In some situations, you may notice a difference between the energy or power delivered by the charger and the charged energy shown in your car infotainment system or app. This is most common on AC charging, but with some vehicles it can also be seen on DC charging.
In this article, we explain the charging process. We hope it is as interesting to you as it is to us. A bit of dry theory first.
AC/DC
Besides being the name of a legendary Australian band, AC/DC are also abbreviations for two forms of electric current:
- AC - Alternating Current
- DC - Direct Current
The electrical grid (your home socket or the cable feeding a charger) supplies alternating current (AC).
Your EV battery stores energy as direct current (DC).
The EV motor(s) operate on AC. The inverter, powered by the battery, converts DC back to AC and drives the motors.
So, for your car to charge and then drive, two things must happen:
- AC from the grid must be converted to DC so it can be stored in the battery.
- While driving, the inverter must convert DC back to AC to power the motor(s).
We will focus only on step 1, converting AC to DC for battery charging. This happens in two different ways depending on whether you use AC or DC charging.
What happens during AC charging?
With AC charging, your vehicle receives current from the charger in its grid form: AC. Inside the car there is a device called an onboard charger, which converts AC to DC so it can charge the battery. The maximum power of this onboard charger is what determines how fast your car can charge on AC stations, typically between 6kW and 22kW depending on the vehicle.
Some EVs use one of their motor inverters for this purpose instead of a separate onboard charger, but the principle is the same.
This conversion process is not 100% efficient and generates heat. Heat is also generated in cables while current flows, which adds extra losses.
Efficiency depends on many factors. Many onboard chargers are quite efficient at full power and less efficient at lower power levels.
| Losses | |
|---|---|
| Best case | 8% |
| Most common | 12% |
| Worst case | 20% |
What happens during DC charging?
With DC charging, the overall process is similar, but conversion from AC to DC happens in the charger itself. Fast chargers are generally more efficient at AC-to-DC conversion, but it still depends on requested power, ambient temperature and other conditions.
| Losses | |
|---|---|
| Best case | 5% |
| Most common | 9% |
| Worst case | 15% |
Important: on DC charging, the station bills based on already converted DC energy. The inefficiency of AC-to-DC conversion is borne by the station operator, i.e. Fines Charging.
Are there losses besides AC-to-DC conversion?
Yes. Your EV may use a small (or sometimes significant) portion of station power to heat or cool the battery. Some vehicles can use up to 10kW for this for short periods, though average battery thermal conditioning is often in the 2kW to 7kW range.
Also, if cabin heating or AC is running during charging, part of the incoming power is used there too. This can briefly reach high values on some vehicles, though typically it stays lower. In many modern EVs, thermal conditioning of the battery and cabin is part of the same system.
Tesla-specific note
Tesla vehicles often display energy that actually went into the battery and may exclude energy used for battery and cabin thermal conditioning. This can make the difference between station-delivered and battery-added energy appear larger.
Conclusion: is the charger "lying"?
No. Charging stations use highly sensitive and precise electricity meters, often much more precise than residential meters. Even small deviations can matter because the vehicle controls how much power and energy it can accept at every moment.
If your vehicle shows a difference versus station readings, it is usually because the car displays battery-added energy, while the station displays energy delivered to the vehicle.
