A battery ages while it sits, not only while it works

Two clocks running at once

Battery degradation is usually discussed in cycles, because cycles are countable and warranties are written against them. But cells lose capacity through two separate processes, and only one of them has anything to do with use. Cycle ageing comes from the mechanical and chemical stress of moving lithium in and out of the electrode structures repeatedly. Calendar ageing comes from slow, spontaneous chemistry that continues whether or not anything is happening.

The dominant mechanism in calendar ageing is growth of the layer that forms at the interface between electrode and electrolyte. That layer is necessary, it forms early in a cell’s life, and it keeps thickening slowly over years. Thickening consumes lithium that is then unavailable to carry charge, and it raises internal resistance.

This is why a cell left in a drawer for years is not the cell it was. Nothing was drawn from it, and it still lost capacity, because the reaction never needed a load to proceed.

Temperature is the accelerator

Calendar ageing follows the pattern most chemistry follows: it speeds up with temperature, and it does so steeply rather than proportionally. A modest rise in average temperature can meaningfully shorten the life of a cell held under otherwise identical conditions, and the relationship compounds over years.

That has a very practical consequence for where a home battery is installed, which is a subject of its own. The short version is that a cool, stable location is worth more to the battery than almost any setting in its app, and a hot loft or a wall in direct summer sun is the opposite.

Cold is a different matter. Low temperatures do not accelerate calendar ageing; they slow it. What they do is reduce available power and, critically, make charging damaging below a threshold, which is why properly designed systems refuse to charge a cold pack or warm it first. That behaviour is protection, not a fault.

State of charge matters more than most owners realise

A cell held at a high state of charge ages faster than one held mid-range, because the electrode sits at a higher potential and the parasitic chemistry proceeds more readily. A cell held at very low charge has its own problems, including the risk of dropping into a range where damage becomes permanent.

This is the reasoning behind the advice, familiar from phones and cars, to avoid leaving a pack sitting full for long stretches. For a home battery it mostly takes care of itself, since a system that charges during the day and discharges each evening spends little time parked at either extreme.

Where it becomes relevant is a battery that habitually fills by early afternoon and then sits at full for many hours, which happens when the unit is oversized for the household’s evening demand. Some systems can be configured to delay charging so the pack reaches full closer to sunset. Whether that is available and whether it is worth configuring depends on the equipment, and the manufacturer’s guidance governs.

What this does to a warranty

Storage warranties usually state two limits and honour whichever is reached first: a number of cycles or a number of throughput units, and a number of years. The second exists precisely because of calendar ageing, and it is the term that catches lightly used systems.

Consider what that means for a battery cycling far less than the warranty anticipates. It will never approach the cycle limit, and the years will expire regardless, and the retained-capacity guarantee will lapse having covered nothing. The household paid for a warranty measured in cycles it never used.

That is a reason to size a battery to be used rather than to be spacious, and it is one of the clearer arguments against the instinct to buy larger for safety. A battery that empties most evenings is converting its warranty into savings. One that hovers half full is converting it into time.

What to expect in practice

The two mechanisms are not additive in a simple way, and manufacturers model them together rather than separately. For a home system in a temperate location, cycling once a day at moderate depth, calendar effects and cycle effects are broadly comparable in scale, and neither dominates.

Degradation also is not linear. Capacity typically falls a little faster early on, settles into a long slow decline, and in some chemistries turns downward again late in life when a mechanism changes character. Which is why a guarantee quotes a retained percentage at a date rather than a rate per year.

The reasonable expectation is that a battery is noticeably smaller in a decade than it is on day one, that it will still work, and that its decline will be gradual enough to be invisible day to day. Anyone told a home battery will hold its capacity indefinitely is being told something the chemistry does not support.