EVs in the heat: charging, cooling, parking

A lithium-ion cell works best in a temperature window of roughly twenty to forty degrees. Below it the cell is sluggish; above it, it performs well but ages faster. The side reactions that cost capacity speed up as temperature rises, and not evenly but ever more steeply the hotter it gets.

Unlike cold, heat leaves marks that stay. A cold battery gives its capacity back as soon as it is warm. A battery that has spent many summer hours hot and fully charged has lost part of its capacity for good. The combination does the most damage: high temperature and high charge at the same time, over long periods.

What counts is the temperature of the cells, not of the air. A car in the midday sun on dark tarmac heats up from above and below. A battery after a long motorway run and a fast-charging stop has been warmed by its own current. You influence the first through where you park, the second through how you drive and charge.

Most current electric cars have liquid cooling for the battery, linked to the air-conditioning refrigerant circuit. When it gets too warm, a heat exchanger cools the coolant and the cells stay in their window, while driving, while charging and, on some models, while parked. That cooling costs energy, but it is the most important protection a car has against heat.

Some older models cool the battery only passively, through air and the casing. On those, the heat from driving and fast charging stays in the cells, and on hot days the battery management noticeably limits charging power, often from the second fast-charging stop of a trip. For a car that lives in the south or does many long summer trips, the type of cooling is therefore a buying criterion.

Whether and when the cooling runs while parked differs from maker to maker. Some cars cool the battery when parked once it reaches a critical temperature, provided there is enough charge; on the cable, the energy comes from the grid. So a car standing in the sun in high summer with a nearly empty battery has less room to protect itself.

Fast charging produces heat in the cells, and on a hot day it meets a battery that is already warm. The battery management keeps the cells below their limit by cutting charging power. A stop after a long, fast motorway run in August can therefore be slower than the same stop in spring, even though the car arrives with the same charge. That is not a fault; it is protection.

The charger can limit too. Its power electronics and cable need cooling as well, and in great heat and direct sun some units reduce their output. A charger in the shade or under a roof is often the quicker choice on a hot afternoon.

Winter preconditioning has a summer counterpart: entering the charger in the navigation system makes many models bring the battery into its window before arrival, which in the heat means cooling it down. Add a few simple rules:

Cooling costs less than heating. The air conditioning moves heat with a compressor rather than generating it, and the difference between inside and outside is usually smaller in summer than in winter. Consumption therefore rises less in heat than in cold. How much depends mainly on the start of the trip: a cabin that has stood in the sun must first be cooled down, and that draws the most power in the first few minutes.

In town this share weighs more than on the motorway, because the air conditioning draws a roughly constant power while the drivetrain needs little at low speed. On the motorway aerodynamic drag dominates and the air conditioning hardly shows. What helps:

For ageing, what counts is how long the battery sits hot and full. The charge level is in your hands: set a daily charge limit that covers your needs, charge to full only before long trips, and then so that charging ends shortly before departure. The range the manufacturer recommends for daily use is in the owner's manual; with some cell chemistries, regular full charges are explicitly intended.

Where you park is only partly in your hands. A garage or multi-storey car park is much cooler in summer than an open lot, shade helps, and light-coloured ground heats up less than dark tarmac. If you park in the heat for longer, at the airport or during the holidays, leave the battery neither full nor nearly empty: full speeds up ageing, nearly empty leaves the car no reserve for cooling and on-board electronics.

Heat also affects parts outside the battery. Tyre pressure rises when the tyres are warm, so measure and set it cold, in the morning or after the car has stood in the shade for a while. The 12-volt battery ages faster in the warmth, even if it often shows its weakness only the following winter. Plastics, seals and wiper blades suffer in the sun.

An electric car from southern Spain, southern Italy or Greece is not automatically worse than one from the north, but its battery has a different history. Mileage says less than on a combustion car, because calendar ageing runs faster in the warmth even if the car was hardly driven. What counts is a recent battery report showing remaining capacity, not the range display on the dashboard.

Ask the seller how the car was kept and charged: garage or street, on the cable or not, with what charge limit, how often at fast chargers. The answers cannot be proven, but they can be checked against the battery report. Also check whether the battery has active liquid cooling, and test the air conditioning on the test drive; on many models the battery cooling shares the same circuit.

Outward signs complete the picture: faded plastics, cracks in the dashboard, brittle seals, tyres with sidewall cracks despite good tread. If the car crosses a border, the import comes into it too, along with the question of whether the battery warranty applies in the new country; some warranties are regionally limited. On Carvexia a seller can state whether the car was imported from another country. That is the seller's statement, not checked; ask to see the battery report in any case.