The short answer: if your iPhone is charging slowly or has stopped charging, the cause is usually somewhere before the battery — a worn cable, a weak adapter, lint packed into the port, or heat. Work through those four first. The battery itself is the last suspect, not the first, and "my battery is broken" is the most expensive wrong guess you can make.
Change one thing at a time. If you swap the cable, the adapter, and the socket all at once, you will know it works again but not why — and the problem will come back the next time you travel.
Three situations, three different causes
| What you see | Most likely cause | First move |
|---|---|---|
| Charges, but very slowly | Worn or non-certified cable, weak adapter, port debris | Swap the cable, then the adapter |
| Stops part-way, often at 80% | Optimized Battery Charging, or the phone got hot | Read the lock-screen notice, or cool it down and retry |
| Not charging at all (no reaction) | Port, cable, adapter, socket — or a real hardware fault | Try another cable + adapter + socket before anything else |
Check it in this order (each step is 30 seconds)
① The cable. This is the single most common cause. Cables fail invisibly: the connector oxidises, the internal wires break at the bend, and the phone still shows "charging" while drawing a fraction of the current. Use a cable you know works, ideally an Apple-certified (MFi) one.
② The adapter. A 5W adapter from an old phone will charge a modern iPhone very slowly — that is not a fault, it is the adapter. So will a cheap adapter whose rated output is optimistic. If you have a higher-wattage Apple or certified adapter, try it.
③ The port. Pocket lint, dust, and fabric fibres compact at the bottom of the Lightning or USB-C port. Charging becomes intermittent or slow, and the cable feels like it does not click in. With the phone off, look into the port with a light — do not dig at it with metal, a toothpick, or a pin. A soft dry brush or a blast of air is the safe option; if the debris is packed hard, let a service point clean it.
④ Heat. A hot phone charges slowly on purpose. iOS reduces or pauses charging current when the battery is warm, because heat is what actually ages a cell. Charging in a thick case, under a pillow, on a wireless pad, or in a hot car all raise the temperature and slow the charge down. Unplug it, take the case off, let it cool, charge again. See why charging generates heat and why the phone gets hot in the first place.
⑤ Software, briefly. Restart the phone once. If it will not charge at all, that one step rules out a stuck process, and it costs nothing.
⑥ Battery health, last. Open Settings → Battery → Battery Health & Charging. If Maximum Capacity is below 80%, or the screen shows a service message, that is the one case where the battery itself is the answer — see how to read battery health.
Why a hot phone charges slowly
Charging is energy conversion, and the part that is not stored becomes heat. When the battery is already warm, iOS deliberately throttles the charging current — the slower charge you are seeing is the protection working. That is why the advice is not "force it to charge faster" but "let it cool and it will charge at normal speed".
This also means a hot phone and a slow charge usually arrive together, and they are one problem, not two. If the phone is hot whenever you charge it, the environment or the charger is the thing to change.
What not to do
- Don't put the phone in the fridge or freezer. Condensation forms inside, and liquid damage is far worse than a slow charge. Let it cool in the shade at room temperature.
- Don't install "battery repair" or "fast charge" apps. They cannot change the charging current. iOS controls it, and no app can override the safety limit.
- Don't push the connector in hard if the port feels blocked. You will damage the pins, and a port repair costs more than a cleaning.
- Don't charge under a pillow or blanket. Trapped heat is the worst case: the phone throttles, charges slowly, and ages faster at the same time.
When it is genuinely the phone
If a known-good cable, a known-good adapter, and a clean port still will not charge it, and the phone does not respond on any socket, then you have a hardware fault — port, or charging circuit. At that point stop swapping parts and get it looked at. If the phone charges slowly while also getting unusually hot, and you want to know whether the heat is normal or a real problem, that is what our diagnostic does: try WhyMyPhone and answer two questions.