Electric Performance
The Battery Temperature Window: Preconditioning Explained
A cold pack has high internal resistance and cannot deliver peak current. A hot one is being protected. Full performance exists only in a band, and preconditioning is how a car gets there.
An electric car's full performance exists only inside a temperature band, and outside it the control unit reduces available current for two different reasons. Cold cells have high internal resistance, so drawing peak current would drop the voltage and, worse, risk plating lithium on the anode. Hot cells are being protected from accelerated ageing. Preconditioning exists to place the pack inside that band before it is asked for anything.
The cold end, and why it is a hard limit
A lithium cell's internal resistance rises sharply as temperature falls. Two consequences follow, and the second is the reason the limit is not negotiable.
- Voltage sag. Drawing current through a higher resistance drops terminal voltage, and since power is voltage multiplied by current, the pack simply cannot deliver its rated output. This one is physics and the car has no choice.
- Lithium plating. At low temperature, lithium ions intercalate into the anode more slowly. Pushed hard, they can deposit as metallic lithium on the surface instead, which is permanent capacity loss and, in the worst case, a safety concern. This is why charge acceptance is restricted even more aggressively than discharge when cold.
The practical result is a car that feels ordinary on a cold morning and transformed twenty minutes later. Nothing has warmed up in the way an engine does. The control unit has simply raised a limit.
The hot end, which is a choice rather than a wall
| Condition | Discharge power | Charge acceptance | Reason | Nature of the limit |
|---|---|---|---|---|
| Cold | Reduced | Heavily reduced | Internal resistance, plating risk | Physical and protective |
| In the window | Full | Full | Cells at their design point | None |
| Warm from use | Full | Reduced | Charging generates further heat | Protective |
| Hot | Reduced | Heavily reduced | Accelerated ageing, thermal runaway margin | Protective |
The asymmetry in the third row matters on a long journey. A pack warmed by hard driving still delivers full discharge power while its ability to accept charge has already fallen, which is why fast charging after a spirited drive is slower than the same charge after a gentle one.
Where the bands sit
Exact thresholds are manufacturer specific and rarely published, but the shape is consistent across lithium ion chemistries in road vehicles.
| Cell temperature | Discharge power | Fast charging | What is happening |
|---|---|---|---|
| below 0 °C | Heavily limited | Generally blocked | Plating risk, high internal resistance |
| 0 to 10 °C | Reduced | Heavily reduced | Resistance still high, warming under load |
| 20 to 40 °C | Full | Full | The design window |
| 40 to 50 °C | Full | Reduced | Charge limit falls first |
| above 50 °C | Reduced | Heavily reduced | Ageing protection |
Two things follow from the table. The window that gives full performance is roughly 20 °C wide out of an operating range spanning 70 °C or more, so a car spends a fair amount of its life outside it. And the charge and discharge limits are not symmetrical: charging is restricted at both ends before discharge is, which is why a car can feel fully powerful and still charge slowly.
What preconditioning actually does
Preconditioning uses energy to move the pack into its window before it is needed, and the direction depends on the situation. In cold weather it heats the pack, using a resistive heater, a heat pump or, in some designs, deliberately running current through the cells so their own internal resistance warms them from the inside. Before a fast charge in warm weather it may cool the pack instead, so that charge acceptance is at its highest on arrival.
Two consequences follow. Preconditioning costs range, because the energy comes from the pack. And it takes time, because the pack's thermal mass resists being moved quickly, which is why navigation-linked preconditioning starts many minutes before arrival rather than at the charger.
The thermal mass is worth quantifying, because it explains the timescale. Lithium ion cells have a specific heat capacity of roughly 1,000 J per kg per kelvin, so a 600 kg pack needs about 600 kJ to raise its temperature by 1 °C, and 6 MJ, which is 1.67 kWh, to move it by 10 °C. A 5 kW heater would take 20 minutes to do that, and a 10 kW one still needs 10 minutes.
Warming the cells from the inside is faster and cheaper. At 200 A through 50 milliohms of internal resistance, the pack dissipates 2 kW within itself, with no heat exchanger in the path at all. Some designs do this deliberately by circulating current that performs no useful work, which is why a cold car can be brought into its window simply by being driven for a few minutes.
Why this shapes every published figure
A quoted 0 to 100 km/h time assumes a pack inside its window at a good state of charge. So does a quoted charging curve. Neither states the assumption, and both are reproducible only when it holds.
It also explains why a set of records produced in one session is a stronger claim than a single figure. Rimac's 23 performance records in a single day, independently verified, demonstrate that the thermal system can hold the pack in its window through repeated full power events, which is a different and harder claim than any one acceleration time.
Questions readers ask
What battery temperature does an EV need for full performance?
A band rather than a single value. Below it, internal resistance rises and current is limited to avoid voltage sag and lithium plating. Above it, current is limited to protect the cells from accelerated ageing. Full power exists only between the two.
Why is my car slower when cold?
Because cold cells have high internal resistance, so drawing peak current would drop the pack voltage, and because pushing current into cold cells risks depositing metallic lithium on the anode. The control unit reduces available power for both reasons.
What does preconditioning do?
It moves the pack into its temperature window before it is needed, heating it in cold weather or cooling it before a fast charge in warm weather. It costs range, because the energy comes from the pack, and it takes time because of the pack's large thermal mass.
Why is charging slower after a hard drive?
Because charge acceptance falls before discharge power does. A pack warmed by hard driving can still deliver full output while its ability to accept charge has already been reduced, and charging itself generates further heat.
Does preconditioning use a lot of energy?
Enough to be visible on the range display, since it comes from the pack. Heat pump systems are considerably more efficient than resistive heating, and some designs warm the cells from the inside by running current through them, using their own internal resistance.
Sources
- Rimac Nevera sets 23 performance records in a single day, on repeated full power events under independent verification.
- UNECE Regulation No. 85, on power measurement for electric drivetrains and the conditions it specifies.
- WLTP range against reality, on cold weather range shortfalls of 10 to 23 per cent, which share the same underlying cause.