Fast-charge thermal limits in the real world
Why the charging curve tapers, and how thermal limits interact with SOC limits.
- Explain fast-charge thermal limits in the real world at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real vehicle
Overview
The charging curve you see on a road trip is the visible output of several thermal and electrochemical limits working together, thermal management being one of the biggest.
In depth
Estimate a fast-charge session
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.
A single fast charge rarely challenges the thermal system. Several in quick succession — as on a long road trip — accumulate heat and reveal how well a design manages sustained thermal load, not peak load.
Key takeaways
- The charging curve reflects both electrochemical and thermal limits.
- Sustained back-to-back fast charging is a harder thermal test than a single session.
- Distinguishing thermal from electrochemical tapers helps explain unexpected slowdowns.
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