An electric car plugged into a fast charger takes on energy rapidly at first and then progressively slower. The taper is not a fault or a limitation of the charger but a deliberate consequence of how lithium cells behave.
Charging current is limited by cell voltage
A lithium cell's voltage rises as it fills. Pushing current into it requires the charger to hold a voltage above the cell's own, and the gap available shrinks as the cell approaches full.
Beyond a defined ceiling the cell begins to degrade rapidly, so the battery management system holds voltage at that limit and lets current fall away instead.
The result is a charge curve that is high and roughly flat at low states of charge, then bends downwards through the upper half of the pack's capacity.
Lithium plating is the real danger
Charging moves lithium ions into the graphite structure of the negative electrode. That structure accepts ions at a finite rate, and the rate falls as it fills up.
Push current faster than the electrode can absorb it and lithium deposits on the surface as metal instead. That metal is permanently lost capacity, and in the worst case it grows structures that can short the cell internally.
The management system cannot see plating directly, so it works to a conservative model that reduces current well before the condition would occur.
Heat sets the other ceiling
Every amp pushed through a cell produces heat through internal resistance, and the heat rises roughly with the square of the current. Fast charging therefore generates a great deal of it.
Cooling systems can remove heat only so quickly. Once the pack reaches its temperature limit, the only remaining option is to reduce the current until the cooling catches up.
This is why a second rapid charge on the same journey is often slower than the first: the pack starts warm and reaches its limit sooner.
Why charging times are quoted from ten to eighty
The published figure covers the portion of the curve where charging is fast. Filling the last fifth can take almost as long as the first four fifths.
That is also the practical advice for long journeys. Two shorter stops in the fast part of the curve usually beat one long stop waiting for a full pack.
The last few per cent involve a very low current while cells are balanced against each other, which is useful at home overnight and close to pointless at a motorway charger.
What the driver can influence
Pack temperature is the main variable within reach. Many cars will precondition the battery to its optimum window if a rapid charger is set as a navigation destination.
Arriving with a lower state of charge also helps, since the car spends more of the session in the steep part of the curve rather than the taper.
Charger power matters less than people expect once these limits bind. A car that accepts a given peak will not exceed it regardless of how powerful the post is.