How do I tell whether it is the cable or the phone?

Try your cable in someone else's phone and a known good cable in yours. If the other cable works in your phone, the cable was the cause. If no cable works but wireless charging does, there is almost always lint packed into the charging port at the bottom of the phone.

Do I have to use an Apple cable?

No, but pick one labelled MFi, which means Apple has approved it. Very cheap unbranded cables often stop working after a few months and tend to give you a charge that starts and stops over and over.

The phone gets warm while charging. Is that normal?

Mildly warm is normal, especially with fast charging. If it gets too hot to hold, the phone pauses charging on purpose until it cools down. Put it somewhere cool and take the case off. If it gets very hot, smells burnt or the screen bulges, unplug it and take it in.

Battery health says 78 percent. Should I replace the battery?

Below 80 percent counts as worn. The phone still works, but it runs out sooner and can feel slower. A battery replacement at Apple or an authorised repair shop costs far less than a new phone.

iPhone not charging? Here is how to get it going again

When an iPhone will not charge, it is nearly always the cable, the plug adapter or dust packed into the charging port at the bottom of the phone. The battery itself is rarely the cause. The steps below take about ten minutes, and most people are charging again by step one or two.

Start by leaving the phone in the charger for a while. A completely flat iPhone can need fifteen minutes before the screen lights up at all, so a black screen right after you plug it in does not mean charging has stopped.

Try this first

Change the wall socket. If the charger is in an extension lead or a power strip, plug it straight into the wall instead. That socket in the strip may have stopped delivering power.
Change the cable. Borrow one you know works, or try your cable in someone else's phone. Cables break inside, usually right at the end, and you cannot see it from the outside.
Change the adapter, the small white block that goes in the wall. Try another one, or plug the cable into a computer or a USB socket in a car.
Shine a light straight into the charging port at the bottom of the phone. If you see grey lint sitting there like a little ball of wool, that is the most common cause of all.
Turn the phone off and gently lift the lint out with a wooden toothpick or a straightened paper clip. Take a little at a time and do not scrape. Never use a needle, water or cleaning fluid in the port.
Plug the cable back in and listen. A charge that has started gives a short chime and a lightning bolt next to the battery symbol at the top right.
Leave the phone charging for at least 30 minutes before you judge whether it worked.

If that did not help

Restart the phone. If your iPhone has no home button: press and release volume up, press and release volume down, then hold the side button until the Apple logo appears. If it has a home button, hold the side button and the home button together until the logo appears. This deletes nothing.
Is the phone warm, or has it been in the sun or a hot car? Let it sit at room temperature for half an hour, take the case off and try again. The phone pauses charging on purpose when it is too warm or too cold.
Try a wireless charging pad if you have one. If it charges wirelessly but not by cable, the cable or the port is the cause. If it charges by cable but not wirelessly, take the case off and place it centred on the pad.
Check the battery under Settings > Battery > Battery Health and Charging. Below 80 percent means the battery is worn, and replacing it often makes the phone both faster and more dependable.
Look on the same page for Optimised Battery Charging. That feature stops at 80 percent overnight on purpose and tops up just before you usually wake. In that case the phone is charging exactly as intended.

Stop charging and take the phone in if it shows nothing at all after an hour in the charger, the charging port feels loose or looks damaged, the phone becomes very hot, smells burnt, or the screen starts to bulge. A swollen battery should not be charged any further. Book a time with Apple or an authorised repair shop. Ask us first if you want help explaining what to say when you hand it in.