Battery Degradation

Battery degradation is the slow, permanent erosion of an electric vehicle battery's ability to store energy and deliver power, accumulating gradually over the pack's service life. Unlike a temporary loss of range in cold weather, degradation is irreversible: the chemistry of the cells changes in ways that cannot be undone, so the battery that leaves the factory with a given capacity will hold progressively less as the years and miles accrue.

Two broad mechanisms drive the process. Calendar ageing occurs simply with the passage of time, as side reactions within the cells slowly consume the lithium and corrode the electrodes even when the car is parked. Cycle ageing results from repeated charging and discharging, each full cycle causing tiny, cumulative structural and chemical changes such as the growth of the solid electrolyte interphase layer and the gradual loss of active material. Together these reduce both usable capacity, which shortens range, and power capability, which can blunt acceleration and slow charging.

In practice the rate is reassuringly modest for modern packs, commonly around one to two percent of capacity lost per year, with the steepest decline often occurring in the first year before the curve flattens. This means a well-treated battery may still retain the large majority of its original capacity after a decade of use, comfortably outlasting many owners' periods of ownership.

Several factors accelerate the decline, and most are within the driver's influence. Sustained heat is the greatest enemy, which is why parking in shade and avoiding repeated high-power sessions helps. Frequent DC fast charging stresses the cells more than gentle AC charging, and dwelling at extremes of charge, whether sitting at one hundred percent or running close to empty for long periods, hastens wear. The widely recommended practice of keeping the everyday charge between roughly twenty and eighty percent reflects this, reserving full charges for journeys that genuinely need them.

Manufacturers counter degradation through sophisticated thermal management that keeps cells within an ideal temperature window, through generous charge buffers that prevent true full or empty operation, and through warranties. A typical guarantee promises that the battery will retain at least around seventy percent of its capacity over eight years or 160,000 kilometres, replacing or repairing packs that fall below this threshold.

Degradation is quantified by the state of health, which expresses remaining capacity as a percentage of the original, and is managed by the battery thermal management system. It varies with chemistry too: lithium iron phosphate cells tend to tolerate full charging and frequent cycling better than other lithium-ion types, an important consideration when comparing the long-term durability of different batteries.