Battery Science
How EV batteries age — explained with sources, uncertainty and practical meaning.
Fundamentals
How EV batteries age
An EV battery loses capacity gradually through two overlapping processes: calendar ageing, which happens with time, and cycle ageing, which happens with use. How fast each one progresses depends on temperature, state of charge, charging current and cell chemistry.
State of health (SOH)
State of health describes how much of its original capability a battery still has, most often expressed as remaining usable capacity relative to new. It is a useful indicator, but every SOH figure is an estimate with its own limitations.
Equivalent full cycles (EFC)
Equivalent full cycles express how much energy has passed through a battery, counted in units of its usable capacity. They make different charging patterns comparable, but EFC alone does not predict degradation.
Battery buffers
The 0 to 100 % shown on your dashboard is usually not the full electrochemical range of the cells. Manufacturers typically keep buffers at the top and bottom, which affect how your charging habits translate into cell stress.
Ageing mechanisms
Calendar ageing
Calendar ageing is the capacity a battery loses simply with the passing of time, even while the car is parked. It is driven mainly by temperature and by the state of charge the battery spends its time at.
Cycle ageing
Cycle ageing is the wear caused by charging and discharging the battery. It depends on how much energy passes through the cells, the depth and position of each cycle within the SOC window, the current and the temperature.
Time at state of charge
For calendar ageing, what matters is not whether your battery reaches a high SOC, but how long it stays there and how warm it is. Charging to 100 % shortly before a trip adds little time at high SOC; leaving the car at 100 % for days adds much more.
Temperature and battery life
Temperature is one of the strongest influences on battery ageing. Lithium-ion cells tend to age slowest around room temperature; heat accelerates SEI growth, while cold raises the risk of lithium plating during charging.
Lithium plating
Lithium plating occurs when lithium deposits as metal on the anode surface instead of entering the graphite. It is most likely when charging at low temperature, high current or high SOC, and it can cause rapid, partly irreversible capacity loss.
Charging
Charging power and C-rate
C-rate expresses charging or discharging current relative to battery capacity. It is a better measure of cell stress than charger power alone, because the same number of kilowatts means something very different for a small and a large battery.
DC fast charging
DC fast charging can add some battery stress, but the effect depends far more on the vehicle's charging curve, pack temperature and SOC than on the charger's nameplate power. Occasional fast charging of a well-managed pack is generally a limited factor; frequent cold fast charging is where the risk rises.
Battery preconditioning
Preconditioning brings the battery to a suitable temperature before fast charging or driving. It can reduce lithium plating risk and speed up charging in cold conditions, but it does not make charging free of ageing.
Chemistries
LFP (lithium iron phosphate)
LFP cells use an iron-phosphate cathode and are known for long cycle life, good thermal stability and lower sensitivity to high SOC than nickel-based cells. They store less energy per kilogram and have a flat voltage curve that makes SOC estimation harder.
NMC (nickel manganese cobalt)
NMC cathodes combine nickel, manganese and cobalt to deliver high energy density and good power. They are widely used in EVs but are generally more sensitive to high SOC and temperature than LFP, especially in nickel-rich variants.
NCA (nickel cobalt aluminium)
NCA is a nickel-rich layered oxide cathode with aluminium as a stabiliser, offering high energy density. Its ageing behaviour resembles that of nickel-rich NMC, with clear sensitivity to time at high SOC and temperature.
LMFP (lithium manganese iron phosphate)
LMFP adds manganese to the LFP cathode to raise its voltage and energy density while keeping much of LFP's stability. Public automotive ageing data for LMFP is still limited, so any lifetime model is experimental.
Sodium-ion batteries
Sodium-ion batteries replace lithium with sodium, a far more abundant element. They promise lower material cost and are often reported to handle low temperatures well, but public long-term ageing data for automotive use is limited and lifetime models are experimental.
Ownership
Battery warranties
Most EV battery warranties guarantee a minimum remaining capacity for a set number of years or kilometres, whichever comes first. The exact terms are set by each manufacturer, and Voltgevity's predictions are not warranty assessments.
Your ownership horizon
Whether a stricter charging strategy is worth it depends heavily on how long you will keep the car. The battery benefits accumulate slowly over years, while the convenience costs are paid from day one.