EV charging and onboard chargers

A battery can only accept DC. Mains supply is AC. Every question about charging speed comes back to one thing — where that conversion happens, and what constraints apply in that place.

Where the conversion happens

On AC charging, the wall supplies alternating current and the car rectifies it internally using its onboard charger. That unit is carried in the vehicle every mile of its life, so it is kept small, light and cheap. Typically 7 kW on a single phase or 11 kW on three.

On DC charging, a large rectifier in the forecourt cabinet does the conversion and feeds the pack directly. That cabinet is bolted to the ground, mains-cooled and has no mass budget, so it can be as large as the economics allow. The car’s onboard charger is bypassed entirely.

What the pins actually do

The power contacts are the obvious part. The signalling pins are where the intelligence lives, and they are deliberately crude so that they cannot fail obscurely.

Pin	What it carries
Control pilot	A 1 kHz square wave. Its duty cycle encodes how much current the supply can provide; the car pulls its voltage down through resistors to announce “connected” and then “ready”
Proximity pilot	A resistor in the plug encoding that cable’s own rating, so the car never exceeds the cable regardless of what the supply offers. It also inhibits driving away
Protective earth	Connects first and disconnects last, by pin length
Power contacts	Two for DC, or line and neutral, or three phases

Notice that basic AC charging needs no protocol at all. A duty cycle and two resistor values are enough to charge safely. Everything cleverer is layered on top of that, and if the clever layer fails the simple one still works.

Connectors

• Type 2 / CCS2 — the European AC connector, with two extra DC pins below it for fast charging. One port serves both.
• Type 1 / CCS1 — the older North American equivalent of the same idea.
• NACS (SAE J3400) — the North American convergence. Smaller, because the same two pins carry AC or DC rather than having separate sets.
• CHAdeMO — an early DC standard, now largely confined to Japan, and notable for having supported bidirectional flow long before anyone else.
• GB/T — the Chinese standard, separate AC and DC ports.

The charge curve

To push current into a cell, the applied voltage must exceed the cell’s own resting voltage. As the cell fills, that resting voltage rises — so the headroom shrinks, and the current must come down to stay within safe limits and avoid plating lithium onto the anode.

This is why a rate is always quoted 10 to 80 percent. The last fifth takes as long as the first three fifths, and on a journey it is nearly always faster to leave and stop again.

Two things move the curve:

• Temperature. A cold pack is limited from the first second, no matter what the charger offers. Pre-conditioning on the approach usually pays for itself several times over.
• Pack voltage. Heating in the cable and contacts goes with current squared. An 800 V pack needs half the current for the same power, so a quarter of the loss — which is what makes rates above about 150 kW practical without heavily cooled cables.

ISO 15118 and what digital communication adds

The analogue pilot can only say how much current is available. ISO 15118 carries a digital protocol over the same wire, and that changes what is possible:

• Plug and charge. The car presents a certificate and is billed automatically. No card, no app, no account at that particular network.
• Scheduled charging. The car and the supply agree a plan, so charging can follow a tariff or a grid signal rather than starting the instant it is plugged in.
• Bidirectional negotiation. Power flowing the other way needs an agreement about how much, for how long and on what terms. That conversation is what the protocol provides.

Sending power back out

Vehicle to load treats the car as a generator running an isolated island — a tool on a site, lights at a camp, a house disconnected from the grid. Comparatively simple.

Vehicle to grid exports into the live mains, and that is a different problem:

• The output must synchronise with grid frequency and phase.
• It must stop instantly when the grid fails. Otherwise a line that everyone believes is dead stays energised while someone works on it. This anti-islanding requirement is why grid export is regulated and vehicle to load largely is not.

The attraction is real — a parked fleet is a very large distributed store, and the grid’s hardest problem is matching supply to demand minute by minute. The obstacles are cycle life on the pack, metering and settlement, and the simple fact that the vehicle’s owner needs it to be charged when they want to leave.

What to take away

• A battery takes only DC. AC charging converts inside the car and is limited by a unit the car carries everywhere; DC charging converts outside and bypasses it.
• The pilot pins are deliberately crude — a duty cycle and two resistors — so that safe charging does not depend on a protocol working.
• The charge curve tapers because cell voltage rises as it fills, not because the charger is holding back. Hence 10 to 80 percent.
• Cold throttles charging from the first second, so pre-conditioning usually saves more time than it costs.
• 800 V halves the current for the same power and quarters the cable heating, which is what makes very high rates practical.
• ISO 15118 adds identity, scheduling and the negotiation that bidirectional flow requires.
• Exporting to the grid must synchronise and must stop the instant the grid fails.

That completes the electric vehicle and high voltage module. If the pack side of it is still hazy, battery cells, chemistries and packs and battery management systems are where the limits that govern charging are set.

Next, how the vehicle perceives the world around it — ADAS sensing and perception.