ISO 15118-20 and Bidirectional Charging

By Michael Samuel Naeem · January 15, 2026 · 5 min read

Fleet operators want chargers that pull power from vehicles during peak tariff windows — but only if the car, charger, and grid operator agree on discharge limits in milliseconds, not via a phone app. ISO 15118-20 is the updated vehicle-to-charger communication stack that adds bidirectional power transfer (BPT) messaging, refined certificate flows for plug-and-charge, and modular documents separating AC from DC semantics. If you are building SECC firmware on a charge point or integrating EVCC on a TCU, 15118-20 is the path from "dumb Level 2 socket" to grid-aware energy asset.

Protocol stack overview

Application (15118-20 EXI messages: SessionSetup, ChargeLoop, BPT...)
        │
   TLS 1.2/1.3 + mutual cert auth (Plug & Charge)
        │
      TCP/IP (typically IPv6 on PLC per ISO 15118-8)
        │
   ISO 15118-3 / DIN 70121 (PLC physical layer)

Session flow (simplified):

1. SDP — SECC IP discovery
2. SupportedAppProtocol — negotiate 15118-2 vs -20
3. SessionSetup / ServiceDiscovery
4. Authorization — EIM (external) or PnC (certificate)
5. PowerDelivery / ChargeLoop — dynamic limits, BPT schedules
6. SessionStop

Bidirectional power transfer (BPT)

15118-20 defines BPT variants for AC and DC:

• AC_BPT — discharge from onboard inverter back to grid
• DC_BPT — discharge via DC link (vehicle-dependent architecture)

Key message concepts:

<!-- conceptual EXI element names, not literal schema -->
<BPT_ScheduledMode>
  <DepartureTime>2026-01-15T17:00:00Z</DepartureTime>
  <EVTargetEnergyRequest>-5.0 kWh</EVTargetEnergyRequest> <!-- negative = export -->
  <EVMaximumDischargePower>7.4 kW</EVMaximumDischargePower>
</BPT_ScheduledMode>

EV sends discharge capabilities; SECC merges grid operator limits from EMS; both sides enforce ramp rates.

Implement state machines carefully — mode transitions mid-session require renegotiation without unsafe power steps.

Plug and Charge certificates

PnC uses V2G PKI:

• OEM provisioning certificate (PCID) installed at manufacture
• Contract certificate from mobility operator
• SECC leaf certificate on charger

15118-20 refines CertificateInstallation and CertificateUpdate flows for contract cert rotation without dealer visit. Misconfigured trust stores cause the silent fallback to RFID — test cross-OEM chains in interoperability plugs.

SECC implementation notes

Embedded Linux SECC stack typical components:

// pseudocode session state
typedef enum {
    SECC_IDLE,
    SECC_TLS_HANDSHAKE,
    SECC_SESSION_ACTIVE,
    SECC_BPT_DISCHARGE,
    SECC_ERROR_STOP
} secc_state_t;

void on_charge_loop(BPT_ChargeLoopReq *req) {
    grid_limit_w_t grid_cap = ems_get_export_limit();
    res->EVSEMaxDischargePower = min(req->EVMaxDischargePower, grid_cap);
    res->EVSEProcessing = Ongoing;
}

Integrate with ISO 15118-3 PLC modem driver; latency jitter on HomePlug affects TLS timeout tuning.

Log EXI messages hashed for audit — full payload logs may contain contract identifiers (GDPR).

Differences from 15118-2 deployments

Field chargers often support dual stack — detect vehicle capability and branch.

Testing and certification

• CharIN test cases for interoperability events
• Hardware-in-loop simulators (e.g., EVerest, RISE V2G) for SECC regression
• Grid code coupling tests with simulated IEEE 1547 ride-through

Certification paths vary by market — UL 9741, CE grid codes — protocol compliance is necessary not sufficient.

EMS integration for V2G

ISO 15118-20 handles vehicle-to-charger communication. Grid integration requires an Energy Management System (EMS) layer:

Grid operator ←→ EMS ←→ SECC ←→ EVCC ←→ Vehicle BMS
                  ↑
            IEEE 1547 / EN 50549 limits
            Tariff signals (OpenADR, EEBUS)
            Fleet scheduler API

The SECC must merge three constraint sets in real-time:

1. Vehicle limits — max discharge power from BMS via EVCC
2. Charger limits — hardware capability of the power module
3. Grid limits — export cap from EMS based on grid code and tariff

void compute_discharge_limit(BPT_ChargeLoopReq *ev_req) {
    float ev_max = ev_req->EVMaximumDischargePower;
    float evse_max = hardware_get_max_discharge();
    float grid_max = ems_get_export_limit();  // from IEEE 1547 ride-through

    res->EVSEMaxDischargePower = min(ev_max, evse_max, grid_max);
    res->EVSEMaxDischargeCurrent = res->EVSEMaxDischargePower / voltage;
}

If EMS communication fails, default to zero export — never discharge without grid authorization.

Open source SECC stacks

Building SECC from scratch is years of work. Open source stacks accelerate development:

• EVerest — modular C++ charging station framework with ISO 15118-20 support, OCPP integration, and simulated EVCC for testing
• RISE V2G — Java-based reference implementation for EVCC and SECC development/testing
• Solidstudio EV — commercial stack with CharIN certification support

Use simulators for regression testing before hardware-in-loop:

# EVerest: start SECC with simulated EVCC
everest start --config configs/15118-20-ac-bpt.yaml
everest trigger --module evse_manager --action start_charging

Interoperability testing

Cross-OEM interoperability is the hard part — Tesla, VW, Hyundai, and Ford each implement 15118-20 with vendor variations:

• CharIN testival — industry interoperability events; bring SECC + multiple EVs
• Certificate chain testing — PnC fails silently to RFID if trust store doesn't include OEM root CA
• SAP negotiation — vehicle offers 15118-2 only; SECC must fall back gracefully
• BPT mode transitions — switching charge→discharge mid-session without power spike

Maintain a compatibility matrix: vehicle model × firmware version × test result. Update after every firmware release on either side.

Failure modes

• PnC certificate chain mismatch — silent fallback to RFID; user thinks plug-and-charge is broken
• BPT without EMS grid limit — uncontrolled export trips grid protection relay
• 15118-2 vs -20 SAP failure — session doesn't start; need dual-stack support
• PLC link drop mid-BPT session — power must ramp down safely within ISO 15118-3 timing
• GDPR in EXI logs — contract certificates contain personal identifiers; hash, don't log raw

Production checklist

• Dual-stack 15118-2 and -20 SAP negotiation supported
• EMS integration with grid export limits enforced
• PnC certificate trust store includes target OEM root CAs
• BPT mode transitions tested without power spikes
• Open-source simulator regression tests in CI
• CharIN interoperability event attended before market launch
• EXI message logging GDPR-compliant (hashed identifiers)

Resources

• ISO 15118-20 standard (ISO store)
• CharIN e.V. — ISO 15118 industry group
• EVerest open-source charging stack
• V2G Clarity / Hubject PnC ecosystem
• IEEE 1547 — interconnection standard for DER