A Glossary of Charging Terms

Charging is the part of electric vehicle ownership with the most vocabulary per unit of understanding. Two people can argue for an hour about how fast a car “charges” while using the same word for power, energy, and time — three different quantities.

This glossary is ordered by how much confusion each term causes.

Power and energy

Kilowatt (kW). A rate of energy transfer. A charger’s output and a car’s charging speed are both measured in kilowatts. It is a flow, like litres per minute.

Kilowatt-hour (kWh). A quantity of energy. A battery’s capacity is measured in kilowatt-hours. It is a volume, like litres.

The practical consequence: a 100 kWh battery charged at 100 kW takes about an hour in theory, ignoring losses and the fact that no car charges at a constant rate. Most confusion about charging follows from mixing these two up — a “350 kW charger” delivers no energy at all until a car is plugged into it and asks for some.

C-rate. Charge or discharge current expressed relative to the battery’s capacity. A 1C rate would charge a pack in one hour at constant current; 2C would take half an hour. Cells have C-rate limits that vary with chemistry, temperature and state of charge, which is why the same car charges differently on a cold morning.

AC and DC, and what the car actually contains

Alternating current (AC). What comes out of a wall. Home charging and most workplace charging is AC, typically at modest power.

Direct current (DC). What a battery stores. Public fast charging is DC.

Between them sits hardware: an onboard charger, which converts AC to DC inside the car. Its rating is the ceiling for AC charging — a car with an 11 kW onboard charger will never take more than about 11 kW from a wallbox, whatever the wallbox can supply.

DC fast charging bypasses the onboard charger and feeds the pack more directly, under the control of the battery management system. That is why DC power figures are much higher than AC ones, and why the car’s own limits — cell temperature, state of charge, thermal strategy — dominate at a fast charger.

The curve beats the peak

Charging curve. The plot of accepted power against state of charge. It rises to a peak, holds there briefly, then tapers as the pack fills.

Taper. The deliberate reduction in power at higher states of charge, which protects the cells from damage. Taper is engineering, not laziness.

Peak power. The highest point on the curve. It is the number in the press release and the least useful figure for planning a journey.

The useful summary is average power across the window you actually use — usually 10 to 80 percent. A car that holds a moderate figure across that window will beat a car with a spectacular peak that collapses early, on the same road trip, at the same charger.

Preconditioning. Heating or cooling the pack before a fast charge so it can accept more power. It is why a car that has been driving for an hour charges faster than one that has been parked in the cold.

A charging curve is a negotiation between the charger, the pack and the temperature. The peak is only the opening offer.

Connectors, briefly

J1772. The North American AC connector. It carries AC only.

CCS1. The North American DC standard: J1772 with two additional pins for DC. It is what most non-Tesla fast charging has used.

CCS2. The European equivalent, built on the Type 2 AC connector instead.

Type 2. The European AC connector.

NACS. The connector formerly known as Tesla’s, now standardised through SAE as J3400 and adopted by most manufacturers for North America. It handles both AC and DC in one shape.

CHAdeMO. An older DC standard, still present on some legacy vehicles and a shrinking number of chargers.

Plug and charge. Authentication handled by the car and the charger rather than an app or card, using the ISO 15118 family of standards. It works when both sides support it.

Words that mean less than they sound

“Ultra-fast.” Undefined. Ask for kilowatts, and for the state-of-charge window in which they are delivered.

“Charges to 80 percent in X minutes.” True under stated conditions and misleading outside them: starting from a low state of charge, at a specific temperature, with a preconditioned pack, on a charger that can supply the power.

“350 kW capable.” A property of the charger, not the car. The car will take what it can take.

“Free charging.” Usually a commercial arrangement with a time limit, a power limit, or a network restriction. Read the terms.

How to use the vocabulary

Two habits will keep you out of most arguments.

First, separate power from energy in every sentence you read or say: a charger supplies kilowatts, a battery holds kilowatt-hours, and time is what connects them.

Second, when someone quotes a charging figure, ask for three things: the charger’s power, the starting and ending state of charge, and the temperature. Without those, the number is a claim rather than a measurement.

The vocabulary is not complicated once the units are separated. The marketing around it is complicated because a peak number is easier to print than a curve — and because a curve requires you to know what you are looking at.