@stupac start by cleaning the lightning port and try a different cable as well. You must also make sure that you have the right charger and that the charger is working properly. If all that is okay, you may need to replace the lightning connector, follow [guide|18227|this guide]. While you are at it, replace the battery. Once you got all that accomplished, re-evaluate. Should it still do the same thing, you will have to take a closer look at the components on your logicboard. It is possible that your power management IC has failed or any of the components that make up your charging circuitry. Try the easy (relative) stuff first.