Rapid Charging: Why Charging Your Electric Car May Not be as Rapid as You Think

Two of the fundamental pieces of knowledge anyone has with their electric car is how quickly it can charge and what types of connections it can use.

Drivers know that if they plug into, say, a CHAdeMO rapid charge station their car will take around 20 minutes to charge from 0 – 80 per cent but how true is this? More and more we are hearing about cases where this isn’t what happened and people have been plugged in much longer than they thought they would be.

We run down the most common reasons why below.

Battery temperature

The nearer the optimum your battery temperature the less throttling of power you'll see.

The nearer the optimum your battery temperature the less throttling of power you’ll see.

The temperature of your car’s battery plays a big role in how fast it can be changed. Too cold and the charger will restrict the power flowing to it, too hot and it will do the same. It is true that on a 22C day in the middle of the UK summer a car may be able to charge from 0 – 80% in twenty minutes. But when the external temperature is 2C and was sub-zero over night, the charger just simply can’t put energy into the battery as fast.

Simply put, when it is too cold, your battery will take longer to fill up.

Some cars, like the Tesla Roadster/Model S and the Ford Focus EV have thermal battery management meaning that even in the depths of winter their battery should be at the optimum temperature for charging giving their users a more stable charging time.

The same applies when it is too hot out. The car won’t allow the battery to be charged as fast, or at all in some cases, until its cells have cooled down. We have seen some Tesla Roadsters in hot climates after a day of driving sit cooling the battery for upwards of 30 minutes before the car allowed any power to flow to the batteries.

Where in the pack you are charging

Ever wondered why rapid charging times are always quoted as 0 – 80% rather than from flat to full? That’s because the higher the state of charge in the battery the slower they have to charge. Overcharging a battery is not something that you would want to do, not that you’d be able. Electric cars have many safeguards built in to make sure this doesn’t happen and one of them is to throttle down the charging rate the nearer to 100% the battery gets.

In fact, when using a rapid charger at the very top of the pack, it is often not putting in any more power than the car’s standard on-board charger would.

When using a rapid charger the final 20% of a battery can take as long, if not longer than the first 80%. It’s for this reason that rapid charging times are quoted as they are.

There is one take away point from this: If you are waiting for a 100% charge and you are using a rapid charger, if someone else comes along do move to a standard ‘fast’ charger rather than hogging the rapid. You’ll be getting the same amount of power and the next person will be able to take advantage of the increased speed from their battery being more empty than yours.

Power available to the station

The power you think you are getting may not always be available at that site.

The power you think you are getting may not always be available at that site.

In the grand scheme of things 40 – 50kW of power isn’t that much. But as most rapid chargers are installed at sites that already have a high demand on their power supply, for example motorway/highway service stations and car dealerships on industrial estates, sometimes mustering up 40kW is an issue.

In these cases, a driver pulling up will see a normal rapid charger that is lower powered than they presume. 20 or 22kW CHAdeMOs can be found in some locations that will take twice as long – or more – than a standard fully powered charger. There’s no easy way to identify these units from looking at them. One would hope that the charging network that installed them would know the power level of their charging stations but in our experience this isn’t always the case.

Sometimes displayed on the charging station you’ll see the voltage and amperage of the power being sent to your car. In these cases a quick bit of maths will help you work out the current power level. Volts times amps equals kilowatts. Just remember that you won’t get the full power a charging station can supply all the time.

Three-phase and single-phase

When it comes to rapid and fast AC charging, one area to further complicate things is the difference between single and three-phase power supplies especially when charging stations are referred to with their amperage.

A 16A charging station would be 3.7kW single-phase but 11kW three-phase. A 32A charging station would be 7.3kW single-phase but 22kW three-phase.

Only certain cars can make use of all three phases of power, for example the Renault Zoe and the European Tesla Model S. If you don’t have one of these cars, if you plug in to a 22kW charging station you’ll only get the single phase level of power.

It comes down to knowing a small amount about how your car works and some basic understanding of electricity.

Do you know of any charging stations that work at a lower power level than you would expect? Have you ever waited longer than you thought due to one of these reasons or one not listed? Let us know below.


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