Heat is the main driver of calendar ageing in a lithium battery, and it works whether the car is driven or not. A pack parked in the sun at 40°C in Penrith all January is ageing faster than the same pack in a Katoomba garage, and it is ageing whether or not anyone drives it. Kilometres matter. Heat matters more than most buyers think.
What heat does
Chemical side-reactions inside a cell — the ones that slowly consume its capacity — run faster at higher temperatures. Roughly, every 10°C of sustained pack temperature meaningfully increases the rate of loss. A pack that sits at 35°C for months does not fail; it quietly loses more capacity in that year than it would have in a cooler one.
Heat and a high state of charge compound each other. A nickel-chemistry pack sitting at 100% in a hot car park is the worst combination available.
Where it shows up in Australia
Western Sydney and the Hawkesbury — routinely the hottest part of the basin, with long stretches over 35°C. Cars that live outdoors here age faster than the same cars on the coast.
New estates with no tree cover — Oran Park, Marsden Park, Box Hill — where a driveway is full sun from dawn to dusk.
Airport and station long-stay parking — a car left at 100% in an open-air car park for a fortnight in summer has had a harder fortnight than one driven daily.
Air-cooled packs — the earlier Nissan Leaf has no active cooling and cannot manage its temperature; hot-climate Leafs are the textbook case of heat-driven loss.
What a liquid-cooled car does about it
Most current EVs cool the pack actively while driving and charging, and some will run the cooling while parked if the pack gets hot enough. That limits the damage from a hot day's driving. It does less for a car parked in the sun for weeks, because the cooling is not running.
How to tell if a car has lived hot
The State of Health figure and cell balance are the result; the temperature history, where the car records it, is the cause. A diagnostic read reports both. A three-year-old car that lived outdoors in Richmond and a three-year-old car that lived in a Mosman garage will often be a few points apart, and it is worth knowing which you are buying.
What to do about it
If you own the car: park in shade where you can, do not leave a nickel pack at 100% in the sun, and let the car precondition before fast charging on a hot day. If you are buying: where the car lived is a real question, and the measured answer is cheaper than the guess.
Suburbs we test in Western Sydney · EV battery degradation per year · Book a test — $249
Verified 15 September 2026.