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Estimating UPS run-time?

G

Gloria West

Jan 1, 1970
0
A UPS rated at 950 VA output (115 VAC sinewave -- yes, sine) with 20 AH, 24
volt battery. How to *estimate* run-time at full load? Presume fresh
batteries. Of course the UPS eats up some in inefficiency...

"Extra credit for showing your work." ;-)

Help is appreciated.

Thanks.
 
E

Ecnerwal

Jan 1, 1970
0
Gloria West said:
A UPS rated at 950 VA output (115 VAC sinewave -- yes, sine) with 20 AH, 24
volt battery. How to *estimate* run-time at full load? Presume fresh
batteries. Of course the UPS eats up some in inefficiency...

"Extra credit for showing your work." ;-)

Help is appreciated.

Thanks.

Insufficient data - what's plugged into it? The rating does not patter,
the actual load is what matters.

If at full load, 20 x 24 / 950 gives the time you'll never actually get,
due to both inefficiency and the fact that the 20AH batteries only give
20 AH of it's withdrawn over 20 hours. Suck it out in half an hour and
you get much less, so you would not get half an hour even with 100%
efficiency.

So, you won't get 30 minutes and 30 seconds. 15 min might be a good
practical guess, but of course the batteries in a UPS are usually so
abused (cheap charging circuits designed to kill batteries and make you
buy new ones) that 5 minutes would be doubtful after a year or two.
 
E

Ecnerwal

Jan 1, 1970
0
Gloria West said:
A UPS rated at 950 VA output (115 VAC sinewave -- yes, sine) with 20 AH, 24
volt battery. How to *estimate* run-time at full load? Presume fresh
batteries. Of course the UPS eats up some in inefficiency...

"Extra credit for showing your work." ;-)

Help is appreciated.

Thanks.

Insufficient data - what's plugged into it? The rating does not patter,
the actual load is what matters.

If at full load, 20 x 24 / 950 gives the time you'll never actually get,
due to both inefficiency and the fact that the 20AH batteries only give
20 AH of it's withdrawn over 20 hours. Suck it out in half an hour and
you get much less, so you would not get half an hour even with 100%
efficiency.

So, you won't get 30 minutes and 30 seconds. 15 min might be a good
practical guess, but of course the batteries in a UPS are usually so
abused (cheap charging circuits designed to kill batteries and make you
buy new ones) that 5 minutes would be doubtful after a year or two.
 
S

Sjouke Burry

Jan 1, 1970
0
BBC said:
Can u plz tel me Why the power in UPS is measured in VA instead of
WATTS?
i'm a begineer,Plz help me out?

I cn hrdl rd/ndrst wt y mewn?
 
R

Rich Grise

Jan 1, 1970
0
BBC said:
Can u plz tel me Why the power in UPS is measured in VA instead of
WATTS?
i'm a begineer,Plz help me out?

It's about reactive power factor; if it's a capacitive or inductive
load instead of a pure resistance, the reactive power can exceed the
real power, and the rest is dissipated in the source. Battery
capacity is measured in Amp-hours; even if you're driving a pure
inductance or pure capacitance, which draws no "real" power, the
battery still has to provide the current.

And, since this isn't the twitland, please speak English rather
than twitspeak.

Good Luck!
Rich
 
R

Rich Grise

Jan 1, 1970
0
Gloria said:
A UPS rated at 950 VA output (115 VAC sinewave -- yes, sine) with 20 AH,
24 volt battery. How to *estimate* run-time at full load? Presume fresh
batteries. Of course the UPS eats up some in inefficiency...

"Extra credit for showing your work." ;-)
How much are you paying for this school? We generally don't do other
people's homework for free.

Good Luck!
Rich
 
E

ehsjr

Jan 1, 1970
0
Globemaker said:
950 WATT / 24 VOLT = X AMPS

t = 20 AH / X = 20 * 24 / 950 = 480/950 = .505 HOURS

Considerably shorter than that. The 20 AH rating is what
the battery can provide over 20 hours - 1 amp x 20 hours.
When you discharge it at a higher rate, in this case at
about 39.6 amps or ~ 40 times the nominal 20 AH rate, it
cannot provide anywhere near the full 20 AH rating.

Ed
 
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