[PLUG] Re: UPS failure/replacement

Brian & Lori Nordlund ble.nordlund at verizon.net
Tue Dec 20 00:37:31 UTC 2005


A poor man's UPS could be built using:

One 12VDC power supply, probably 25 Amp capacity would be acceptable.  This
would not be cheap for a new unit, but if you are hunting for surplus or
will build your own, you might get by without spending a fortune.

One 12V sealed or controlled venting battery.  It would not be good to run a
standard car battery, since they vent hydrogen gas and acid vapor, neither
of which are great to have wafting around your computer room.  You can
usually get a 36AH gel cell for around $50 if you look around.  That size
battery would give you close to an hour and a half of operation, assuming
one computer at less than 300 Watts of load.  If you go with a smaller
battery, just make sure it can source 25 or 30 Amps to operate the inverter
at load (assuming one or maybe two computers).

One 300 to 500 Watt power inverter.  Computer power supplies are pretty
forgiving as to the source of AC power since they are chopping it up anyway.
As a result, you can probably use a simple "quasi-AC" inverter, many of
which go on sale for $50 or so, especially in the automotive market, like GI
Joe's.

Since this is the simple approach, the system would be on-line all the time,
which leads to the requirement for the power supply to be able to provide
the large amount of power to operate the computer system as long as you have
AC power.  If you want to get slightly tricky, you could experiment with an
AC-powered relay, that you would wire so that the computer is connected to
the commercial AC power as long as there is power.  When the relay coil is
deenergized, the contacts would swing you over to the output of the
inverter, which is already sitting idle ready to go.  This would do two
things: one is it would reduce the average load on the inverter, extending
it's useful life.  The other thing is that the power supply could be
replaced with a simple float charger for the battery, which would only have
to maintain the battery and keep the inverter powered on in the idle state.
I have not made any measurements, but a 1 Amp source would probably be
plenty.  Of course, for this to work, your computer would have to be able to
sustain itself during the momentary interruption of power while the relay
swings over to the inverter.  Because this approach has a pretty attractive
cost benefit, due to eliminating the huge power supply required for an
on-line system, the experiment would be worth a try.  I would be surprised
if it didn't work just great.

If you delve into this, remember the DC side of things runs pretty high
current (25 to 30 Amps), so you should be running 10 gauge wire or so.  The
AC side is peanuts at 2 or 3 amps.

If it were me, I would do this in a heartbeat.  I currently have a small
off-the-shelf UPS that is hooked up all the time, but I also have 750AH of
batteries under my desk that are always kept at float voltage with a small
2A supply.  When I decide to watch TV during a power outage, I just plug in
the inverter and get about 30 hours of operation.  I just haven't bothered
to make it permanent.  Of course, if you wire something up and it makes big
sparks, I didn't tell you to do it.................

Just some thoughts,
Brian.



-----Original Message-----
From: plug-bounces at lists.pdxlinux.org
[mailto:plug-bounces at lists.pdxlinux.org]On Behalf Of Kevin Cosgrove
Sent: Monday, December 19, 2005 11:57 AM
To: General Linux/UNIX discussion and help; civil and on-topic
Subject: Re: [PLUG] Re: UPS failure/replacement



On 18 December 2005 at 9:42, plug_0 at robinson-west.com wrote:

> Why buy an APC unit if the batteries are proprietary?

After I recycled an old (very old) UPS I found out that Battery
Exchange on TV Hwy in Beaverton carries UPS batteries.  If you're
shopping (I missed the original posting) for a UPS, why not check on
replacement battery availability and cost as part of the purchase
decision?  I do the same regarding printer consumables when selecting
a printer.

Best-o-luck....

> Why not build your own off of standard car batteries that you
> can pick up just about anywhere?
>
> Does anyone know of a kit with all the needed circuitry pre done
> for a do it yourself UPS?


--
Kevin


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