This was originally posted on APC forums on 7/14/2012
I have a Backup ES 750 that has started sounding a continuous raspy (not normal) beep and provides no power. LED is green, no other malfunction lights. Cannot turn the unit off with on off button. Beep continues even if nothing is plugged into it. Beep only stops when the battery is removed (of course) but starts again when battery is re-inserted (and unit turned on) even with nothing plugged into the unit. Battery voltage tests 13.5 VDC. After inserting a replacement battery, the unit operates normally. Software indicates the only power interruption occured one week before this beeping began. What gives - why doesn't the battery work if it tests fully charged and where did this ugly rasping sound come from?