Calendar aging is the gradual loss of battery capacity and performance that occurs over time, even if the battery is not being heavily used. Unlike cycle aging (which is driven by charging and discharging cycles), calendar aging happens during storage or light use and is strongly influenced by temperature, state of charge (SoC), and time.
What Is Calendar Aging?
Calendar aging refers to long-term chemical changes inside a battery that reduce:
– Usable capacity (kWh available)
– Power capability (how much current the battery can safely deliver/accept)
– Efficiency and charging performance (more heat, earlier taper)
It affects EV batteries, stationary storage (BESS), and even spare battery modules stored in warehouses.
Why Calendar Aging Matters in EV Charging
Calendar aging matters because battery condition influences charging behavior and customer experience. As batteries age:
– Maximum charging power may decrease due to higher impedance
– The charging curve can taper earlier, increasing charging time
– Thermal limits can become more restrictive under fast charging
– Real-world range and usable energy decline over years
For fleets and high-utilization charging sites, understanding calendar aging helps plan charging strategy and total cost of ownership.
What Drives Calendar Aging
The main drivers include:
– Time: aging happens continuously, even without cycling
– Temperature: higher temperatures accelerate chemical reactions and degradation
– High SoC storage: keeping a battery near 100% SoC increases stress and side reactions
– Chemistry and design: different cell chemistries and pack designs age at different rates
– Battery health and history: older batteries age faster due to accumulated wear mechanisms
In general, a battery stored hot and near full charge will age much faster than one stored cool at moderate SoC.
Typical Real-World Examples
– EVs parked for long periods at high SoC in hot climates losing capacity faster
– Fleet vehicles that are always charged to 100% “just in case” seeing faster long-term degradation
– BESS systems kept at high SoC for backup readiness requiring careful thermal and SoC management
– Second-life batteries requiring testing and grading because calendar aging differs by history
How Calendar Aging Is Managed
Common mitigation strategies include:
– Avoid storing batteries at very high SoC for long periods when not needed
– Use scheduled charging so vehicles reach high SoC closer to departure time
– Maintain controlled temperatures via BTMS in vehicles and HVAC in BESS
– Use SoC limits (e.g., daily cap below 100% where operationally acceptable)
– Monitor state of health (SoH) and adjust charging policies over time
For EV charging operators, it’s important to note that many of these controls are vehicle-side (OEM) rather than charger-side.
Key Benefits of Understanding Calendar Aging
– Better fleet charging policies that preserve battery life
– More accurate forecasting of long-term performance and charging speed
– Improved battery degradation modeling for business planning
– Lower lifecycle cost through reduced premature capacity loss
– Better planning for battery second life and recycling timing
Limitations to Consider
– Calendar aging varies by chemistry, OEM control strategy, and real-world conditions
– Exact degradation rates require lab data or long-term field measurement
– “Best SoC for storage” depends on operational needs and temperature control
– Charging behavior alone cannot prevent calendar aging if storage conditions are poor
Related Glossary Terms
Battery Aging
Cycle Aging
State of Charge (SoC)
State of Health (SoH)
Battery Impedance
Battery Degradation Modeling
Battery Thermal Limits
Battery Thermal Management System (BTMS)
Charging Curve
Battery Second Life