What is top-up charging?
This is the method that most EV drivers take advantage of, which is to plug in and charge whenever you get an opportunity, usually at a public charger on the street or in the supermarket car park. So rather than waiting for the battery to run flat completely, it’s actually easier and quicker to simply keep the cells topped-up, which means you’ll usually have plenty of charge and so rarely suffer from range anxiety. Owners usually combine this technique with a full charge overnight using a domestic slow or rapid charger.
How much range do you get per hour of charging?
As you’d expect, the answer to this depends on the charger you’re using, and to a lesser extent your car (not all are capable of rapid charging). However, as a guide you can expect a standard 3kW slow charger to add 10 miles of range after an hour. Step up to a fast charger with a delivery of 7kW, which is the rate most domestic wallboxes charge at, and after 60 minutes you’ll have added up to 30 miles. By contrast, a 50kW rapid charger can theoretically add 175 miles of range in this time – although you’ll need a large battery to achieve this, as with smaller units you’ll be getting into the last 20 percent of capacity when charging slows. It’s a similar case with 150kW chargers, but as an indication of how effective they are, you can add 100 miles of range in as little as 10 minutes.
Factors that affect charging speed
There are a number of factors that can have an influence on charging speed, particularly when it comes to a full charge. For starters there’s the size of the battery, with the larger the capacity the longer it takes to be replenished.
Then there’s the amount of charge that’s already in the battery, with the first 80 percent of capacity being filled much more quickly than the last 20 percent – this is when the charging rate slows considerably, especially on fast or rapid chargers.
Your car’s onboard adapter will also dictate the rate of charging, so if it’s rated at 50kW that’s maximum you’ll get, even if you plug into a 150kW charger. The same is true of the charger you use, so a car with 150kW capability will only charge at 7kW on a fast charging wallbox. You’ll also see slower charging when the weather is cold, particularly on a rapid charger. Battery heating systems try to keep the reduction in speed to a minimum, but they draw extra current when doing so, which adds to the slightly increased charging time.