BATTERY AGING & country dependence [Technical Note]
Calendar Aging: SOC & Temperature:
SOC: State of Charge — Is the battery's fill level, expressed as a percentage of its usable capacity. 100% SOC is fully charged, 0% is fully discharged. High SOC means the cathode sits at high voltage, which is where electrolyte oxidation and cathode structural instability accelerate. SOC is one of the two main drivers of calendar ageing. The other is temperature.
Temperature: SOC & Temperature compound and take a toll on your battery’s health. A cell parked at 100% SOC in 45 °C heat degrades far faster than the same cell at 50% SOC and 25 °C. This is an argument against sizing a battery so large that it spends summer afternoons sitting full at 100% while the PV array is still producing. Under net billing you want the battery cycling — charging in the morning, discharging into the evening peak — not parked at full charge in the heat. Most decent BMSes let you cap charge at 90–95% for exactly this reason, at a small cost in usable capacity. This combination follows roughly Arrhenius behaviour: degradation rate roughly doubles for every ~10 °C rise. A cell at 45 °C ages several times faster than the same cell at 25 °C, doing nothing at all. Combine that with high SOC and the two multiply rather than add.
Do not confuse SOC with SOH (State of Health), which is the capacity remaining relative to when new — the thing that degrades over the 15–25 year life.
Cycle ageing: Throughput / Depth of discharge (DoD) / C-rate: Beyond calendar ageing (SOC & Temperature) the next category is cycle aging.
Throughput : Total energy pushed through the cell over its life, usually the warranty metric (e.g. 8,000 cycles to 80% SOH). Every kWh in and out costs you a little capacity.
Depth of discharge (DoD): Shallow cycles are disproportionately gentle. Cycling 30–80% instead of 0–100% can multiply cycle life several-fold, because you avoid both the high-voltage stress at the top and the mechanical strain at the bottom.
C-rate: How fast you charge and discharge. Higher current means more internal heating and more lithium/sodium plating risk. Home ESS (Energy Storage System) at 0.2–0.5C is benign; this matters far more in EVs than in your application.
The practical point for a 15–20 kWh system (Cyprus): calendar ageing will likely dominate over cycle ageing, because you'll cycle once a day at most while the battery sits in ambient heat for 8,760 hours a year. That inverts the usual assumption. In Germany you'd size for cycles; here you should size for heat and siting, and be sceptical of any warranty quoted purely in cycle count without a temperature condition attached — check what ambient range the guarantee actually holds for.
