Charging Systems: AC On-Board Charger, DC Fast Charge, and EVSE

Charging Systems: AC On-Board Charger, DC Fast Charge, and EVSE

How EV Charging Works at a System Level

An electric vehicle draws energy from the electrical grid through a piece of infrastructure called the Electric Vehicle Supply Equipment (EVSE). The EVSE does not itself convert power; it safely delivers grid power to the vehicle and manages the communication handshake that authorizes and controls the charge session. Understanding this division of responsibility — EVSE manages safety and signaling, vehicle converts and manages energy — is the foundation of charging system service.

Charging Levels and Connector Standards

Level 1 (AC, ~120 V) uses a standard household outlet and the SAE J1772 connector. Charge rates are slow, typically under 2 kW, making Level 1 suitable for overnight top-up or plug-in hybrid vehicles with small battery packs. The J1772 connector has five pins: two AC power pins (L1 and Neutral), a ground, a Control Pilot (CP), and a Proximity Pilot (PP).

Level 2 (AC, 208–240 V) also uses the SAE J1772 connector in North America, delivering up to roughly 19.2 kW depending on the EVSE rating and the vehicle's on-board charger capacity. The same five-pin J1772 inlet handles Level 1 and Level 2 because the difference is only the supply voltage and circuit ampacity, not the connector.

DC Fast Charge (DCFC) bypasses the on-board charger entirely, delivering high-voltage DC directly to the battery through the vehicle's DC charge port. Three connector standards are in common North American use:

• CCS Combo 1 (CCS1): Combines the upper J1772 AC portion with two large DC pins below it. Uses Power Line Communication (PLC) over the pilot circuit for the charging protocol (ISO 15118 / DIN SPEC 70121).
• CHAdeMO: A separate circular connector with its own inlet; uses a CAN bus-based communication protocol. Common on older Nissan and Mitsubishi vehicles.
• NACS (North American Charging Standard, SAE J3400): Developed by Tesla and released as an open standard; a compact connector handling both AC Level 2 and DC fast charge through the same inlet. Increasingly adopted as an industry standard.

The J1772 Control Pilot Handshake

The Control Pilot (CP) line is a 1 kHz square wave generated by the EVSE at ±12 V. Its duty cycle encodes the maximum current the EVSE can supply, allowing the vehicle to know available power before drawing it. The vehicle's charge inlet circuit places a resistor across CP and ground to signal its state. The IEC 61851-1 standard defines pilot states (often labeled A through F):

• State A: No vehicle connected; CP steady +12 V DC.
• State B: Vehicle connected, not ready; CP oscillates, vehicle pulls to +9 V.
• State C: Vehicle ready, requesting charge; vehicle pulls to +6 V (ventilation not required).
• State D: Charge with ventilation required; vehicle pulls to +3 V.
• State E/F: Fault conditions; EVSE shuts off power.

The duty cycle translates to available current linearly: for duty cycles between 10 % and 85 %, current rating (A) = duty cycle (%) × 0.6. For example, a 25 % duty cycle signals 15 A available.

The On-Board Charger (OBC)

The OBC converts the AC supply from the EVSE into the DC voltage and current required by the high-voltage battery pack. It contains two main functional stages:

1. Power Factor Correction (PFC) stage: Converts AC to an intermediate high-voltage DC bus while drawing current from the grid in a near-sinusoidal waveform, minimizing reactive power. This stage protects the grid supply.

2. Isolated DC-DC converter: Steps the intermediate DC to the exact voltage commanded by the Battery Management System (BMS) and provides galvanic isolation between the AC mains and the HV battery — a critical safety feature.

The BMS monitors cell state-of-charge, temperature, and voltage balance. It sends charge-current and charge-voltage set-points to the OBC via an internal CAN network. The OBC follows these commands, ramping current up during bulk charge and tapering it as the battery approaches full capacity.

Safety Protocols — Non-Negotiable

High-voltage charging circuits carry potentially lethal energy. Observe these rules without exception:

• PPE: Wear Class 0 insulating gloves (rated 1,000 V) tested and in-date, with leather protectors. Use insulated tools rated for the voltage class of the vehicle.
• Lockout/Tagout (LOTO): Before accessing any HV component, remove the manual service disconnect (MSD), apply a lock, and verify absence of voltage with a CAT III or CAT IV meter. Wait for the capacitor discharge interval specified in the OEM service information before touching HV conductors.
• Training vehicle only: Perform all live pilot-signal observations only on designated training vehicles with instructors present. Never probe live HV circuits without authorization and proper metering equipment.
• Orange conductors = HV: On any hybrid or BEV, treat orange-jacketed cables as energized until proven otherwise.
• Shock hazard from EVSE output: Even the EVSE output cable is energized after handshake completion. Follow manufacturer commissioning procedures.

DC Fast Charge: What Changes

During DCFC, the vehicle disables the OBC and opens the DC charge contactors directly to the battery pack. The EVSE (in this context, a DC charging station) contains its own large rectifier. The communication protocol (CCS uses ISO 15118/DIN SPEC 70121 via PLC; CHAdeMO uses CAN; NACS uses its own protocol) negotiates voltage, current limits, and session authentication before the contactors close. The BMS still governs maximum current and voltage; if thermal or voltage limits are reached, it commands the charger station to reduce output. This is why DCFC rates taper as the battery fills and heats up.