When an electric car stands for a long time

A lithium-ion battery ages along two independent paths. One depends on use: every cycle changes the electrodes a little. The other runs regardless and is called calendar ageing — it happens while the car stands, and it happens always.

Calendar ageing depends chiefly on two quantities: cell voltage, which is to say state of charge, and temperature. A high state of charge means a high voltage at the electrode surface and therefore stronger side reactions in the electrolyte. High temperature accelerates those reactions further, because chemical processes speed up as it gets warmer.

From which follow the worst and the best ways to leave an electric car. Worst: fully charged, in the sun, in a car park in summer. Best: at a middling state of charge in a cool garage. Over months the difference between those two cases is measurable in remaining capacity — and at resale that is not an anecdote but a figure in the battery report.

Almost every electric car has two batteries. The big one provides propulsion; the small one feeds control units, locking, lighting and — the decisive point — the contactors that connect the high-voltage system at all. Without the small battery the car stays dead even when the big one is full. That is the commonest failure after a long stand, and it surprises people every time.

In normal use the car tops the small battery up periodically from the big one. To do that it has to wake, and whether and how often it does depends on the car. Some models manage it reliably for months; others drop into a deep sleep after a while, or stop topping up once the main pack falls below a certain level. Which rule applies is in that car's handbook.

In practice: the small battery is the component that decides how a long stand ends, and it is also the cheapest thing in the car. A conventional charger on the designated terminals is the simplest insurance against coming back to a car that will not even unlock.

A modern car never sleeps entirely. It listens for the key, it maintains a mobile connection for the app, it monitors the alarm, and it wakes itself at intervals to take readings. Each of those functions draws little; together and over weeks they draw a great deal.

Then there is the cells' self-discharge, which in lithium-ion chemistry is small — considerably smaller than what the car's electronics draw. Finding a noticeably lower state of charge after three weeks is not a battery problem but a car that stayed awake enough to remain reachable the whole time.

Most cars therefore have a transport or deep-sleep mode that switches exactly those functions off. It exists for workshop stays and shipping, and it can equally serve your own absence. The price is that the app can no longer reach the car — which is precisely the point.

Neither full nor empty. Full means high cell voltage and therefore accelerated calendar ageing. Empty means the car goes on drawing energy for weeks and eventually reaches a level where it stops supplying the small battery — or, at worst, deep-discharges the cells. A middling charge keeps both away and leaves a reserve for the draw while parked.

Where the manufacturer names a window for extended standing, that window governs, not the general rule. It is in the handbook, and it is one of the few points where the handbook is genuinely car-specific — different cell chemistries behave differently, and what suits one is not optimal for another.

Leaving it plugged in is the most robust arrangement, provided two conditions hold: there is a settable charge limit, and the installation may be left unattended. A proper charging point may be; a mode 2 cable in a domestic socket for weeks is not intended for it. Plugged in, the car can also run its thermal management without draining the pack.

For ageing, cold is the pack's friend: side reactions in the electrolyte run slower, and a car that spends a winter in a cold garage ages less by the calendar than one in a warm climate. For operation, cold is the adversary — but only until the pack is back in its operating window.

From which follows what applies after a cold stand: drive the first few kilometres gently, expect little regeneration, and do not head straight for a rapid charger. The battery management protects itself and limits; that is not a fault but precisely the protection that prevents charging a freezing pack.

Heat is the more serious case, because it acts continuously. A car standing outdoors fully charged through several summers shows it in its remaining capacity. Where no shade is available, a lower state of charge already helps a great deal — the two factors act together, and only one of them is under your control.

The pack is not the only component that notices a long stand. Tyres lose pressure and develop flat spots at the contact patch, which a heavy car works out more slowly than a light one. Brake discs corrode, and on an electric car they start from a worse baseline, because the friction brakes see little use anyway.

Then the small things that are the same in any car: damp in the cabin, seals that stick, a pollen filter that smells in a humid vehicle, and an electric parking brake that can seize if left applied for months in a damp place. That last one is where a long stand turns into a workshop bill.

The first drive afterwards is therefore an exercise in itself. Check tyre pressures before moving. Brake firmly once on an empty stretch to clear the rust from the discs. And ignore the range display for a while — it calculates from the consumption of the last few kilometres, and there have not been any for weeks.

The effort is small and the order simple. What follows applies to three weeks as much as to six months — only the question of whether somebody can look in meanwhile grows more important with time.

Low mileage for the age is a good sign in a combustion car and an open question in an electric one. Calendar ageing runs regardless of distance, and a car that spent three years fully charged in the sun may have less remaining capacity than one that was driven for the same period.

So the question to a seller is not only how far it was driven but how and where it stood. Garage or street, which climate, plugged in or not, with what charge limit. The answer cannot be proved, but it can be set against the battery report — and when the two agree, that says something good about the seller's account in general.

Imported cars are a special case. Months can separate the previous owner from the new one, spent in a compound, in a port or on a ship — in a climate nobody chose and at a state of charge nobody monitored. That is no reason to walk away, but it is a reason to see the battery report before the purchase rather than after it.