welcome to the forums kaseiryu.
This sounds like a short.
There are several places this can be happening from, so lets start with the easiest. The battery pack.
Take the pack out and make sure the connectors are all clean, peek inside the hilt to and make sure the 3 prongs are clean. Inspect the pack itself. One of the springs may be loose, or tab may be broken. .
Do you have a similar saber that u can swap the battery packs out with to see if the pack itself is faulty?
Let's start there, if all is fine with the pack, you may need to check out the switch/power button, but that gonna require you taking the saber apart, ...a little. ;)