What charging really is
Moving energy from the grid into a pack, and everything that stands in the way.
- Explain what charging really is clearly and accurately
- Read the numbers that define the behaviour
- Apply the idea to a real charging session
Overview
Charging looks like plugging in a kettle. Underneath it is a negotiated, monitored, actively cooled power transfer with three computers agreeing on every ampere.
In depth
Who decides the power?
The car does. The charger advertises what it can supply, the cable declares its rating, and the BMS requests a current within its own limits. The lowest number in that chain always wins.
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.
Key takeaways
- The car requests, the charger supplies.
- The weakest link sets the power.
- Power is voltage times current.
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