"How many years will this battery last?" is the most frequently asked — and most frequently dodged — question in storage procurement. The datasheet says 6,000 cycles; the salesperson says "16 years, no problem". Several layers of assumptions sit between those two statements. Let's take them apart.

Cycle life ≠ service life

Cycle life is the number of full charge-discharge cycles a battery completes before capacity falls to a defined threshold (usually 80%, called EOL — end of life). 6,000 cycles ÷ one cycle per day = about 16 years. The arithmetic is right, but it assumes three things: every cycle under standard conditions, a constant 25℃ ambient, and no calendar aging. Reality rarely satisfies any of the three.

Two kinds of aging happen at once

Batteries age along two tracks: cycle aging (accumulated by charging behavior) and calendar aging (accumulated by time itself — a battery slowly degrades even sitting unused, especially hot and at full charge). Real life is governed by whichever arrives first. For a home system cycling daily, cycle aging dominates; for backup systems used a few times a year, calendar aging takes over. That's why backup buyers should scrutinize storage conditions and calendar-life declarations instead.

Four variables that speed up aging

• Temperature: the biggest enemy. Sustained operation above 35℃ roughly doubles the aging rate. Liquid cooling that keeps cell-to-cell spread within 3℃ is, in essence, insurance for lifespan. Reading the thermal design tells you more than reading the cycle count.
• Depth of discharge (DOD): draining from 100% to 0% every day costs far more than from 90% to 20%. Many datasheet figures correspond to 80% DOD — always ask what conditions the claimed cycles were tested at.
• C-rate: sustained high-current fast cycling ages electrode materials faster. Systems designed with power headroom simply last longer.
• Time spent at full charge: full charge plus heat is the most damaging combination. An EMS that supports a "float band, full charge only when needed" strategy is a genuine plus.

Reading degradation curves and warranties

Reputable vendors publish degradation curves: for example, "97% after year one, then about 2.5% per year, guaranteed 70% at year ten." Warranty terms deserve equal scrutiny — distinguish product warranty (free repair/replacement for defects, typically 5–10 years) from capacity warranty (a promise that remaining capacity stays above a threshold). Three questions to ask before signing: Is the warranty based on 70% or 80% EOL? Is it limited by throughput or cycles, whichever comes first? If capacity falls short, is the remedy augmentation, replacement or refund? If it isn't in the contract, it doesn't exist.

A practical way to estimate

To translate a datasheet into years: confirm the test conditions behind the claimed cycles (DOD, temperature, rate), then adjust for your real usage pattern — assume about 0.8 equivalent full cycles per day for variable consumption, and take another 20% off in hot climates. The simpler route: hand your electricity bill and installation environment to a supplier for a lifetime simulation, and get the commitment written into the capacity warranty. Lifespan isn't calculated — it's guaranteed by design, usage and contract together.