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Author Topic: Building a UPS (pwrgate, AGMs, inverter...)  (Read 3453 times)

Posts: 3

« on: August 04, 2008, 09:09:51 PM »

Hello All -

I'd like to build a UPS to save money and take advantage of parts I have here already.  I've got a Super PwrGate from WestMountain and a couple of new AGM deep-cycle batteries.

What's different here is that I want this for AC, not DC.   When power from the mains goes out, I want to auto-switch to an inverter running off of the AGMs to continue supplying AC power, and then when mains power comes back on, have it switch back.  I'm thinking I can use the PwrGate to keep the batteries charged and ready.  The load is small, 200 Watts or less.

What advice might you have for me?  I hate to spend $$$ on an APC UPS with limited battery capacity.

Mike de K5MTF  

Posts: 3746

« Reply #1 on: August 05, 2008, 08:04:40 PM »

hi mike,

you don't have to spend big money on a ups.

get a nice used ups w/o batteries at the next
hamfest or on ebay from a local seller so ship fees
will be low or free for pickup.

you will find nice full sine wave ups from
liebert, apc and emerson at very reasonable prices.

you can add cable and powerpole connector to your
external batteries.  the ups will keep the batteries
charged up and ready to go. Most of these units will
use 24, 36 or 48 VDC as input to the inverter.

If you don't exceed the specs for the ups
you can run as long as your batteries can last
and then powerpole them out with fresh pack.

I've got two just for field day, they were used at
local supermarket for the pos cash registers.
Just needed to blow out all the dust bunnies.

73 james

Posts: 548

« Reply #2 on: September 22, 2008, 09:37:30 AM »

When calculating load, make sure to factor in any start-up transients which may occur, especially in motor driven devices. These can often be close to 10 times the steady sate load. Things like air conditioners, well pumps, refrigerators, boilers, furnaces, and TV sets tend to have relatively high start-up demands.

I run a 5.5 KVA (10 Kva surge) sine wave inverter, which is fed from 24 2 volt, 660 Ah cells, set up as a 48 volt bank. the inverter has a built-in charger and transfer switch. Everything is fully automatic. The transfer switch works so well, I can't even tell if the power goes out (unless I try running the clothes washer or dishwasher, which I chose not to conenct to the inverter)!

I realize you're looking for something small, but be sure to factor in EVERYTHING you could possibly want to run during a power outage before buying equipment, and then oversize a little for start-up surges. It's always cheaper to buy the right things once than to start small and then have to scrap everything and buy bigger capacity equipment.

73, de Tom, KA1MDA

Posts: 6275

« Reply #3 on: December 07, 2008, 10:21:38 AM »

I picked up an old 800RT UPS by APC at a hamfest a couple of years ago for $20.  The seller told me the batteries were bad, and he didn't want to mess with the thing since he got a new one which was bigger.

I stripped it down, cleaned it up and modified it with an internal cooling fan and connections for an exterior backup battery (using #6 gauge wiring).  I bought two large AGM batteries (the unit is set up for a 24 volt battery) so I have a longer back-up time.  I use it on my computer system desk (including the desk light, printer and phone/fax)--it still works well--and this unit can even start my computer system if the power is out!
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