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Five things going on behind the kilowatts. Most of them aren’t the charger.

By Steven Day, Plug-n-Go · Reading time: approx. 5 minutes

You plug in at one charger and it ships energy into the car at 150 kW. Same model of charger somewhere else the next week, same car, same time of day — you’re getting 70. Or you turn up at the very same site two days running and the second session is slower. What’s going on?

Charging speed isn’t a single number that lives in the charger. It’s the result of a brief negotiation between four or five things, several of which change minute to minute. Once you know what they are, the variation stops feeling random — and you can usually do something about it.

We covered the headline equation in our other blog on what kW really means: actual rate = the lower of what the charger can deliver and what your car can accept. That’s still the big one. But four other variables decide how close to the headline number you actually get.

The charger and the car — a quick refresh

The charger has a maximum kW it can deliver. Your car has a maximum kW it can accept. The session runs at the lower of the two.

Quick numbers: a Renault 5 E-Tech caps at 100 kW on DC. A Tesla Model Y at around 175 kW. A current Hyundai Ioniq 5 at roughly 240 kW. Plug any of them into a 350 kW ultra-rapid and the session runs at the car’s number, not the charger’s. We unpacked all of this in a recent blog — read that if you want the long version.

But that’s just the ceiling. What actually happens in the first ten seconds — and what happens in minute fifteen — depends on the next four things.

Where you are in the charge

The most important hidden variable. Batteries don’t accept energy at a constant rate. They drink it in great gulps when they’re nearly empty, and sip it cautiously as they fill up.

A typical curve: from 10% to maybe 50%, the car holds its peak rate. From 50% to 70%, the rate eases off slightly. From 70% to 80%, it drops noticeably. From 80% upwards, it’s a trickle. By 95% you might be charging at 30 kW on a charger rated 150.

This is deliberate — covered properly in a future blog on the charging curve — and it’s why journey-stop strategies aim for 10% to 80%, not 0% to 100%. If you plug in at 75% and unplug at 95%, you’ve spent the whole session in the slow part of the curve. That’s not the charger letting you down. That’s just where you happened to plug in.

How warm the battery is

Cold batteries charge slowly. It’s not a fault, it’s chemistry — lithium-ion cells want to be at about 25–35°C for full-speed charging, and at near-zero they’re working at roughly half pace.

If you’ve ever pulled into a service-area charger on a frosty Monday morning after the car’s been parked outside overnight, you’ll have seen this. The number on the screen is half what it was on a Tuesday in May. Nothing’s broken. Your battery’s just cold and trying its best.

Hot weather is rarely a problem in the UK, Guernsey or Gibraltar — modern EVs cool the battery actively with their own thermal management. Forty-degree Andalucían afternoons can briefly throttle a session, but you’ll know about it (and probably want to step out of the sun anyway).

Whether someone else is using the hub

Some DC rapid sites share a single power supply across multiple bays. A site advertised as “150 kW” might mean 150 kW total — split between however many cars are plugged in at the moment.

A session that runs at 150 kW when you arrive can drop to 75 kW the moment a second car pulls in. Not a fault — designed behaviour, to keep the site delivering steadily under load. You can spot it: the kW number drops with no other explanation. Walking to a different (less popular) bay in the same hub sometimes solves it, if the bays are independently powered.

Whether your car got the memo

The variable people forget. Your car’s behaviour at a charger depends on what it knows about the journey.

Set a rapid charger as your destination in your car’s satnav and the car pre-conditions the battery on the way, warms or cools as needed, and arrives ready to drink. Pull up cold, off-route, no notice, and the car arrives unprepared. Same charger, same temperature outside, very different first ten minutes.

Worth getting into the habit. Two extra taps before you leave the house can mean a faster, cheaper session at the other end. And in winter, it can mean the difference between fifteen minutes and forty.

The charger and the car set the ceiling. State of charge, battery temperature, how busy the hub is, and whether the car was pre-warmed decide how close to that ceiling you actually get. None of these is random; all of them are knowable; most of them are partly under your control.

The Plug-n-Go take

When you find a site in the Plug-n-Go app, the kW rating, connector type, live availability and (where it applies) power-sharing notes are all visible before you arrive. Pair that with your car’s known maximum AC and DC limits and its pre-conditioning behaviour, and a session you’re about to start becomes mostly predictable.

If a session is meaningfully slower than you’d expected and you can’t work out why, our driver-support line on +44 (0)330 232 1111 is answered 24/7, by humans, every day of the year. Free from the UK; standard international rates apply from Guernsey, Gibraltar or elsewhere. Sometimes there’s a fault on the charger. More often it’s one of the five variables above, and a thirty-second chat saves a fruitless half-hour at the bay.

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