Understanding the charging curve: why 150 kW is not 150 kW

When a maker states 150 kilowatts, it means that under favourable conditions the car briefly reaches that figure at some point during charging. It does not mean it holds it, still less that it holds it from ten to eighty per cent. Power is not constant across state of charge but follows a curve that peaks early and falls afterwards.

Two cars with the same headline figure can therefore differ by half in the time they take. One reaches 150 kilowatts at fifteen per cent and holds it to fifty; the other touches it at twenty per cent for two minutes and is down to half by forty. The brochure figure is the same number and describes two entirely different cars.

If you want to see a car's curve, specialist media measurements and driver-maintained databases have them. What they show is a measurement under particular conditions rather than a promise — but it is orders of magnitude more informative than a single number.

The C-rate is the ratio of charging power to battery capacity. A 50 kilowatt-hour pack charging at 100 kilowatts sees a C-rate of 2 — at that power it would fill in half an hour if the power stayed constant. The same pack at 50 kilowatts sees 1 C, and a 100 kilowatt-hour pack at 100 kilowatts sees 1 C as well.

That is the interesting number, because the stress on a cell depends not on the kilowatts but on the current per cell. A large battery spreads the same power over more cells; each one works more gently, runs cooler and ages more slowly. So at the same kilowatt figure a large pack not only holds high power for longer, it also charges more gently.

A practical consequence for comparisons: divide peak power by capacity. A result around 1 is a moderately specified car. Well above 2 means the battery is designed for fast charging — which can be excellent, but presupposes effective cooling.

The fall in charging power as state of charge rises is called taper, and it is not a manufacturer's economy measure but chemistry. During charging, lithium ions move from the cathode into the anode and intercalate between the graphite layers. The fuller the anode, the fewer free sites near the surface and the longer the ions take to reach deeper layers.

Force more current in than the anode can take and lithium deposits as metal on the surface instead of intercalating. This plating is largely irreversible: the material is no longer available for storage, capacity falls permanently, and in the worst case dendrites grow through the separator. The battery management prevents it by pulling the current back.

There is an electrical reason too. Cell voltage rises with state of charge. The charging electronics run at constant current first and switch, once the permitted voltage is reached, to a constant-voltage phase in which the current falls by itself. Power is voltage times current — as current falls, so does power. That is why the last twenty per cent are always the slowest, on every lithium-ion battery, in every car.

The published curve applies to cells inside their operating window. Below and above it looks different. Cold cells have high internal resistance and take little current, because otherwise exactly the plating the taper exists to prevent would occur. Hot cells age faster, so the management limits at the top end as well.

Between those limits lies a window that, depending on cell chemistry, sits roughly between twenty and forty degrees. Cars with active liquid cooling hold the cells inside it even through repeated fast charging. Cars with air cooling alone cannot do that on a hot day after the second stop — power falls, not because of state of charge but because of temperature.

That is why preconditioning works so well. Enter the charger into the navigation and the car warms the battery to the window on the way. A car that arrives at five degrees of cell temperature spends the first quarter of an hour warming itself up — and at a charger that bills by time you pay for that.

Power is voltage times current. To move a lot of power you can raise the current or the voltage. Current is the expensive route: losses in the cable, connector and cell interconnects grow with the square of the current, and that heat then has to be carried away. Which is why cables for high currents are liquid-cooled and still heavy.

A vehicle architecture at around 800 volts rather than around 400 moves the same power at half the current and a quarter of the ohmic losses. That is the real reason some newer cars charge so fast — not better cells but a higher voltage level. The side effect: the charger has to be able to deliver that voltage, and older units cannot always do so.

On the other side stands the charger with its own current limit. Many fast chargers cap at a few hundred amps. A 400-volt car hits that ceiling at a given power whatever the battery could do. So charging at the highest power your car allows needs a charger that matches its voltage level — and that figure is printed on the unit.

Two charging points in one cabinet often share a power module. With a second car there, each gets half, or the one plugged in first takes priority and the second whatever is left. Which rule applies is rarely written on the unit; you notice it when power drops as soon as the neighbouring bay is taken.

The second point is the site's own grid connection. A charging park with many points rarely has the capacity to run them all at full power. Some sites buffer with a stationary battery, many simply share out what they have. At a full motorway services on the first weekend of the holidays, the car's curve is not the issue.

And finally the cable: an uncooled one limits current to what it can take thermally. So at the same cabinet a cooled cable can deliver more than the one beside it. Given the choice, take the thicker, shorter, cooled one.

The most useful figure in daily use is not peak power but the energy that goes into the battery in a given time. Twenty minutes is a realistic break; how many kilowatt-hours arrive in those twenty minutes decides your journey time. That figure integrates everything: peak, taper, temperature and voltage level.

The time from ten to eighty per cent is also common. It is better than peak power but has a catch: it flatters small batteries. A small pack reaches eighty per cent sooner and still has less energy on board. Comparing two cars, it is better to work in kilowatt-hours per unit of time, or directly in kilometres added per unit of time.

From the taper follows the rule that saves most time on a long trip: two short stops beat one long one. The curve is steepest at the bottom, so you charge where it is fast and move on before power collapses. Ten to sixty per cent twice takes appreciably less time than ten to ninety once — for the same energy added.

The exception is the final leg. If no charger follows the stop, you charge as far as you need and accept the slow percentages. The second exception is winter: there an extra stop costs another cooling of the battery if the stretch in between is short.

Planning that needs no app, just two numbers: the rough shape of your car's curve and its consumption at the speed you drive. Everything else follows.

On a car charged mostly at home, the curve barely matters. On one that regularly covers long distances it matters more than range. These are the points to settle before the test drive:

The answers are rarely in the listing but can be found: the first three in the data sheet for that model year, the fourth in published measurements, the fifth in the battery report for that particular car.