What damages a phone battery the most?

Heat and deep discharge cycles. Charging in the 20–80% band and keeping the device cool roughly doubles cycle life; fast charging matters less than either.

Battery degradation

Your phone battery, five years from now.

Lithium-ion cells fade two ways at once — every charge cycle, and every day just sitting warm. Tune your habits on the right and watch the capacity curve bend.

— health · yr 5

Year 1

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Year 3

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Year 5

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your habits careful user 80% "needs service"

Habit preset

Tune the habits

Charging speedSlow (5W)

faster charging plates lithium · medium effect

Avg charge depth (DoD)60%

deeper cycles strain the anode · big effect

Avg battery temperature22°C

side reactions ~double per 10°C · big effect

Charge cycles per day0.6

how hard you lean on the battery daily

What's costing you capacity

Heat—

Charge depth—

Fast charge—

How to use

Pick a preset or drag the four sliders. The amber curve is your habit; the grey ghost is a careful baseline (slow charge, 20–80%, cool). The cost bars rank which habit is bleeding the most capacity right now — heat and charge depth usually dominate, fast charging less than people fear.

What kills lithium batteries

Capacity fade comes from two clocks running at once: cycle aging (each charge/discharge) and calendar aging (time spent warm, even idle).

SEI growth: a solid-electrolyte interphase thickens on the anode, fastest near 100% charge — this is the calendar clock.
Heat: above ~30°C side reactions accelerate roughly doubling per 10°C, driving both clocks.
Lithium plating: fast charging, especially when cold, deposits metallic lithium irreversibly.

Industry baseline: ~20% loss after 500 full cycles (Apple, Tesla spec). Real-world spread is huge because habits vary.

Which habits actually matter

Big effect:

Staying in the 20–80% band roughly doubles cycle life.
Keeping the device cool — don't charge under a pillow or on a hot dash.

Medium effect:

Slow (5W) vs fast (65W+) charging — a 10–25% lifetime difference.

Small effect:

Overnight charging (modern firmware throttles the trickle), Lightning vs USB-C (negligible).

Apple's "Optimized Charging"

Recent iPhones (15+) and Pixels learn your routine and hold at 80% until shortly before you unplug. It works — Apple reports 80% capacity retained at 1,000 cycles versus 500 historically. Leave it on.

Citations

Vetter, J. et al. (2005). "Ageing mechanisms in lithium-ion batteries." Journal of Power Sources 147(1–2): 269–281.
Wang, J. et al. (2011). "Cycle-life model for graphite-LiFePO₄ cells." Journal of Power Sources 196(8): 3942–3948.
Birkl, C.R., Roberts, M.R., McTurk, E., Bruce, P.G. & Howey, D.A. (2017). "Degradation diagnostics for lithium ion cells." Journal of Power Sources 341: 373–386.
Keil, P. et al. (2016). "Calendar aging of lithium-ion batteries." Journal of The Electrochemical Society 163(9): A1872–A1880. (Calendar-aging mechanism at elevated temperature.)

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