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Old computer UPS for emergency power

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Posted by nospam4me on December 7, 2006, 8:06 pm

I can get my hands on a cheap used 500W computer UPS. The battery is
dead in it, so I was thinking of replacing the battery with marine deep
cycle battery. I would leave the unit in my garage plugged into grid to
keep  battery fully charged. Once the power goes out, I would just run
extension cord to power necessary items. I want to use deep cycle
battery because it has much larger capacity than the original battery
and is cheaper.
My question/ concern is with using deep cycle battery, can it be used
for that type of application? I am worried about the UPS unit
overcharging the battery causing explosion and burning down my house.

Posted by clare at snyder.on.ca on December 7, 2006, 11:16 pm

On 7 Dec 2006 12:06:39 -0800, nospam4me@mailcity.com wrote:

No problem there - but make sure the UPS has a fan. Most 500s don't.
Most that are useable for this purpose require 24, 26, 48, or 60 volts
too. Best one to look for is an old Exide/Powerware Prestige EXT. They
are set up for extended run, are full sine wave, and are available in
750, 1000, and 1500va sizes. They've been made for about 15 years, so
there are some used ones with bad batteries available at reasonable

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Posted by Steve Cothran on December 8, 2006, 1:08 pm

In another life I worked at a manufacturing plant where we used
scarfed ups'es with dead batteries to power portable computers on
electric carts, by hooking up to the 12/24v batteries in the carts.
Nifty and handy but...

If nothing else, be sure you fuse the thing at the battery terminal as
the UPS can likely not handle the bazillion amp fault current supplied
by the bigger battery if (when) something goes south in the ups.

Posted by sylvan butler on December 8, 2006, 5:30 pm


I wouldn't worry about that.  The deep-cycle battery is so much larger
than the original battery, a more likely problem is that the UPS will
not charge it fully.  Unless you expect to regularly exercise it, a deep
cycle may not be ideal.  A regular starting battery is more typical of
UPS usage patterns.

Will the UPS "cold start" (turn on and provide 120vac even if 120vac is
not present)?  Some will only continue the output, but not start the
output after the input power goes off.

Also note the good points raised by others...  Does the UPS use a 12v
battery?  500w is a common point for 24v battery UPS systems.  Fuse the
battery (50amp to 100amp fuse at 12v, half of that if 24v).  And be
aware that some UPS will overheat if run for a long time (as made
possible by your much larger battery).


Wanted:  Omnibook 800 & accessories, cheap, working or not
sdbuse1 on mailhost bigfoot.com

Posted by clare at snyder.on.ca on December 9, 2006, 12:25 am

On Fri, 8 Dec 2006 10:30:43 -0700, sylvan butler


 Overcharging will not be a problem, even IF the charger is properly
sized, because gel batteries are charged at a lower voltage than AGM
or flooded cell batteries.
If you use a "dual conversion" UPS run time is a non -issue because in
normal use they run 100% of the time. That's why I reccommend
thePowerware elite (9000 series).

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