In theory, calculating the charging time of an electric car is very simple. Indeed, all you have to do is divide the capacity of the vehicle’s battery, noted in kilowatt hours (kWh) by the power of the terminal in kilowatts (kW), then you get the time required.
As a demonstration, let’s take a 2023 Chevrolet Bolt with a 65 kWh battery. Here is the time required to fully charge this car from an empty battery on different types of charging stations:
120V residential outlet with dedicated 15 amp (1.4 kW) fuse: 46 hours and 26 minutes
240V level 2 charging station (7.2 kW at 30 amps): 9 hours and 2 minutes
Intermediate power terminal (24 kW): 2 hours and 42 minutes
Fast charging station (50 kW): 1 hour and 18 minutes
Simple, right? In theory yes, but in practice not really. While the above estimates are pretty good for trickle charging, it’s not the case when ramping up.
The charging curve of an electric car: never linear
As mentioned in a previous article, fast charging is controlled by the vehicle’s charging management system (BMS). The BMS sets the charging current according to criteria of temperature, battery cell voltage, charge level, etc. For example, if the cell temperature is below a threshold predetermined by the manufacturer’s engineers, the vehicle will accept less charging power. Only by warming the battery can the current increase.
This results in a variable power curve depending on temperature and time. Typically, the power will increase rapidly following the initial connection, then, after a certain level of charge, will begin to decrease. The biggest power drop usually occurs around 80%. As we can see on the Electrical Circuit graph below, we see that the curve follows a bell shape:
Photo: The electrical circuit
Winter gets involved
As seen in the graph above, a very cold ambient temperature (-20 degrees) will considerably decrease the charging power accepted by the BMS of the Chevrolet Bolt. However, a major flaw of the Bolt is that even if the car has just been driven on the highway, the maximum power will not exceed 30 kW, even if you plug into a more powerful terminal.
A colleague of mine, a new owner of Bolt, made the following comments after experiencing this situation:
-I can’t wait to download software that will allow me to access the BMS and linearize the charging curve so my Bolt can take 50kW regardless of temperature!
-Yes, but you will invalidate your 8-year warranty!
-No big deal, I paid for a battery, I’ll use it all.
Note that this hacker budding is smarter than it appears:
“Meanwhile, before arriving at a fast terminal, I rinse my Bolt: pedal to the floor, maximum regeneration, several strong accelerations and braking, and presto, the batteries become hot and I obtain 46 kW of power at the DCFC, even in cold weather! »
How modern problems require modern solutions…
And there are more modern solutions, indeed!
It should be noted that not all BMS are programmed as conservatively as the Bolt’s. For example, its direct competitor, the electric Hyundai Kona, can still reach 50 kW at -20 degrees. Additionally, several manufacturers now offer battery pre-conditioning: while the vehicle is in motion, the system uses energy to warm the battery to a level that allows for optimum charging speed.
And finally, some manufacturers will reconfigure the BMS from one model year to another in order to unlock a faster charging speed. Depending on the vehicle, this update can also be performed wirelessly (over the air or OTA). With a little luck, my colleague may receive such a proposal from GM for the Bolt!