal derating). This loop is where all the active charging physics occurs.
When the EVCC decides to end the session (target SOC reached, user unplug request, or fault), it sends a PowerDeliveryReq with ChargeProgress=Stop. The SECC ramps down output current and voltage. The EVCC opens its main contactor. Session termination messages complete the ISO 15118 session close.
EIM vs Plug and Charge: How Authentication Works
ISO 15118-2 defines two authentication modes for DC fast charging sessions:
EIM (External Identification Means) — the user authenticates through an external method: RFID card, NFC tap, or mobile app QR code. The ISO 15118 session uses TLS for encryption but does not use vehicle-level certificate authentication. All Indian CCS2 fast charging today uses EIM.
Plug and Charge (PnC) — the vehicle authenticates automatically using an X.509 certificate provisioned into the car's secure element by the OEM or charging network. The TLS session includes mutual authentication — both the charger and the car present certificates verified against a root CA. If validation succeeds, the charging network backend authorises the session and billing begins automatically. No RFID card or app needed.
| Aspect | EIM (India today) | Plug and Charge (ISO 15118-2) |
|---|---|---|
| Authentication method | RFID card / QR code / app | X.509 certificate in car's secure element |
| Requires network backend infrastructure | OCPP session management | Full PKI: root CA, sub-CA, certificate provisioning |
| User action required | Tap card or open app | None after initial setup |
| Session start time | 3–8 seconds after authentication | Automatic on plug-in |
| Privacy | Depends on network operator policy | Certificate-based pseudonymity possible |
| Availability in India | All CCS2 stations | Not currently deployed |
| OEM requirement | None beyond CCS2 hardware | Must provision vehicle with ISO 15118 certificate |
The PKI (Public Key Infrastructure) required for Plug and Charge has three layers: an ISO 15118 Root CA (operated by an industry body or consortium), Sub-CAs operated by charging networks, and leaf certificates provisioned into individual vehicles. Establishing this infrastructure requires coordination between OEMs, charging network operators, and a root CA operator — coordination that has been achieved in Europe (CharIN's Certificate Management System, or CMS) but not yet in India.
The IEC 61851-1 Fallback: What Happens When ISO 15118 Fails
ISO 15118-2 specifies a fallback mechanism for when PLC communication fails to establish within 5 seconds of plug insertion. In this case, both devices revert to IEC 61851-1 PWM control pilot mode:
- The charger sets its CP PWM duty cycle to indicate maximum available current (e.g., 16% duty = 10A, 50% = 25A, 96% = 80A)
- The car reads the duty cycle, applies its own BMS current limit, and requests power delivery by pulling the CP voltage to 6V (closing an internal resistor)
- The charger delivers DC power at the negotiated level
- No Plug and Charge, no digital parameter negotiation, no smart charging profile — just a current limit handshake
The practical implication: most "communication error" failures reported by Indian CCS2 users are actually ISO 15118 PLC failures that successfully fall back to IEC 61851-1. The charging session proceeds, but slower (IEC 61851-1 has less granular current control) and without advanced features. A failure that does not fall back — where the session simply does not start at all — indicates hardware: a damaged CP pin, a failed HomePlug modem in the charger, or a vehicle EVCC software fault.
Indian monsoon conditions are a specific reliability challenge for CCS2 PLC communication. Moisture on the connector surface increases impedance on the CP pin, attenuating the HomePlug Green PHY PLC signal. SLAC can fail in wet conditions even with an undamaged connector. If you experience repeated CCS2 communication failures during monsoon season, wipe the connector and inlet with a dry cloth before inserting — this simple step reduces moisture-related PLC attenuation significantly.
V2G: Why the Protocol Exists But the Infrastructure Does Not
ISO 15118-2 includes support for V2G (Vehicle to Grid) power transfer — where the EV battery supplies energy back to the grid rather than receiving it. The protocol provides this through the V2GTP (V2G Transfer Protocol) and a bidirectional CurrentDemandReq model where the SECC can request negative current (power export from vehicle).
The protocol is there. The infrastructure is not, in India, for several compounding reasons:
Hardware: V2G requires a bidirectional DC-DC converter in the charger that can both supply DC to the battery and accept DC from the battery and invert it back to grid-compatible AC. CCS2 chargers installed in India today are unidirectional — designed only to supply power. The additional hardware adds 30–50% to charger cost.
Grid regulations: Indian DISCOMs operate under a regulatory framework that does not currently permit customer-to-grid energy export from EV batteries. The Electricity Act 2003 and its subsequent amendments define net metering (for solar PV) but not EV battery export. Without regulatory approval, a V2G charger cannot be commercially operated.
Metering and billing: Exporting energy from an EV battery to the grid requires a bidirectional energy meter at the customer connection point and a tariff structure that compensates the EV owner for exported energy. Most Indian residential connections use unidirectional meters.
Vehicle support: No currently manufactured EV sold in India supports V2G energy export — the bidirectional power electronics required are not present in any mainstream Indian-market vehicle's charging system.
Some third-party V2H (Vehicle to Home) adapters are available in India for certain EV models — these use the car's existing DC charging inlet to run a portable inverter from the battery, powering appliances without grid connection. While useful for backup power, these devices operate outside any AIS-138 certification framework. Using them does not violate the car warranty in most cases (check the OEM's specific statement), but they are not subject to the same safety testing as BIS-certified charging equipment. Use only devices from established manufacturers and do not attempt to connect them to household wiring — use only isolated outlet connections.
ISO 15118-20 and India's Trajectory
ISO 15118-20 (2022) extends the protocol to address second-generation use cases:
- BPT (Bidirectional Power Transfer): Full V2G and V2H communication at the protocol level, including dynamic current negotiation in both directions
- Wireless Power Transfer (WPT) communication: Defines the data link for inductive charging systems (ISO 15118-8 provides the physical layer)
- AC bidirectional: V2H via AC-coupled systems
- Enhanced service discovery: More efficient session setup
India's trajectory toward ISO 15118-20: the AIS-138 Part 2 amendment references ISO 15118-2. An update to reference ISO 15118-20 would require a fresh BIS rulemaking process, industry consultation, and certification test procedure development. Given the pace of the 15118-2 adoption — which took from 2014 (standard published) to 2022 (AIS-138 mandate) — expecting India to formally adopt ISO 15118-20 before 2027–2028 would be optimistic. The practical gateway is not the protocol standard but the DISCOM regulatory framework for V2G export — which is the slower-moving dependency.
CharIN's Test Specification for CCS2 and ISO 15118-2 requires interoperability testing between vehicle and charger implementations before certification. India's BIS testing under AIS-138 Part 2 references this test specification for the communication layer. The implication: CCS2-certified equipment in India should, in principle, have passed interoperability tests. When sessions fail in practice, the causes are typically connector degradation, firmware bugs introduced by post-certification software updates (both in chargers and vehicles), or SLAC/PLC signal quality issues from physical damage or moisture — not fundamental protocol incompatibility between certified devices.
Key Takeaways
- Every CCS2 fast charge session involves a structured ISO 15118-2 protocol exchange: SLAC network setup, TLS session, service and parameter discovery, cable check, precharge, and a continuous current demand loop — all before the first ampere flows. Understanding this sequence explains every category of session failure.
- The CP pin carries three simultaneous signals on different frequency bands: IEC 61851-1 PWM (1 kHz), HomePlug Green PHY PLC (1.8–30 MHz), and DC offset state voltage. CP pin degradation — from corrosion, moisture, or mechanical wear — is the most common root cause of CCS2 session failures in Indian field deployments.
- Plug and Charge is defined in ISO 15118-2 and technically supported by CCS2 hardware in India, but not operationally available because Indian charging networks have not deployed the required PKI backend infrastructure.
- V2G is architecturally supported by ISO 15118-2 but absent from India for hardware, regulatory, metering, and vehicle capability reasons. Commercial V2G deployment in India is at minimum 3–5 years away.
- When ISO 15118 PLC handshake fails, both devices fall back to IEC 61851-1 PWM control pilot. Most session failures recover via this fallback and proceed with basic DC charging — the loss is advanced features, not the charging session itself.