DC fast charging and the vehicle inlet
What is different when a station, not the on-board charger, does the conversion.
- Explain dc fast charging and the vehicle inlet at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real drive system
Overview
DC fast charging moves the AC-to-DC conversion off the vehicle entirely, which is what allows the enormous power levels — hundreds of kilowatts — that would be impractical to package inside a car.
In depth
The DC fast charger negotiates directly with the vehicle's BMS over a communication protocol (CCS, NACS or CHAdeMO depending on region and platform), then supplies DC current straight to the battery through contactors, bypassing the on-board charger's conversion stages entirely.
Charging Time Calculator
How long a session takes between two states of charge, including charger efficiency.
t (h) = capacity × (target − start) ÷ 100 ÷ (power × efficiency)Only the energy between the two states of charge has to be delivered. Losses in the cable, on-board charger and cells mean the wall power is higher than the power reaching the cells, so we divide by the efficiency.
Example: A 60 kWh pack charged from 20% to 80% needs 36 kWh. At 50 kW and 92% efficiency: 36 ÷ (50 × 0.92) ≈ 0.78 h ≈ 47 minutes.
- Real DC sessions taper above ~60% SOC, so the result is optimistic for target values above 80%.
- AC charging is limited by the car's on-board charger, not by the wallbox.
- Cold packs accept far less power until preconditioning has warmed them.
Fast-charging power is not constant: it tapers as the pack fills and as cell temperature or voltage limits are approached, which is why fast-charge calculators and real sessions diverge above roughly 60–70% state of charge.
Key takeaways
- DC fast charging bypasses the on-board charger, sending DC directly to the pack.
- The vehicle's BMS still negotiates and limits the session in real time.
- Charging power tapers well before 100% due to voltage and thermal limits.
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