The video below will walk you through it. Every now and they the tabs that make contact between the battery and chassis can bend inward and create connectivity issues. This is a fairly common and a relatively easy fix. For starters remove your grip to expose the battery compartment. Remove the battery. Insert something behind the tab, it can be anything, butter knife, screw driver, or the allen wrench provided with your saber. Bend the tabs inward so they apply more pressure on the battery and make better electrical contact.
https://www.youtube.com/watch?v=wQAoUsUN7YQ
