EV charging systems

Every EV battery charges on DC. The only question is where the grid's AC becomes DC: inside the car, in a small on-board charger, or in a large cabinet beside the parking space. That one choice explains the power levels, the connectors and most of the cost of a charging site.

AC charging: the charger is in the car

Level 1 uses a 120 V outlet at 12 A, about 1.4 kW, a few miles of range per hour. Level 2 uses 208-240 V at up to 80 A, 7-19 kW: 32 A at 240 V is 7.7 kW and 80 A is 19.2 kW. The wall box, properly the EVSE, is not a charger at all. It is a contactor, a ground-fault monitor that trips at 20 mA, and a signalling circuit. The car's on-board charger (OBC) does the AC-to-DC conversion, and its rating, commonly 7-11.5 kW, sets the ceiling because every kilowatt of converter rides around in the car. In Europe, Type 2 sockets add three-phase power: 400 V at 16 A is 11 kW and at 32 A is 22 kW.

The EVSE talks to the car over the control pilot, a 1 kHz, ±12 V square wave whose duty cycle advertises the most current the car may draw: amps = duty × 0.6 between 10 % and 85 % duty, so 53.3 % means 32 A. The car answers by loading the pilot: 9 V means plugged in, 6 V means ready for power.

DC fast charging: the charger is the cabinet

A DC fast charger (50-350 kW) moves the converter off the car. Grid AC, usually 480 V three-phase in North America, is rectified with power-factor correction to a DC link of roughly 800 V. An isolated DC-DC stage (a high-frequency inverter, transformer and rectifier) then produces whatever voltage and current the car requests. The transformer isolates the battery from the grid, so the car's high-voltage system stays floating and an insulation monitor can catch a single fault. Cabinets stack identical power modules of tens of kilowatts each, so one design scales and a charger can keep working at reduced power when a module fails. At 95 % efficiency a 150 kW charger still turns about 8 kW into heat, so fans or chillers are part of the design.

The cable is a thermal problem too. Above roughly 200-250 A a passively cooled cable gets too thick and heavy to handle, so high-power dispensers use liquid-cooled cables that carry 500 A or more through a thin, flexible conductor, with temperature sensors at the connector pins that cut the current if they get hot.

Connectors

Tesla opened its connector as the North American Charging Standard in November 2022 and SAE standardized it as J3400; most automakers have since announced a move to it, with adapters bridging the CCS1 fleet. China uses its own GB/T connectors.

Protocols: ISO 15118 and OCPP

ISO 15118 (and its predecessor DIN SPEC 70121) is the conversation between car and charger, carried as powerline communication over the pilot wire. The car states its limits, the charger states its own, and during charging the car requests current in a continuous loop that the charger follows. ISO 15118 also defines Plug & Charge: the car stores a contract certificate and is authenticated and billed automatically, with no card or app; ISO 15118-20 adds bidirectional power. OCPP (Open Charge Point Protocol; 1.6 and 2.0.1 are the most widely deployed versions) is the conversation between charger and the operator's backend: JSON over a WebSocket for authorization, transactions, meter values, status, firmware and smart-charging profiles that cap a charger's power. Because OCPP is open, a site owner can change backend providers without replacing hardware.

Designing the site

Charger internals drive the site design. Power flows from the utility's medium-voltage line through a pad-mount transformer (for example 12.47 kV to 480Y/277 V), main switchgear and feeders to the power cabinets and dispensers. The NEC treats EV charging as a continuous load, sized at 125 %, but NEC 625.42 lets an automatic load management system set the design load instead of the nameplate sum. That is how eight 150 kW dispensers can share 600 kW of cabinets: at any moment most sessions are tapering or idle, so the transformer and service are sized near 600 kW rather than 1.2 MW, and the utility's demand charge on the monthly peak shrinks with them.

Accessible stalls are designed in, not added later. In California, CBC Table 11B-228.3.2.1 sets how many accessible EV spaces a site needs and Sections 11B-812.6 and 11B-812.7 their geometry (2022 edition, unchanged in 2025): every space is 216 in (18 ft) long, a van-accessible space is 144 in (12 ft) wide and a standard accessible space 108 in (9 ft) wide, each with a 60 in (5 ft) access aisle, and an ambulatory space is 120 in wide with no aisle. Operable parts, including the connector, must sit within accessible reach ranges and work with one hand, so cable weight, reach and cable management matter as much as amps; lighter liquid-cooled cables help.

V2G, V2H and interoperability

With a bidirectional charger the car becomes a battery for the home (V2H) or the grid (V2G). CHAdeMO did this first; ISO 15118-20 brings it to CCS and NACS, and AC versions need a bidirectional on-board charger. Exporting to the grid needs utility interconnection approval and certified equipment, such as UL 1741-SB inverters in the US. Interoperability is the industry's sore point: many failed sessions are communication failures (SLAC timeouts, certificate errors, a failed insulation check) rather than broken power hardware. The US NEVI program's minimum standards (23 CFR 680, in force since 2023 and kept by FHWA's 2025 guidance revision) require at least four DC ports that deliver 150 kW each at the same time (680.106), OCPP and ISO 15118 conformance (680.108) and 97 % uptime per port (680.116).

Play the DC session, then raise the other stalls' load until the site cap, not the dispenser, limits this car. Switch to AC to see the pilot duty cycle change with the power the load manager can give.