Why is my electric car charging so much slower than the charger says?
The charging curve that governs everything, why arriving with a full battery is the commonest mistake, and the five other reasons a 150 kW post delivers 40 kW.
- Difficulty
- beginner
- Time
- 15 min
- Read
- 6 min
Short answer
A charge post's rating is its maximum, not a promise. Your car decides the actual rate, mainly from how full the battery already is and how warm it is. Charging is fastest when the battery is fairly empty and at working temperature, and it tapers steeply as the battery fills — which is why the last twenty per cent can take as long as the first eighty. Arriving at a rapid charger with a half-full cold battery is the commonest reason for a disappointing session.
This is the biggest single gap between what an electric car advert implies and how one behaves, and it causes a great deal of unnecessary irritation. The number on the charge post — 50, 150, 350 kW — is what that unit can supply. What actually flows is negotiated by the car's battery management system, moment by moment, and its priorities are cell temperature, cell balance and long-term battery health, not your schedule. The result is a charging curve: a rate that starts high when the battery is low, holds a plateau, then tapers away steeply as the battery fills. Every EV has one and no manufacturer advertises it prominently.
Step by step
- Understand the curve before blaming the charger.Charging rate is highest at a low state of charge and falls as the battery fills, because the battery management system reduces current to control voltage, heat and cell balancing. The taper usually begins somewhere around half to two-thirds full and is steep beyond about eighty per cent. This is physics and chemistry, not a fault.
- Arrive at a rapid charger with the battery low.The practical consequence of the curve is that a session from ten to sixty per cent is far faster per unit of energy than one from sixty to ninety. On a long journey, two shorter stops from a low state of charge beat one long stop to full — this is the single biggest lever a driver has.
- Stop at around eighty per cent on a journey.The time taken to go from eighty to one hundred on a rapid charger is often comparable to the time taken from ten to eighty. Unless you genuinely need the range to reach the next charger, unplugging at eighty and driving on is faster overall.
- Precondition the battery if the car offers it.A cold battery cannot accept high current safely, so the car limits it — sometimes severely. Most modern EVs will warm the pack on the way to a charger if you navigate to it using the car's own system, or offer a manual preconditioning button. In winter this can be the difference between 40 kW and 130 kW.
- Check what your car can actually accept.The car's own maximum DC charging rate is a hard ceiling. A car that peaks at 50 kW will never charge faster than that on a 350 kW post. Similarly, AC charging is limited by the car's onboard charger — commonly 7 kW or 11 kW — so a 22 kW AC post gives many cars no benefit at all.
- Notice whether the post is shared.Many rapid chargers are installed in pairs sharing one power cabinet, so two cars charging simultaneously each get roughly half. Some are labelled; many are not. If a session started fast and halved when another car plugged in, that is what happened.
- Allow for temperature at both ends.Very cold batteries charge slowly until warmed. Very hot ones — after a long fast motorway run in summer, or after back-to-back rapid charges — also charge slowly, because the car is protecting the cells from further heat. A moderate battery temperature is the sweet spot.
- Look at the cable and the connector.Some rapid posts have liquid-cooled cables that support the highest rates and some do not. A post rated at 150 kW with an air-cooled cable may be limited in practice. A dirty or damaged connector, or one not fully seated, also reduces the rate.
- Read the session, not the advert.The car's own display usually shows the instantaneous rate in kilowatts. Watching it for the first minute tells you whether you have a good session or should try a different post — far more useful than the number painted on the unit.
Common mistakes
- Charging to 100 per cent at a rapid charger on a journey — The last twenty per cent is the slowest part of the curve, and it often takes as long as the whole of the rest. Unless you need it to reach the next charger, it is time thrown away.
- Judging a charger by its rating rather than by the car's display — The post's number is a ceiling shared between the post, the cable, the car and the battery's condition. The car's own instantaneous reading is the only figure that reflects what is actually happening.
If it doesn't work
Rapid charger delivering a fraction of its rating on a cold morning
Cause: Cold battery, current limited to protect the cells — Fix: Precondition on the way, or accept a slower session. Rate usually improves as the pack warms during charging, so the first ten minutes are the slowest.
Charging fast, then abruptly halved
Cause: Another vehicle plugged into the paired unit — Fix: Nothing wrong. If time matters, move to a standalone post; the rate should recover when the other car finishes.
AC charging at 7 kW on a 22 kW post
Cause: The car's onboard charger is the limit, not the post — Fix: Expected on most EVs. Check your car's AC rating before paying a premium for a fast AC unit.
Charging much slower after two rapid stops in a row
Cause: Battery temperature is now too high — Fix: The car is protecting the pack. A slower session, or a break, lets it cool. Plan longer journeys with slightly longer gaps between rapid stops.
Charging speed fine but the cost is much higher than expected
Cause: Some networks charge by time as well as by energy, or apply overstay fees — Fix: Read the tariff on the unit or in the app before starting. Overstay penalties after a session ends are common and are enforced.
Session will not start at all
Cause: Payment, authorisation, connector seating or a network fault — Fix: The catalogue's public charging guide covers the sequence. Reseating the connector firmly resolves a surprising proportion.
Questions people ask
Is slow rapid charging damaging my battery?
No — the slowing is the protection working. The battery management system reduces current precisely to avoid the heat and voltage stress that would damage cells. A session that tapers is a car looking after itself.
Why does the last 20 per cent take so long?
As the battery fills, the car holds the voltage constant and lets the current fall, and it also balances the individual cells so they finish level. Both processes are inherently slow and neither can be rushed without shortening the pack's life.
Should I always precondition?
Whenever you are heading to a rapid charger in cold weather, yes — it is the biggest single improvement available. It is less useful before a slow AC charge at home, where the rate is low enough that battery temperature barely matters, and it uses energy to do it.