My CyberPower LX1500GU seemed to have dead batteries, given the Self Test button in the CyberPower personal edition of the software caused everything to power off! Yet, batteries were listed as fully charged, etc. etc. when it was powered back up.
After removing the old batteries, shaking them sounded like a maraca! Yes, I failed to test regularly and these were the original batteries. This time the Self Test button actually worked after replacing. It was also handy that these came charged!
The only difficulty was one connector took a while to disconnect. I was worried about ruining the connector. and wire. But connecting the new batteries back up was easy.