A battery cycle is one full discharge equivalent (e.g., 50% drain twice = 1 cycle). Lithium-ion batteries typically last 500–1000 cycles before capacity drops below 80%.
A battery cycle is one complete charge/discharge loop, measured by accumulated energy (Watt-hours), not number of plug-in events. Discharging 50% (e.g., 100% → 50%) then recharging = 0.5 cycles. Two such 50% discharges = 1 full cycle. This accounting method allows realistic degradation prediction independent of user charging patterns. A device drained 50% daily and fully recharged takes two days per cycle.
**How battery degradation works technically:** Lithium-ion batteries degrade due to irreversible electrochemical reactions in the electrolyte and electrodes. Each charge/discharge cycle creates microscopic crystal structures (lithium dendrites) on the anode, trapping ions and reducing available capacity. Higher temperatures accelerate these reactions (Arrhenius equation: reaction rate roughly doubles per 10°C increase). Full 0–100% charges stress the electrodes more than shallow 20–80% cycles because extreme voltages promote side reactions. Fast charging (high current) generates more heat and pushes ions faster than the electrode can accommodate, creating structural damage. After 500–1000 cycles (depending on conditions), capacity typically drops to 80% of original, and internal resistance rises, slowing charging.
**Why cycle counting matters to buyers:** A 3000 mAh battery with 1000-cycle rating lasts 3 million mAh before degrading to 80%. Divide by daily consumption: charging a phone 50% daily (1500 mAh/day) = 2000 days before reaching 80% capacity (5.5 years). Warranty: Apple guarantees 1000 cycles (iPhone 15+), some Android manufacturers 500–800 cycles. Real-world: heavy users (100% discharge daily) hit 80% capacity in ~1.5 years; light users (50% daily) hit it in 5 years.
**What to look for / common pitfalls:** - Temperature is the primary degradation factor (more than cycle count); keeping phone cool extends battery life 20–40% - Fast charging (100W+) daily causes visible capacity loss within 2 years vs slower overnight charging (10W) over 5 years - Wireless charging heat (70–80% efficient) adds thermal load; long-term wireless-only charging degrades battery faster than wired - "Optimized Battery Health" (iOS) / "Adaptive Battery" (Android) learn your usage and halt charging at 80% overnight, extending lifespan - Battery replacement cost: $50–150 OEM (Apple, Samsung), $20–50 third-party (reliability/durability risk) - Replacement timing: typical iPhone/Samsung phones reach 80% capacity around 2–3 years with normal use, replacement not urgent unless < 50% capacity
Real-world 2026: iPhone 15 Pro (1000 cycles to 80%), Galaxy S24 (typically 800–1000 cycles), budget phones (500–800 cycles). Heavy power users reaching 80% capacity in 18–24 months; moderate users 3–4 years. Battery replacement becoming common service, driving repairability concerns and right-to-repair movement.