Battery backup

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From - Thu Jul  1 14:14:42 1999
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From: jeffl@comix.santa-cruz.ca.us (Jeff Liebermann)
Newsgroups: comp.unix.sco.misc
Subject: Re: UPS suggestions
Date: Thu, 01 Jul 1999 17:56:31 GMT
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On Thu, 01 Jul 1999 06:15:39 -0400, -bill- <bill@TechServSys.com> wrote:

>I'll take the lecture.

OK.  However, you get an abbreviated version or I'll be late for what
will hopefully be a free lunch.  Unfortunately, the door price is a box
of RTL8029 based ethernet cards.  Yech.

>The big problems with 48v power supplies as I see it are:
> cost
> needing special ventilation for the batteries/special battery room
> a 48 v battery pile represents a large danger.

Yep.  However, have you ever seen how really big UPS's are setup?  They
don't just drop some car batteries on the floor and run clip leads to the
equipment.  The battery vents are on a manifold, the equipment cabinet is
desiccated and explosion proof, and the whole mess complies with
applicable NEC codes.  There are fuses, cut-outs, fire detectors, and
instrumentation that can be added.  It's not that hard but you can't do
it in the middle of an office or in most server rooms.  I also would not
attempt to build a large DC operated system without professional and
experienced help.  As a general rule, you can build a box around gel
cells, but wet cells should be desiccated and ventilated.

For small installations (<3 machines), the batteries can be sealed gel
cells on a palette.  Basically, it's similar to solar alternative energy
installation.  A good fast fuse will protect the battery pile from the
inevitable screwdriver across the terminals.  Out gassing from dessicate
gel cells is minimal and is not a problem in a large enough room.  I
would not do it in an unventilated closet mostly because of the heat
build up, and not the hydrogen accumulation.  Almost any kind of
protective enclosure will suffice.  Look at the way the larger Best and
APC SPS's are packaged for a clue.

Where I get into trouble is when I have a growing installation that
starts out small and adds a new server every few months.  I had one
customer with 6ea DC operated servers running on a bank of 24ea 2volt
Trojan batteries charged by an enormous fork lift battery charger.  I did
not design this mess but will confess to being involved in making it
work.  Were it not in a well ventilated warehouse, I would be worried.  I
successfully lobbied against the aesthetics committee's desire to enclose
it with a wall for safety reasons.

I also use DC power on mountain top radio sites.  Besides the completely
unreliable power, there are generator limitations at some sites.  Even
with a generator, the computers tend to be more sensitive to power
glitches than the radios.  Since many sites run on solar power anyway, DC
powered computers are an obvious expedient.  However, that's not what
inspired me to use DC.  It was because the common SPS product of several
years ago was not terribly RF proof.  My logs showed false tripping,
erratic operation, and uncontrolled output that I suspected was caused by
the high RF fields.  The solution was to simply remove the SPS from the
picture.

Another wonderful surprise is that according to the EPA, anyone storing
more than about 500 lbs of batteries is now the proud owner of a
hazardous material and toxic waste dump and therefore must comply with
environmental regulations.  This is largely being ignored with clean
installations, but I slammed into the EPA when one customer made a huge
messy pile of dead batteries, in plain sight, and let them sit for 6
months.  Eventually someone complained and the EPA arrived in force.  My
next car will NOT be another pickup truck.

Let's do some math and guesswork.  The typical server uses about 170
watts (1.5amps) at 117VAC.  This does NOT include the monitor which will
run off the usual APC UPS.  170 watts at 24VDC is 7 amps.  I use 2ea deep
cycle type 27 marine gel type batteries rated at about 100 Amp-hours.  I
can usually run these down to 75% of capacity without damage and 60% if I
don't care about battery lifetime.  75% of 200 A-Hr is 150 A-Hr or 21
hours run time for a single server that uses 7 amps.  I've run tests and
this is roughly what I get.  Now, try to buy a 117VAC SPS that will run a
server for 20 hours for under $2000.  Also, with such a long runtime, I
don't bother with automatic shutdown scripts.

I'll spare you my list of home power related URL's.  You should be able
to find lots on alternative power generation.  The safety issues are
often discussed.

The biggest headache is finding a suitable battery charger.  The
Statpower 12volt incantations in the marine supply and altenative energy
catalogs are fine.  However, I want 24V and 48VDC and these are difficult
to find in low power versions.

I'm late, more later if anyone is interested...


-- 
Jeff Liebermann  150 Felker St #D  Santa Cruz CA 95060
(831)421-6491 pgr (831)426-1240 fax (831)336-2558 home
http://www.cruzio.com/~jeffl   WB6SSY
jeffl@comix.santa-cruz.ca.us   jeffl@cruzio.com

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