Does overnight charging kill your battery? The chemistry, minus the folklore

The phone battery advice you got from the group chat is half folklore from the nickel-cadmium era and half marketing from the chip makers, and the chemistry underneath is simple enough to hold in your head for one paragraph - which this page does before the habits, so you can retire the myths with reasons instead of vibes.

The two things that actually age a lithium cell

• Cycles: a battery's life is measured in full discharge-equivalents (the 0-100 count), and a cell that "dies" at 80 percent of its original capacity has simply spent its rated cycles. Partial charges do not consume fractions of cycles the way the folklore fears; the number that matters is the total energy pushed through the cell.
• Calendar ageing, and its heat coefficient: time ages a cell even on the shelf - faster when it sits at the top of its voltage (the "full 100 percent for days" state), and much faster when it sits there warm. The desk's one-line physics: a hot battery is an old battery; the rest is engineering.

The myths, filed

"Overnight charging overcharges it": no - the phone stops the current at full and the charger idles; the myth is a nickel-era ghost. What is real about the night: the phone sits at its full voltage for six more hours in a bed where the warmth has nowhere to go - so the fix is not the plug, it is the 80-percent cap most modern phones ship as a setting (the same setting that costs you the last 15 percent of capacity you rarely needed, and buys back it back in years - the trade the software makers now recommend out loud). "Use the original charger only": mostly superstition - the negotiation of voltage and current is the standard's job; a good third-party charger that keeps the phone cool is the same chemistry, and a no-name one that cooks it is the risk the folklore half-remembers. "Drain to zero to recalibrate": the cell's gas gauge does not need exercise; deep discharges just spend cycles.

The three habits that survive the evidence review

1. Keep it cool: the charge-through-the-pillow case, the sunlit-dash mount, the play-while-fast-charging triple-heat - the desk's ranking of every habit on this page, and the free one. Remove the case when it is hot; that is the whole tip.
2. Live in the middle band on the days you can: the 20-80 "range" the EV people use is genuinely the cell's comfort zone, and the phone's own optimised-charge setting is this habit automated; the person who tops up at the desk all day without ever hitting 100 is living exactly the chemistry's preferred life.
3. Replace the cell, not the guilt: the 80-percent health report is a service item, not a verdict - a fresh battery is the cheapest upgrade the phone allows, and the desk's standing advice to the worried is the same as the mechanic's: when the number says replace, replace, and stop paying the folklore's tax while you decide.

The summary in the group chat's own register: the night charge is fine; the hot night is not; and the cap in the settings menu is the phone telling you it learned the chemistry too.