Back-UPS ES 700 power supply Strange noise

[Blackball]

I puchased a while ago a Battery Back up power supply. I havnt used it for a while (About a year). I went to turn it on and there is no power so I assume it is flat. I have plugged it in to charge it up and the power on button lights up, but am getting an unusual noise. (Not high pitched) It has been stored in a dry place. Any ideas? I will take it back to the electrical shop I bought it from, but just wondered if anyone else might know.

[Teken]

A slight buzzing / hum is normal when charging or when the UPS is running off the battery. Sometimes you may hear what sounds like a hissing noise too. That hissing sound is the battery heating up and off gassing and being vented out the battery terminal. Cells that are left fully depleted can and will deform (swell & bulge) when recharged which makes it very hard to remove. 

Lastly, repeated charging of a bad cell will shorten and damage the UPS charging system.