A new phone lasts all day; two years later it's dead by mid-afternoon. You've heard you should “drain it fully before charging” and “never charge overnight.” Which is true?
Most battery folklore describes batteries from decades ago. Understand the actual mechanism and care gets simple.
1. What to actually do. Three things — ① heat is the enemy ② living between 20% and 80% extends life ③ avoid full discharge. Everything else is minor.
2. Where the risk is. A swollen battery is not something to live with — the regulated part is quoted in full below. Do not open it yourself; take it to an authorised service centre.
3. What not to believe. “Drain it fully before charging” and “never charge overnight” describe batteries from decades ago. Lithium-ion works differently — below, what is actually regulated is separated from folklore.
1. Batteries age by cycles, not plug-ins
Lithium-ion life is measured in charge cycles, where one cycle means 100% of capacity used — not one visit to the charger.
- Two 50% top-ups = one cycle
- Five 20% top-ups = one cycle
- So frequent small charges cost you nothing — they're actually preferable
After several hundred cycles capacity noticeably drops, which is why the change becomes obvious around year two.
2. What actually ages a battery
| Factor | Impact · what to do |
| Heat | Worst — no hot cars, no charging under bedding |
| Sitting at 100% | High — prolonged full charge is stressful |
| Full discharge to 0% | High — especially leaving it there |
| Extreme cold | Moderate — temporary; sudden shutdowns in winter |
| Fast charging | Low–moderate — generates heat, no need every time |
| Overnight charging | Low — modern phones stop at 100% |
“Overnight charging kills batteries” is outdated. Phones stop charging at 100% and many delay the final 20% until just before you wake. What still hurts is charging on a pillow or duvet, where heat can't escape.
3. Charging habits
| Do | Don't |
| Stay roughly between 20% and 80% | Drain to 0 then fill to 100 every time |
| Top up whenever convenient | Believe you must “fully discharge first” |
| Charge on a hard surface | Charge under bedding or cushions |
| Ease off heavy gaming while charging | Play demanding games mid-charge |
| Use certified chargers | Use unbranded cheap adapters |
| Store long-term at ~50% | Leave it at 0% or 100% for months |
4. Settings that stretch today's charge
| Setting | Impact · how |
| Screen brightness | Biggest — enable auto-brightness or drop a notch |
| Screen timeout | — set to 30 seconds |
| Dark mode | — genuinely saves on OLED screens |
| Background refresh | — disable for apps you rarely use |
| Location services | — switch “always” to “while using” |
| Notifications | — fewer screen wake-ups |
| Low power mode | — instant effect when needed |
The screen consumes the most power. Lowering brightness alone often beats every other tweak combined.
5. Checking your battery health
- Most phones show maximum capacity under Settings → Battery.
- Below 80% is the usual point to consider replacement — it's also a common warranty threshold.
- The same screen shows which apps drain most. The top entry is often a surprise.
| Drains quickly | Ageing battery — check health |
| Shuts off around 30% | Significant degradation — replace |
| Device swelling | Stop using immediately — fire risk |
| Excessive heat when charging | Check charger and cable; if it persists, get it serviced |
Never press or puncture a swollen battery. If the back panel lifts or the screen bulges, stop using the device and take it to a service centre. Damaged lithium cells can ignite.
6. Reviving an older phone
- Check battery health — under 80%, replacement is the real fix
- Review top battery-draining apps and restrict or remove them
- Adjust brightness and timeout
- Update the OS — power optimisations often improve
- Free up storage — a full device slows everything
A battery replacement costs a fraction of a new phone. If nothing else bothers you, it can buy another year or two. Keeping the handset and cutting the monthly bill instead is worth a look too.
“A swollen battery is dangerous” — the regulated part
Battery care is a topic where manufacturer guidance and folklore blur together. What follows pulls out only what the US Federal Aviation Administration (FAA) and the US Consumer Product Safety Commission (CPSC) actually state, separated from what is merely repeated.
Taking one on a plane — the FAA has numbers
From the FAA PackSafe page:
“Lithium ion (rechargeable) batteries are limited to a rating of 100 watt hours (Wh) per battery.”
“up to two spare larger lithium ion batteries (exceeding 101–160 Wh)”
Here is the rule most often broken: spare batteries cannot go in checked baggage. The FAA text says they “must be carried in carry-on baggage only.” And when a cabin bag is gate-checked, spare batteries “must be removed from the bag and kept with the passenger” — the bag goes, the batteries stay in your hand.
Terminal protection is a rule too — “Battery terminals … must be protected from short circuit.” The methods the page accepts are leaving batteries in their retail packaging, taping the terminals, or using a battery case. In other words, do not drop a power bank into the same pocket as your keys and coins.
Damaged or recalled batteries cannot fly at all. From the FAA: “Damaged or recalled batteries and battery-powered devices … must not be carried aboard an aircraft (e.g., carry-on or checked baggage) unless the damaged or recalled battery has been removed, or otherwise made safe.” Packing a swollen power bank is a rules violation, not just a bad idea. What else to check before packing is in the packing guide.
Not verified — the FAA page does not state outright whether batteries installed in a device may go in checked baggage; it refers the reader to a separate entry (“portable electronic devices, containing batteries”) that we did not open, so we make no claim about it. These are also US rules; we did not check Korean or other national carriers.
Swollen batteries — cases the CPSC has actually acted on
“Swelling is dangerous” is a sentence this article had been carrying without a source. In the CPSC's own documents we found that exact sentence used as grounds for a recall.
| CPSC document | Date | What it says |
|---|---|---|
| Power bank recall Super Off-Road solar power banks | 25 June 2026 | The “lithium-ion battery can swell and overheat, posing a burn hazard” — two reports of swelling, no injuries. Consumers are told not to bin or recycle it but to take it to a municipal household hazardous waste centre. |
| Stop-use warning Garberiel lithium-ion chargers | 10 October 2024 | They “can overheat, resulting in arcing and causing fires” — six incidents, one death. The CPSC's instruction: “immediately unplug the charger and dispose of it.” |
| Loose-cell warning 18650 lithium-ion cells | 8 January 2021 | “Loose cells can overheat and experience thermal runaway … resulting in fires, explosions, serious injuries and even death” — “Do not buy or use loose 18650 lithium-ion battery cells.” |
All three point the same way. The “bad habit” listed above as using a cheap charger of unknown origin is not a matter of taste — it is an item with a reported death attached to it. And not throwing a swollen battery in the bin is explicit CPSC guidance: it goes to household hazardous waste.
These are actions against specific products, though. We did not find any statistic in CPSC documents on how often batteries swell in general.
The rest of this article — the 20–80% band, charge cycles, dark-mode savings — was not checked against an agency original. It summarises manufacturer guidance and commonly accepted explanations. Only the FAA and CPSC items above are confirmed against regulation.
Where the battery advice splits
Do I really need to stop at 80%?
It helps longevity, but daily inconvenience isn't required — enabling optimised charging is enough for most people.
Is fast charging bad?
It's fine. It runs warmer, so use it when you're in a hurry and normal charging otherwise.
Is wireless charging worse?
It generally produces more heat. Use it somewhere ventilated to reduce the effect.
Do battery-saver apps help?
Most do nothing or make things worse. Built-in OS features are sufficient.
How do I look after a power bank?
Same principle — don't leave it flat for months, top it up a few times a year. If it swells, it goes to household hazardous waste, not the bin, per the CPSC recall notice.
Anything special in summer?
The car is the danger. A phone on the dashboard gets far hotter than the outside air. Indoors, while you're adjusting the air conditioning, move the charging spot somewhere ventilated. For trips, read this with the travel preparation guide.
Does the charger really matter?
This one is not a preference. The CPSC told consumers to unplug and discard a particular lithium-ion charger, and that document records one death. Use certified equipment. To review device and account security in one pass, see the password security guide.
Eighty percent of battery care is simply not letting it get hot. Stop charging on the bed and you'll notice the difference.
Sources
- US Federal Aviation Administration — agency original — PackSafe, “Lithium Batteries” (updated 13 April 2026; read July 2026). Source for the 100 Wh limit, the 101–160 Wh two-spare allowance, “carry-on baggage only”, and the ban on damaged or recalled batteries.
- US Federal Aviation Administration — agency original — PackSafe, spare (uninstalled) lithium batteries. Source for “protected from short circuit” and for removing spares from a gate-checked bag.
- US Consumer Product Safety Commission — agency original — Super Off-Road solar power bank recall (25 June 2026). Source for “swell and overheat”, the two incident reports, and the hazardous-waste disposal instruction.
- CPSC — agency original — Garberiel lithium-ion charger stop-use warning (10 October 2024). Source for overheating, arcing and fire, six incidents and one death, and “immediately unplug … and dispose of it.”
- CPSC — agency original — Warning on loose 18650 cells (8 January 2021). Source for “thermal runaway” and “Do not buy or use loose 18650 … cells.”
Where to check further
- Your own airline's battery rules. The figures here are FAA (US), and whether a battery installed in a device may go in checked baggage was deferred to a separate FAA item we did not open. Check your airline's dangerous goods page before flying — carriers differ.
- The 20–80% band, cycle counts and dark-mode savings. These summarise manufacturer guidance and common practice, not agency text. Your device maker's battery support page is more precise for your specific model.
- How common battery swelling actually is. The CPSC document is an action on a specific product, not a population statistic. Check CPSC recall search and your manufacturer's notices for your own model.
Written as of July 2026. The items confirmed from FAA and CPSC originals are quoted directly in the body; the remaining care advice was not checked against an agency original. This is general information; features and names vary by device and manufacturer. If you suspect a battery fault, don't open the device — contact an authorised service centre. To cut the monthly bill as well, see the MVNO guide; for trips, the packing guide.
