This Battery Aging & Calendar Life Estimator calculates long-term battery degradation that occurs simply due to time, even when the eBike battery is not being used. This type of aging is known as calendar aging, and it is one of the most important long-term factors that determine battery lifespan, total capacity, and future performance.
What Is Calendar Aging?
Calendar aging describes the natural chemical degradation that happens inside lithium-ion cells as time passes. Even without cycles or usage, battery materials slowly react and deteriorate, causing the battery to lose capacity year after year. Calendar aging occurs inside every lithium cell and cannot be avoided, but it can be slowed dramatically with proper storage conditions.
- Electrolyte decomposition
- Growth of the SEI layer
- Loss of active lithium ions
- Increased internal resistance
These microscopic chemical reactions slowly reduce how much energy the battery can hold, how much current it can deliver, and how efficiently it can discharge.
What Factors Accelerate Calendar Aging?
Calendar aging depends strongly on temperature, storage charge level (SOC), and time. In fact, calendar aging is often more damaging than cycle aging in eBikes that are used occasionally or stored for long periods.
Main calendar aging accelerators
- High temperature (above 30°C)
- High state of charge (above 70%)
- Long-term storage without cycling
- High internal resistance and old batteries
Important: A lithium battery stored fully charged at 35°C may lose 20–35% capacity in only 12 months.
Typical Calendar Aging per Year
- 25°C at 50% SOC: ~3–5% per year
- 30°C at 50% SOC: ~8–15% per year
- 35°C at 100% SOC: ~20–40% per year
- 40°C at 100% SOC: severe and often irreversible
These numbers vary depending on chemistry (NCM, NCA, LFP), but temperature and SOC always determine long-term aging speed.
Best Way to Store an eBike Battery Long-Term
- Keep storage charge around 40–60%
- Store between 15–25°C
- Avoid leaving battery fully charged
- Avoid leaving it inside a hot garage or car
- Check battery every 2–3 months
Following these recommendations may extend the useful calendar life of a lithium-ion eBike battery by several years.
Calendar Aging vs Cycle Aging
Many people assume battery aging happens only when riding or charging, but research shows that most capacity loss comes from calendar aging. Even a brand-new battery unused for 3 years may lose more capacity than a battery that has 300–500 cycles but stored properly.
- Cycle aging: happens during charging/discharging
- Calendar aging: happens with time, even unused
Both aging types combine to determine the total lifespan of the battery.
Frequently Asked Questions
Q: Does a stored battery age even if not used?
A: Yes. Calendar aging continues even without cycles, especially at high temperature or high charge levels.
Q: Will a lithium battery age faster if kept fully charged?
A: Yes. High SOC accelerates SEI layer growth and causes permanent capacity loss.
Q: What temperature is best for long-term storage?
A: 15–25°C. Above 30°C aging increases dramatically.
Q: How much capacity does a battery lose per year?
A: Most lithium batteries lose 3–5% at room temperature and 40–60% if stored hot and fully charged.
Q: Does cycling reduce or increase calendar aging?
A: Cycling causes its own degradation, but most unused batteries age more due to time and heat.
