Backups BE700G-UK Constant Tone

[Anonymous user]

With the above UPS, last night it started with a constant tone no power indication illuminated, and all devices connected to it powered off. I have already searched for rectification ideas so have tried the disconnect from mains, remove battery, hold power down for 5 seconds, re-connect battery and power back on. The device goes through it's self test and then emits a constant tone with no light indication again. 

The only things connected to the device was a 2 bay NAS which was in standby at the time as it had powered down for the night. An answer phone & 2 Broadband routers (I don't like the fact BT can get into the homehub, therefore potentially into my network) so it shouldn't be overloaded the highest power draw is 90W on the NAS when it under heavy workload.

I have had a voltmeter on the battery while out of the unit that was showing around 13.5 - 13.7 volts, the Ups was about the same when power was applied without the battery connected. 

Any suggestions as my first thought were the battery as it is over 5 - 6 yrs old, and still original, but not keen on spending nearly half the cost of a new unit on something that might not fix the problem.