Re: UPS as backup power

[Bill Wichers <billw at waveform_net>]

>My research tells me for some reason, the load vs
>runtime is not linear.  For example, the chart for
>[snip]
>So I thought a filter at 20-25W would only run 60min
>max.  But you have gotten 12+ hrs!  This is very good
>news for some of us.
>
>Maybe an EE guru can explain why.  My guess is, it has
>to do with 1) Wattage rating on many equipment are
>peak values so the actual consumption varies & is much
>lower, 2) the UPS has to invert DC back to AC so maybe
>the higher the load the less efficient it gets.

It has to do with 3) battery discharge curves. Batteries don't drain in a 
linear relationship to their load. That is why if you look at the Amp/hour 
(Ah) rating of a battery if will always specify a rate (usually 20 hour), 
which can be read as "the number of amps that can be drawn from this 
battery for 20 hours". 1 Ah is supposed to mean you can draw one amp for 
one hour, but if you draw 1/2 amp you might get 160 minutes of runtime. I 
am sure there is a reason based on the battery chemistry, but I haven't 
ever seen it. Also, remember that a discharged battery does not mean "zero 
volts". A discharged battery by definition has reached the point at which 
the voltage has dropped to a level that is considered discharged. If I 
remember correctly that is 10.2 volts for a 12 volt battery.

Wattage ratings on equipment are normally "worst case" loads, like a motor 
running at maximum, or an adjustable fan running on high, etc. The 
conversion from DC to AC in the inverter wastes some energy as heat, but 
isn't a large contributing factor to UPS runtime for our purposes.

You might want to check and make sure your UPS doesn't get too hot when 
running for extended periods of time. Most small UPSes are only expected to 
run for about 7-15 minutes and are not usually designed to run for very 
long periods (many hours). If your unit isn't hot to the touch you are 
probably OK.

>A friendly reminder: Since most UPS use Lead-Acid
>batteries (ie a car battery), logic tells me you
>should keep it fully charged at all times & shouldn't
>do too many of these "full drain tests".  Or you may
>significantly shorten it's life & capacity, just like
>if you leave the car lights on far too many times. :P

No UPS will use a normal lead-acid car battery due to the emissions 
problems during charging and discharging. All UPSes use either sealed lead 
acid or gel-cell batteries. Typical UPS batteries are "deep cycle" type, 
and they are able to be deeply discharged with less chance of damage than a 
typical car battery. Better UPSes should shut down before the battery 
reaches the point of damage. Pretty much all UPSes will charge whenever 
they are getting power, so you shouldn't have to worry about that part.

         -Bill



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Waveform Technology
UNIX Systems Administrator