The battery was new when you got it and it comes with probably less than 50% charge. After you charge it on your nitecore and use it when it's fully charged, it should last longer. Batteries do take long to charge. It should be 100% after 5 hours. To confirm if it's your charger over charging, use a digital multimeter and measure the voltage. A full battery will read 4.2v on a multimeter. If the reading is higher, that means your charger is over charging the batteries and this is dangerous for you and your batteries. If it's less, just let it charge more.
