Why Home Batteries Age: Calendar Ageing Versus Cycle Ageing
- Calendar ageing happens even when a battery sits idle; cycle ageing comes from charging and discharging.
- Heat speeds up calendar ageing, so where a battery lives matters.
- Deeper discharges and faster charging increase cycle ageing.
- The two effects can be added together, which makes lifetime estimates easier to reason about.
- Capacity loss is gradual, not a sudden failure.
Every lithium-ion home battery slowly loses capacity, and it does so for two separate reasons: the passage of time and the work it does. Knowing which is which explains most warranty terms and most of the advice about how to treat a battery.
Two clocks running at once
A battery has a calendar clock and a cycle clock. The calendar clock ticks whether or not you use the battery. The cycle clock only advances when energy goes in and out.
Researchers at the US National Renewable Energy Laboratory (NREL) studied lithium-ion cells and found that the two kinds of fade behave differently, but can be treated as additive: total capacity loss is roughly the calendar loss plus the cycling loss. Their model that simply added the two matched test results within about 1%, while a model that multiplied them together was off by around 16% in one test (NREL technical report on lithium-ion ageing). That is a laboratory result on a specific cell type, not a promise about any particular home battery, but it gives a useful mental model.
What calendar ageing is
Inside a lithium-ion cell, a thin layer called the SEI (solid electrolyte interphase) grows on the graphite electrode over time. Growing it uses up lithium that could otherwise store energy, so capacity drops. The NREL report describes this fade as roughly following the square root of time and as speeding up with temperature.
The practical lesson is that heat matters. A battery in a hot loft ages faster than the same battery in a cool utility room. Where to put it is a real durability question as well as a convenience one, and the home battery storage category pages show the operating temperature range each maker states.
The same report found that calendar fade is not a simple function of how full the battery is. It varied in a non-obvious way across the state-of-charge range, and the pattern changed with temperature. So we would be cautious about any claim that a battery must be kept at one specific charge level.
Good to know
Early in a battery's life, a few percent of capacity can shift up or down in the first months, according to the NREL report. A small early change is not by itself a sign of a fault.
What cycle ageing is
Cycle ageing comes from the mechanical and chemical stress of moving lithium in and out of the electrodes. NREL found that this fade also grows roughly with the square root of the number of full-equivalent cycles, and that it increases with depth of discharge and with charging rate.
In plain terms: emptying a battery deeper and charging it harder both cost more life per cycle. That is why warranties are written in cycles or in total energy throughput, which our battery capacity guides help you relate to your own use.
Interestingly, the report also found that cycling fade got worse at lower temperatures, which it linked to slower lithium movement and greater stress inside the electrode. Cold and fast charging is a bad combination, a point that matters for where you place the battery.
What this means for a home battery
Most home batteries do one gentle cycle a day, often less. At that pace, calendar ageing is a large share of the total, which is why warranties commonly limit both years and energy throughput; check the terms for your product.
Three sensible habits follow from the science, none of which needs special effort:
- Install the battery somewhere that stays cool and stable, within the maker's stated temperature range.
- Do not size the battery far beyond what you will use; a pack that cycles a little each day is not wearing out faster, but you pay for capacity you do not use.
- Prefer the default charging settings unless you have a reason to change them, because manufacturers tune them for the cell.
Common questions
Does a battery age if I never use it?
Yes. Calendar ageing continues with time, and it is faster at higher temperatures. A battery sitting unused in a hot place can lose capacity without a single cycle.
Is a deeper discharge always worse?
In the NREL results, cycle fade increased with depth of discharge. In practice, makers set the usable range so that you cannot discharge past what they consider safe.
Can I measure my battery's ageing myself?
Many systems report a state of health or capacity figure in their app. Treat it as an estimate rather than a precise measurement, because it is calculated, not directly measured.
Should I worry about a small capacity drop in year one?
Not necessarily. Early capacity shifts of a few percent were observed in the research and are separate from long-term ageing. Check your warranty for the capacity threshold it guarantees.
Related guides
Want to know which battery size fits your home?
Use our free calculator to get a size, savings and payback estimate.