Estimating UPS run-time?

[Gloria West]

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.

 

[John Larkin]

On Wed, 27 Apr 2011 07:33:19 -0700, Gloria West
<gloriasbest22@NOSPAMgmail.com> wrote:

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.

RT(hrs) = 0.8 * 20 * 24 / 950 = 0.4

What's the prize?

John

 

[Ecnerwal]

In article
<0001HW.C9DD773F00DA65C2B01029BF@news.eternal-september.org>,
Gloria West <gloriasbest22@NOSPAMgmail.com> wrote:

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20 AH of it's withdrawn over 20 hours.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.

--
Cats, coffee, chocolate...vices to live by

 

[Ecnerwal]

In article
<0001HW.C9DD773F00DA65C2B01029BF@news.eternal-september.org>,
Gloria West <gloriasbest22@NOSPAMgmail.com> wrote:

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.

--
Cats, coffee, chocolate...vices to live by

 

On Wed, 27 Apr 2011 07:33:19 -0700, Gloria West
<gloriasbest22@NOSPAMgmail.com> wrote:

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.

1. Connect intended load to UPS.

2. Unplug UPS.

3. Start stopwatch.

4. When UPS shuts down or load no longer works properly, stop
stopwatch and read backup time available.

That's the best case estimate. It's an estimate because the battery
life will not be the same the next time the UPS is used. Inexpensive
UPS units often trade quick charging of the batteries (to have full
UPS power available again as quickly as possible) for battery life (a
properly controlled charge takes longer but preserves battery life).

See the discharge chart on page 2 or this document
http://www.gamewell-fci.com/datasheets/CS-2500.pdf for info on
discharge rate versus power available. A 26AH battery (20 hour
rating) can only provide 15.6AH when discharged in one hour. That's
only 60% of the 20 hour rating. At an equivalent discharge rate, your
20AH battery can only provide 12AH. 24V * 12AH = Watt hours - that's
perhaps 15-20 minutes. Most UPS units are designed to take a load
until
1. the load can be shutdown properly
or
2. a slower backup device (generator) can be brought online.

If you want long-term power from batteries, you need BIG batteries and
an inverter than can run at full power for days without overheating.
Your UPS is probably only rated for some minutes of power and will
overheat and shutdown (or die) if you try to run it at 950VA for
several hours.

I have a 180 watt APC UPS that powers my home network (wireless router
and a 16 port switch). It's good for something over an hour with a
load of less than 50 watts. This is normally adequate, as outages are
infrequent and usually short.

How do I know it runs an hour plus? A windstorm took out power to the
area for 16 hours last July. I have an 80AH car battery that I could
connect to this UPS, but I think the UPS generates too much heat to
allow it to run for 8 hours and I can live without the internet for a
few hours ;-)

I fired up the smaller of my two generators intermittently - just
enough to keep the fridge and freezer cool. I used a 12 volt fan
connected to an 8AH gell cell battery (good for about 24 hours use)
instead of the ceiling fan in the bedroom for people cooling and I
cooked on the grill outside to avoid heating the house.

John

 

[BBC]

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?

 

[Sjouke Burry]

BBC wrote:

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?

 

[Rich Grise]

BBC wrote:

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

 

[Rich Grise]

Gloria West wrote:

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

 

[Globemaker]

On Apr 27, 10:33 am, Gloria West <gloriasbes...@NOSPAMgmail.com>
wrote:

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.

950 WATT / 24 VOLT = X AMPS

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

 

[ehsjr]

Globemaker wrote:

On Apr 27, 10:33 am, Gloria West <gloriasbes...@NOSPAMgmail.com
wrote:

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.


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