Heat, not charging, is what kills batteries
The habits that meaningfully extend battery life, the myths left over from older chemistry, and when a replacement is the right call.
Every rechargeable device in your house runs on roughly the same chemistry, and that chemistry wears out in a predictable way. A battery holds a bit less charge each month, and one day the phone that used to reach bedtime is dead by four in the afternoon.
You cannot stop that. You can slow it down quite a lot, and most of what slows it down is free. It also helps to know which pieces of received wisdom are left over from battery types nobody has used in twenty years.
Two things do most of the damage
The first is heat. Lithium cells age faster the warmer they are, and the effect is not gentle — a battery kept hot degrades dramatically quicker than one kept cool. A phone left on a car dashboard in summer, a laptop running a long export while sitting on a duvet, a device charging under a pillow: those afternoons cost real capacity, permanently.
The second is time spent sitting at a very high or very low state of charge. A cell held at 100% is under more chemical stress than one at 60%. A cell drained flat and left that way for weeks can drop below the voltage at which it can safely be recharged. The middle of the range is the comfortable place, and the extremes are where the wear happens.
Everything else is a footnote by comparison. Charging to full occasionally, using the device while it charges, topping up from 70% — none of that matters much next to heat and time at the extremes.
What to actually do
- Turn on the charge-limiting feature your device offers. Most phones and laptops now have some version — optimized charging that waits to finish the last stretch until you need it, or a hard cap around 80% for machines that live on a desk.
- Keep it out of the sun and off soft surfaces. A laptop needs air under it; a bed or a lap blanket blocks the intake and the whole machine cooks.
- Take the case off if it gets hot while charging fast, especially a thick case on a phone doing a big update.
- Charge slowly when there is no hurry. Overnight on a modest charger runs cooler than twenty minutes on the fastest one you own. Use the fast charger when you need speed, not by default.
- If a device is going into a drawer for months, leave it around half charged and check it once or twice a year rather than storing it full or empty.
The 20-to-80 habit gets repeated a lot, and it is directionally right — but it is a preference, not a rule. A phone you never charge past 80% and never run below 20% will last longer in the long run and be less useful in the short one. Use the whole battery when you need it. Just avoid parking it at either end for days on end.
Myths from an older chemistry
| "Drain it fully before recharging" | That was for nickel cells with a memory effect. Deep discharges are mildly bad for lithium, not good for it. |
|---|---|
| "Leaving it plugged in overcharges the battery" | Devices stop charging when full. The concern is heat and time spent at 100%, not overcharging. |
| "Closing background apps saves the battery" | It mostly does not, and force-quitting apps you use often can cost more power on relaunch. |
| "You should calibrate it monthly" | A full cycle occasionally helps the percentage readout stay accurate. It does nothing for the cell itself. |
Reading the numbers your device gives you
Phones and laptops report some measure of battery health — a percentage of the original capacity, a cycle count, or both. A cycle is one full battery's worth of discharge, added up: two days of using half the battery counts as one cycle, not two.
Treat the health percentage as a trend rather than a precise figure. Watch it over months. A slow, steady decline is normal and expected. A sudden drop, a device that shuts off at 30%, or a battery that swells enough to lift the trackpad or bow the back panel are different matters — the last one especially. A swollen battery should stop being used and charged, and go to a repair shop or a proper recycling point rather than a bin.
When to replace the battery instead of the device
In our experience a worn battery is the most common reason a perfectly good machine gets retired, and it is usually the cheapest fault on that machine to fix. A battery swap costs a fraction of a new phone or laptop and buys years. On many laptops it is a genuinely simple job; on phones it is fiddly enough that a shop is the sensible choice for most people.
Two cautions if you go the do-it-yourself route. Use a replacement from a supplier with a real returns policy, because cheap cells with optimistic capacity ratings are common and a bad one is a fire risk in your pocket. And never puncture or bend a pouch cell while prying it out — that is how a repair becomes an emergency.
It is worth asking about battery replacement before you buy, too. Whether the pack is held in with screws, pull tabs, or a sheet of adhesive that requires heat and solvent decides whether this is a forty-minute job or a reason to give up on an otherwise fine machine.