Interesting that the fan comes on while charging--and as the batteries degrade, I suppose that happens more frequently.Batteries have a shelf life whether you're using them or not. I have two old APC 1400RMNET units, they're over 15 years old still working. The fans just come on when power is lost or charging. There's an alarm that comes on too so the fan noise is mute at that point. You'll be shutting down PJ anyway. Then I have a small one battery APC CS 500 for AVR and stuff. 3 backup and 3 surge ports.