Electric Vehicles
What actually limits charging speed
A charger rated at 350 kW and a car that peaks at 250 will not give you either number for very long.
Charging headline figures are peak figures. The peak lasts a short time, and everything about the session is governed by conditions the marketing does not mention.
The charging curve
A DC fast charging session does not deliver constant power. It follows a curve.
Power rises quickly at low state of charge, holds near peak briefly, then tapers — steeply above roughly fifty to sixty percent, and steeply again as the pack approaches full.
Which is why the useful metric is not peak kilowatts but the time from ten to eighty percent. That figure captures the shape of the curve rather than a single instant on it.
Charging from eighty to one hundred percent frequently takes as long as ten to eighty, for a quarter of the energy. On a road trip, it is almost always faster overall to stop twice and charge to eighty than once to full.
Why the taper exists
As lithium ions move into the anode during charging, the rate at which they can intercalate is limited.
Push current beyond that limit and lithium plates out as metal on the anode surface rather than entering the structure. Plating permanently reduces capacity and, in severe cases, creates dendrites that can short a cell.
The battery management system therefore limits current as the anode fills, which is what the taper is. It is a safety and longevity constraint, not a conservative software choice.
Temperature
The single largest session-to-session variable.
Cold cells have higher internal resistance and much lower acceptable charge rates. A pack at freezing point may accept a small fraction of its rated power until it warms.
Hot cells are also limited, because charging generates heat and the pack must stay within a safe window.
The optimum is a warm-but-not-hot pack, which is why cars with route-based preconditioning — heating the battery on approach to a planned charging stop — charge dramatically faster in winter than the same car navigating without it.
If your car supports preconditioning, using in-car navigation to route to a charger is worth several minutes per stop. Many owners never discover this.
Pack voltage and the charger
Charging power is voltage multiplied by current.
Chargers are limited by both a maximum voltage and a maximum current. An 800-volt vehicle architecture can draw high power at moderate current; a 400-volt vehicle needs much higher current for the same power, and cable and connector current limits bite.
This is the practical reason 800-volt platforms charge faster: they reach high power without exceeding the current limits of the cable.
Note also that a charger rated at a headline figure may split that power between two connectors when both are in use, so the stall next to yours affects your session.
Cable cooling
Very high current heats the cable. Chargers above roughly 150 kW generally use liquid-cooled cables, which is why they are thicker and heavier.
If the cooling system derates in high ambient temperature, the charger reduces power. This is a real cause of disappointing sessions on hot days at busy sites.
Site-level constraints
A charging site has a grid connection with a fixed capacity.
When several vehicles charge simultaneously, some sites share power dynamically, reducing per-vehicle rates. Others have on-site battery storage specifically to buffer this.
Which means an eight-stall site with a modest grid connection can deliver less per car than a four-stall site with a strong one.
AC charging is a different thing
AC charging is limited by the car's onboard charger, not by the wall unit.
A home unit rated at 22 kW delivers only what the car's onboard charger accepts — commonly 7 kW or 11 kW. Buying a higher-rated wall unit does not speed up a car that cannot use it.
For most owners, overnight AC charging at home covers daily use entirely, and DC fast charging is a road-trip tool rather than a routine one.
What this means in practice
Judge a car on its ten-to-eighty time, not its peak.
Precondition the battery before a fast charge, using in-car navigation.
Arrive at a charger with low state of charge, since the curve is steepest there. Arriving at fifty percent wastes the fast part of the session.
Stop at eighty on long trips.
And do not read a disappointing session as a fault. A cold pack, a shared site and a hot day will each cost you power, and all three together will cost a great deal.
