Degradation — how batteries lose capacity

Two processes run in parallel. Calendar ageing happens whether the car moves or not, and is accelerated by heat and by long periods sitting at a high state of charge. Cycle ageing comes from charging and discharging, and is accelerated by fast charging, deep discharges and low temperatures at the moment of charging.

The shape of the loss is not linear. Most packs shed a noticeable slice in the first months and then settle into a much slower decline, which is why an early reading taken alone tends to alarm people unnecessarily.

Why it matters when buying

For a new car, degradation is a warranty question: manufacturers guarantee that state of health will stay above a stated threshold for a stated period, and the thresholds differ enough between brands to be worth reading before signing.

For a used car it is the single most important unknown, and the only reliable way to resolve it is a diagnostic readout rather than the seller’s description. Some markets have independent battery testing services; where they exist, the cost is trivial against the price of the car.

Chemistry matters here too. LFP packs generally tolerate more full cycles than nickel-based ones, which is part of why manufacturers using them warrant them for longer.

Related: SOH · battery warranty · residual value