In theory, calculating the charging time of an electric car is very simple. All you have to do is divide the capacity of the vehicle battery, given in kilowatt hours (kWh), by the power of the terminal in kilowatts (kW), then you get the time required.
Let’s take a 2023 Chevrolet Bolt with a 65 kWh battery for demonstration. Here is the time it takes to fully charge this car with a dead battery at different types of charging stations:
120V domestic socket with its own 15A (1.4kW) fuse: 46 hours and 26 minutes
Charging station 240 V level 2 (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
Easy right? Theoretically yes, but practically not. While the estimates above are pretty good for trickle charge, this is not the case at boot time.
The charging curve of an electric car: never linear
As mentioned in a previous article, fast charging is controlled by the vehicle’s charge management system (BMS). The BMS adjusts the charging current based on criteria such as temperature, battery cell voltage, state of charge, etc. For example, if the cell temperature is below a threshold specified by the manufacturer’s engineers, the vehicle will accept less charging power. The current can only increase if the battery heats up.
This results in a variable performance curve depending on temperature and time. Typically, the performance increases rapidly after the first connection and then begins to decrease after a certain state of charge. The biggest drop in performance usually occurs at around 80%. As we can see on the electrical circuit diagram below, we see that the curve follows a bell shape:
Photo: The circuit
Winter gets involved
As can be seen in the graph above, a very cold ambient temperature (-20 degrees) significantly reduces the charging power accepted by the Chevrolet Bolt’s BMS. But a big flaw of the Bolt is that even if the car was just driven on the highway, the maximum power does not exceed 30 kW, even if you connect it to a more powerful terminal.
A colleague of mine, a new Bolt owner, 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 charge curve so my Bolt can take 50kW no matter the temperature!
-Yes, but you lose your 8-year guarantee!
– No big deal, I paid for a battery, I’ll use anything.
Note that this would-be hacker is smarter than he looks:
“Meanwhile, I flush my Bolt before arriving at a quick terminal: pedal to the floor, maximum regeneration, several hard accelerations and brakings, and whoops, the batteries are getting hot and I’m getting 46kW of power at the DCFC, even in cold weather! »
As modern problems require modern solutions…
And there are indeed more modern solutions!
It should be noted that not all BMS are programmed as conservatively as Bolt’s. The direct competitor, the electric Hyundai Kona, still achieves 50 kW at -20 degrees. In addition, several manufacturers now offer battery preconditioning: while the vehicle is in motion, the system uses energy to heat the battery to a level that enables an optimal charging speed.
Finally, some manufacturers will reconfigure the BMS from one model year to another to unlock faster charging speeds. Depending on the vehicle, this update can also be performed wirelessly (over the air or OTA). With any luck, my colleague might get such a suggestion from GM for the Bolt!