What Causes Battery Degradation? Calendar Ageing, Cycle Ageing and What Drives Them

Mechanisms, Modes and What You Notice

Researchers at Imperial College London and the Faraday Institution, in a widely cited 2021 review titled Lithium ion battery degradation: what you need to know, describe ageing at three levels. At the bottom are the mechanisms: the physical and chemical changes inside the cell. They group into modes: loss of lithium inventory (less lithium available to carry charge), loss of active material (less electrode able to store it), and rising impedance. At the top are the effects an owner notices: capacity fade, meaning less energy, and power fade, meaning less power.

The same review names three external stress factors that drive these mechanisms: temperature, state of charge and load profile. In general, it says, temperature is the most significant.

The Four Main Ageing Mechanisms

Dozens of mechanisms have been identified, but four explain most of what happens in a working battery. The interactive drawing below shows where each one happens inside the cell.

• SEI growthOn the first charge, a thin protective film called the solid electrolyte interphase forms on the graphite anode. It keeps growing slowly for the rest of the cell's life, even when the battery is unused, trapping lithium and raising resistance. Growth slows over time, roughly with the square root of time, and speeds up with heat.
• Lithium platingWhen lithium arrives at the anode faster than it can slot into the graphite, it coats the surface as metal instead. Cold temperatures, fast charging, high state of charge and high cell voltage all cause it. Some plated lithium is lost for good, and in severe cases it can grow dendrites that pierce the separator.
• Particle crackingElectrode particles swell and shrink on every cycle and eventually fracture. Cracks cut particles off from the circuit and expose fresh surface where more SEI grows. Deep discharge, high currents and large particles make it worse.
• Cathode and electrolyte breakdownAt high voltage and high state of charge, especially in nickel-rich NMC cathodes, the cathode surface can degrade, metals can dissolve and migrate to the anode, and the electrolyte slowly decomposes. High temperature accelerates all of it.

Calendar Ageing vs Cycle Ageing

Sandia National Laboratories' energy storage handbook splits ageing into two kinds. Calendar ageing comes from side reactions that happen even when the battery is not being cycled. Cycle ageing comes from the stress of charging and discharging. Sandia describes the two as largely independent and additive, a useful first approximation, although researchers also stress that they interact.

Calendar ageing depends mainly on temperature and state of charge, and it speeds up with temperature following an Arrhenius relationship: the hotter the cell, the faster the reactions run. Cycle ageing depends on depth of discharge, charge and discharge rate, and temperature.

For stationary storage, the calendar side deserves more attention than it usually gets. An NREL paper on grid-connected batteries notes that while lithium-ion lifetime is commonly discussed in cycles, the calendar life of the cell is often more limiting than its cycle life. A battery that cycles once a day spends many hours waiting, and how warm and how full it waits matters.

The Stress Factors That Matter Most

Put the mechanisms and the two kinds of ageing together and a short list of operating conditions explains most of the difference between a battery that lasts and one that doesn't. Try them in the explorer below.

• TemperatureHeat speeds up SEI growth and electrolyte breakdown; cold makes charging risky because of plating. Sandia describes temperature-driven ageing as a continuum between plating at low temperatures and SEI formation at high temperatures.
• State of charge while restingCalendar ageing studies consistently show faster capacity fade at higher states of charge, across chemistries.
• Depth of dischargeIn Sandia's multi-year comparison of commercial cells, capacity fade increased with depth of discharge for every cell tested, though LFP was the least sensitive.
• Charge rateSandia's handbook notes that high charge rates are far more damaging than high discharge rates, because lithium can only enter the graphite so fast.
• Temperature uniformityCells held at different temperatures age at different rates, so the spread across a pack matters as well as the average.

Why Identical Cells Still Age Differently

Even cells from the same production line, used in exactly the same way, can age at noticeably different rates, most likely because of small manufacturing differences. The Imperial College review cites a study of 24 nominally identical cells cycled with the same load profile that showed a wide spread of lifetimes.

In a real battery the differences are amplified by position: a module near a cooling outlet runs cooler than one at the far end of an airflow path. A three-year study of a grid battery in South Korea, published in 2025, found ageing concentrated in particular racks where airflow from the cooling system was uneven, with cell-to-cell variation growing every year. That is the practical case for monitoring health at module and cell level rather than assuming the whole battery ages as one.

What Software Can and Can't Change

Some ageing is fixed when the battery is built: the chemistry, the cell design, the quality of manufacture, the cooling system. But most of the stress factors above are operating decisions, made every day by the energy management system and the battery management system.

• TemperatureWhen heavy work is scheduled, whether cooling starts before the heat, and whether charging slows when cells are cold.
• State of chargeWhether the battery is filled hours before it is needed or just in time, and where it rests overnight.
• Depth of dischargeWhether every cycle runs end to end, or only when the extra energy is worth the extra wear.
• Charge rateWhether charging is spread across the available hours or rushed.
• UniformityWhether drifting modules and cooling problems are spotted while they are still small.