As of about 4 hours ago, my iPhone has refused to charge and the battery has drained, every time it gets enough power to turn on it displays the lock screen and "9%" battery before promptly dying after a second or two.
I have never had any issues with my battery before today and i'm using the same charger i've always used
EDIT:
It is an iPhone 5s, I have had it for about a year but is second hand, iOS 8.X, it no longer turns on at all, just displays one or other of "charging" or "needs charging" and can not reach the lock screen, I have tried 3 different charger with 2 different cables.
Best Answer
Troubleshooting an iPhone (or iPad) requires access to some known-good parts and accessories. As @gnasher729 suggests, either get a known-working charger and cable or try the ones you have with a working device.
Once (or if) you've eliminated the charger and cable, then the next likely culprit is the battery. Remove the existing battery and if possible, measure the voltage of the battery directly with a multimeter. If you are getting more than 3.2V, then the device should boot on it's own. If the voltage is good and it doesn't boot, then you have a logic board issue.
Finally, if the battery doesn't charge or hold a charge (or if you don't have a multi-meter to test the original battery), replace it with a good quality battery from a reputable vendor or preferably, one that is known-good. If the phone still doesn't boot, once again you have a logic board issue. The only option at that point is a board-level repair, which might not be economically viable, or a replacement.