Question about UPS rating - Volt Ampear hours and VA?

  • Thread starter Thread starter Alanz
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A

Alanz

Hi,

I am just confused about the UPS rating system.
I got my UPS serviced a few days back and the service guy said that he
has installed a 625 VA battery. But my problem is how do I verify that.
I opened up the UPS and saw that on the battery the rating is written
as 12V 7AH.
Does this rating has any connection with the VA rating?
Can i derive the VA rating from this or is it something entirely
different?


thanks.
 
Alanz said:
Hi,

I am just confused about the UPS rating system.
I got my UPS serviced a few days back and the service guy said that he
has installed a 625 VA battery. But my problem is how do I verify that.
I opened up the UPS and saw that on the battery the rating is written
as 12V 7AH.
Does this rating has any connection with the VA rating?
Can i derive the VA rating from this or is it something entirely
different?


thanks.
Click to expand...

VA is the total load that the UPS can take, and it is not a function of
the battery. The battery only decides the backup time. The VA is
dependent on the other components in the UPS, the transformer or
capacitor (not sure). Theoretically the VA rating arrived at by
multiplying the utility power voltage (230/115 V) with the Ampere
rating of the UPS.
 
Alanz said:
Hi,

I am just confused about the UPS rating system.
I got my UPS serviced a few days back and the service guy said that he
has installed a 625 VA battery. But my problem is how do I verify that.
I opened up the UPS and saw that on the battery the rating is written
as 12V 7AH.
Does this rating has any connection with the VA rating?
Can i derive the VA rating from this or is it something entirely
different?


thanks.
Click to expand...

The battery VA rating is then 12x7 = 84 VA. But you wont get 84VA at
the output terminals due to
a) system inefficiency
b) if you want to battery to last, discharge needs to be stopped before
its dead.
c) amphour ratings are not for short discharges, which will yield less
energy

If we allow roughly 10% for the first, 20% for the 2nd, and 30% for the
3rd, you're looking at getting around 40% of those 84VA = 33VA. If your
computer runs at 120w during shutdown, and monitor at 80w, 33VA will
run 200w for 33/200hrs = 10 minutes.


NT
 
What he meant is that the 12V 7AH batttery was the appropriate one for
your 625VA UPS. To see whether he was right, go to the UPS vendor's web
site and check out the specs for your model (or look in the instruction
book). The battery seems to me underpowered but it could be that
a larger battery (or battery pack) might not fit, physically, inside the
UPS. If the battery compartment is full, you've most likely got the
right replacement battery. Check out the adequacy by pulling out the
AC plug and see how long your computer stays up.

12V 7AH means that the battery will, nominally, supply 7 amperes
for one hour at 12 volts; (actually much less). As the demand for
amperes goes down, the battery will last longer than expected. For
example, it the battery has to supply 3.5 amperes, instead of 7, it
might stay at 12 volts for 2.5 hours rather than 2 hrs. Conversely, if
it had to supply 14 amperes, it might last 20 minutes rather than 30.

Totally disregarding all inefficiencies, 625VA divided by 12 volts
equals about 50 amperes. A 7AH battery would last ~3 minutes at that
discharge rate. In reality, much shorter because far more than 625 VA
is needed to produce 625VA (625VA is the output, not the input of the
UPS) and as the battery ages its capacity diminishes until, after 3-5
years it is useless.
 
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