dwight at supercomputer.org wrote:
On Wednesday 26 March 2008 07:49:48 am John Summerfield wrote:
Tom Horsley wrote:
On Wed, 26 Mar 2008 08:35:49 -0500
"David G. Mackay" <mackay_d@xxxxxxxxxxxxx> wrote:
...
Most UPS boxes seem to be about the same. They'll be reporting
self test OK and lots of battery life, then the power actually
fails, and they fall over dead.
...
Really all it has to go on is the potential difference between
positive and negative connectors. The electronics aren't going to
know how fast it goes from 72V (about where my UPS should be) to
something unsatisfactory without actually running it (partially)
down.
While that's technically true, they could also use the age of the
battery. Most (f not all) UPS batteries need replacing after 5-6
years, and decent modern UPS's allow you to set the date that you
installed the battery. This is a fairly good indicator that the
battery needs replacing.
I have found more typical is 3-4 years, I don't think I have ever got a set of
UPS batteries to last much longer.
I bring this point up for the folks out there who have old UPS's and
batteries, and are under the mistaken impression that their UPS is
going to work the next time they lose power.
Some of the UPS self-tests are good enough to catch the batteries going bad and
warn you, and some of them are not, the way you learn about it is when it fails
to work.
I had to find this out the hard way, years ago, unfortunately.
Hopefully someone else won't.
And even new batteries can die from defects, I had to determine which of 4
12V-100Ah batteries (7.5KVa UPS) that I had just installed 6 months ago had lost
a cell and have that battery warrantied. You had to put a voltmeter on each
battery while it was under load to determine which battery had a bad cell, lucky
this UPS had a good enough self test to detect a bad cell, and had enough
overcapacity to run the load with one battery having a bad cell.
Oh, and if you're buying a used UPS off of eBay, find out how old it
is first. You might be able to buy it without the worthless battery,
and save yourself some shipping cost, and possibly lower the price
of the UPS.
I would probably figure on replacing the batteries on a UPS bought like that,
besides the age, if the were stored for too long, or stored in too warm of a
location their life could be even shorter.
-dwight-
