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UPS voltage when running on battery - should it be low?

Posted on 2016-07-25
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Last Modified: 2016-07-30
I cause my own problems most times.  but I don't think  it's this time.

I took an old APC 550VA UPS that had a dead battery and wired the battery leads to a cigarette lighter plug. I wanted to make it into an inverter to get 120V in the car.  I have a couple inexpensive inverters but wanted to try this with an old UPS.  I had opened up the UPS and broke the buzzer so it's not beeping when running on the car battery.

Checking the voltage coming out of the UPS when running on the (car) battery via the cig. lighter jack, it said 90V.  (with both an old sears craftsman digital VOM and a new Harbor freight digital VOM).

I took an unmodified similar but newer APC UPS and measured that with AC power (120v) and when it was on the internal battery (around 90v like on the UPS I modified).

I plugged in some transformer bricks and USB power adapters and they put out their rated voltage (12v and 5v).

I don't have an olliliscope... is the 90v because the UPS makes a different wave rather than the sine wave from power outlets and that looks to cheap VOMs to be less than 120V?!

Thanks!
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Accepted Solution

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BeGentleWithMe-INeedHelp earned 0 total points
ID: 41727930
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Expert Comment

by:Obaid ur Rehman
ID: 41728288
When UPS is not connected to the main power and it is using battery power, the voltage of the batteries will be indicated a little low and power is being drawn out of it. When it is connected to Main, the voltage you measure are actually charging voltage, not the actual voltage of the battery. If you disconnect Main power and measure battery voltage on it's terminals, that is the correct battery voltage.

If a load is present, battery will indicate a little further low voltage as power is being drawn.
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Author Comment

by:BeGentleWithMe-INeedHelp
ID: 41728293
i'm talking the voltage at the 120v receptacles when running on batteries,  not the battery voltage
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Expert Comment

by:Obaid ur Rehman
ID: 41728304
I am having trouble understanding your question. Does it give low out put power (90V AC) on batteries instead of 120V?

APC UPSs are configured with APC Link Protocol. It may give strange behavior if not configured properly but it doesn't necessarily means the UPS is broken or faulty.
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Author Closing Comment

by:BeGentleWithMe-INeedHelp
ID: 41735658
I found the answer on the web
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