At 07:31 AM 7/13/2007, Robert G. Brown wrote:
For small/personal clusters I change my mind again. I tend to buy cheap
UPS's for my house because our power bobbles for 1-5 seconds nearly
every heavy rain/windstorm we have, which is why I know from direct
experience that the batteries in these UPSs are lucky to last two whole
years. I've got something like three of them where I'm plugged into the
surge side because the UPS side is dead (no power at all) or goes down
anyway when power bobbles, to the tune of system crash AND the maddening
beeping. I'd love to find a 10 second PDU/conditioner that used a
really large capacitor instead of a battery to buffer short outages,
Tough.. the energy density of lead acid is really high.
Here's an example using one of those big 1 Farad 12V caps the auto
sound people use.. say the UPS can take voltage drop of 2V on the
"battery". At 12V it stores 72 Joules. At 10V, you've recovered 22J.
That's about 1/10th second, assuming the UPS is 100% efficient, which it isn't.
For 10 seconds, you need 100 Farad.. that's a BIG cap.
The battery life *should* be a whole lot longer (Lead Acid batteries
can have 10-20 year lives, as can NiCd), however, the run of the mill
UPS doesn't treat the battery very well.
especially at mass market prices.
That's the problem.. mass market means race to the bottom for quality
and life, to reduce initial price. You can get a PC UPS which
provides 10 minutes or so at 200-300VA for $50-70 at the local big
box store. If you were willing to pay, say, $300-400, you could
probably get 20 year life.
(you could cobble one together cheaper.. a high quality charger is
probably $50-100, good quality battery is around $20, a high quality
inverter is $200 or so, and then you'd need some sort of transfer switch.
Take a look at products aimed at the sailboat/solar power market,
which are definitely aimed at longevity.
Does anybody know of such a beast? No battery, no toxins, just a big
cap and small price?
rgb
--
Robert G. Brown http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567 Fax: 919-660-2525 email:[EMAIL PROTECTED]
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