BE750G-ES problem : constant tone on power failure, no backup power supplied but battery shows 12V..

[Anonymous user]

I purchased a BE750G a while back, recently the unit died on me and the device is out of warranty as I cant find the exact date of purchase vs manufacture and it's over 3 years old by APC's logs. I have a basic education in electronics repair and feel confident in troubleshooting this device with the manual provided by APC. However I cannot find anything in the manual that seems to match the behaviors that I'm experiencing with my particular unit.

In a nutshell my power went out in a storm and the unit failed to supply backup power to my devices. The laptop power supply was plugged into the master and the master enable LED was lit up along with the power LED, after the power went out the power LED went off, the master enable LED remained on and the unit made a constant tone. I had to press and hold the power button for several seconds to stop the tone and until the power came back at the house if I tried to power on the unit 'cold' it would just emit a droning tone until turned back off, the master enable light would turn off and on if pressed. At no point does the device provide backup power to any socket regardless of master enable being on or off.

In the past when the power goes out the unit would make a tone every so often to let me know it was on battery power and would support my devices for a short period of time. Now it just works as a power strip and when the power goes out the unit just dies, makes a noise until I cut it off and no power except that provided to the master enable LED.

At first I was curious to see if the battery was the problem but the battery is showing 12V when removed from the unit and tested with a multimeter. The battery also seems to hold that voltage if removed and let sit so I don't think the battery is defective. I pressed the breaker reset, removed the battery and held the power button to discharge the unit and reattached the battery, etc - nothing changes the cold start behavior. I'm wondering if the unit's logic board is just shot or if there's something I can do to fix the problem. As the manufacture date is over 3 years old and I have no chance in finding the receipt to try and get a repair I'm looking for any help. If it's a bad fuse or available component I can solder / replace it so that's not an issue if it comes down to it. However I'm not familiar with single points of failure on these devices so any help would be great.

[BillP]

Hi Foodude,

I am by no means an expert on this particular APC UPS or the function of its lights and buttons, but I would be concerned if your battery is only showing 12V when removed from the unit and unloaded.  Float charge level on a 12V battery should be around 13.5 volts or better when it is fully charged.  And with three years already on the battery, it may be time for a replacement.

[BillP]

I was thinking the same. It also sounds like the battery cannot support any load and with this type of behavior after 3 years, it could be needing a battery replacement. What happens if you turn it on and pull the plug with no load on the battery outlets? Does the UPS turn off?

[Anonymous user · Accepted solution]

It is not the battery, I ran some tests and had these results :

1. I obtained a known working APC battery to an identical model UPS unit.

2. I fulled charged the battery with a working unit. Said working unit will beep 4 times every 30 seconds when power is disconnected and supplies backup power as it should.

3. I disconnected the old battery and installed the replacement into the unit that was making the constant tone on power failure hereafter referred to as the malfunctioning unit.

4. I turned the malfunctioning unit on and it proceeded to give me the same beep then constant tone until I turned it off. No change at all.

5. I disconnected said battery, the new and fully charged battery, and installed it in a known working APC UPS.

6. I tested the new, fully charged, replacement battery in a known working unit and it functioned perfectly. 4 beeps every 30 seconds, power supplied to devices.

7. I took the battery from the malfunctioning unit and put it into the known working unit and charged it for 16 hours.

8. I removed the battery, installed it back into the malfunctioning unit and again - beep then constant tone until turned off.

So no matter what battery is in the UPS, it just cuts on and makes this constant tone unless it's being supplied AC power from a wall socket.

I would like to correct my OP, the master enable light will not turn off or on while the unit is on. The unit just shows a light and makes a constant tone.

I do have some college education in electronics fundamentals so I discharged the unit and opened it up with some heavily insulated gloves on and noticed no loose connections, visible damage, burning, swollen / plagued caps, or anything that looks out of the ordinary. The board looks about perfect, not even dust.

Unless someone knows of something I'm just missing, I'm stumped.