Monday, September 10, 2007

Battery-Backed House Circuit

A few weeks ago we had some thunder storms where I live and the power was knocked out for about 4 hours. After the first hour I decided that the utility company was going to take their time fixing the problem and I was tired of sitting in the dark, so I wired a small 12v automotive inverter up to a spare car battery to run a floor lamp.

This quick solution worked well, but I decided I wanted something a little fancier than a dirty car battery sitting beside the lamp. Since my basement is not finished I have access to much of the first floor wiring, and since I've switched to exclusively using a laptop instead of desktop computers I am no longer using my computer UPS's. With those two facts taken together, my next step is fairly obvious.

I spent a couple of hours mapping out the circuits in the house so I knew what went where. As it turns out the overhead light in the dining room and most of the outlets in the living room are all on the same circuit. From the basement I could see the cable for one branch of that circuit, and as it happened it ran right above where I wanted to install a TV for the game room.

I turned off the power to that circuit, cut the cable about two feet from where it disappeared into the ceiling and installed a standard electrical outlet in a galvanized steel box. Onto the wire leading to the ceiling and the rest of the circuit I attached a 15A plug end. There is enough slack in the cable to connect the plug directly to the outlet to power the circuit as before, which I did while I set up the next step.

In this picture I haven't yet screwed down the outlet or tied up the cables.

I dragged out an old 350W Genica UPS I picked up a few years ago for $45. Normally this runs off of a standard 7Ah SLA battery, but clearly that would not be sufficient for my needs. I popped the top and built some leads to connect the battery cables to my spare car battery.

Right away in bench testing I noticed that the power handling components in the battery recharge circuit get very hot, probably upwards of 250F. I don't know if that is normal, but I don't know why the UPS wouldn't limit it's charge current to something it's components could handle. Whatever, I riveted an ugly 1.5"x1.5" scrap of aluminum bar to the tab on the hottest of the TO-220 components and let it run for a bit. The temperature stabilized somewhere around 'uncomfortably warm', down from 'sizzling' on the touch scale.

A week earlier my son ran the lawnmower over it's cord and cut it right in half, so as it happened I had a direct-wire extension cord I could attach to the UPS output to replace the factory outlets, which were built into the plastic top, which I had removed. I then dragged the whole assembly over near the new outlet, plugged the UPS into the outlet and plugged the rest of the circuit into the UPS.

Here I've got the UPS open with everything connected. The heatsink on the recharge transistor is on the right. The orange cord is wire-nutted directly into the UPS output wires, replacing the built-in outlets that were molded into the top of the UPS. I don't intend to use the top, though I probably could have. The car battery is off to the right, connected with some old speaker cable with spade connectors that fit very nicely into the original battery connectors. Obviously I won't leave this cobbled up this way for the final install, but for testing it works nicely.

It was about this point I discovered that the UPS cannot cold start onto the battery the way my APC UPS can. That is a bit of a bummer as it means that once the UPS turns off when the power is out, I cannot start it again. That would be a valuable feature in the event of an extended power outage. I may have to track down the bits of the UPS circuit that monitor the line power and see if I can fool it into doing a cold start.

Once I had gotten the UPS running the circuit off battery I discovered that only half the outlets I had anticipated powering were actually powered. Evidently the circuit has two branches and I was only powering one. At first I was disappointed, but it occurred to me that since it was a 20 amp circuit and I had two 10 amp UPS's that wasn't such a bad thing. I'll just track down the other branch and put the other UPS on it.

I haven't found any loads the UPS doesn't like. It uses what they call a 'modified sine wave' output, which looks to me like a simple square wave, but what do I know. The compact fluorescent bulbs like it, the air pump, filter and heater for the fish tank like it, and the TV and cable box like it, so I guess it'll do.

Before I set up the circuit I load tested the battery and UPS by powering the TV and cable box, which average about 85W. I had charged the battery with the car battery charger, which brought the battery up to about 12.5v. From that level it ran for about 70 minutes. I figure after conversion losses I was probably pulling around 8A or so from the battery. Since 12.5v is a fairly low charge level (the UPS brings it up to about 14.2v before the charger cools down) I'm guessing I can probably get around 20AH capacity from the battery, maybe more if I leave a desulfator on it for a few weeks (that'll be another post). If I can get it up to 25AH and I keep my loads to two compact fluorescent lamps with occasional use of the dining room overhead light (5 CF bulbs) for an average load of about 50W I should be able to get about 6 hours of run time. Eventually I'd like to move up to a 100Ah battery which would give me enough power for a couple of days of lighting, and it'll be happier about doing deep cycle duty than this car battery.

Update: I stopped by Wal-mart on the way home from work today and checked out their deep cycle offerings. They have a group 29 (big) deep cycle battery that works out to about 148Ah for about $75 after taxes. They also have a 115Ah deep cycle for about $65. I've read good things about the brand, so I've noted those batteries on my mental wishlist. I'd love to get about 6 of those and several of the automotive inverters in various sizes and set them up to run all the interesting circuits in the house. I figure that with such a setup I could run most of the lights and the microwave for several days before I'd need to recharge.

Of course at that point I'd be getting into needing a transfer switch and some other equipment too, at which point I might as well just spring for a standard commercial house backup and get true sine output (not that anything I've got seems to give a crap what the waveform looks like).

Update 2009-Feb-9

I have obtained an APC Smart-UPS 1400 from the discard pile at work. Evidently the battery pack was dead and they are moving to a different UPS system anyway, so it wasn't worth it to replace the battery. I pulled the pack which consists of two 17Ah SLAs disconnected them and checked the voltage, 5v on one and 4v on the other. In other words, doorstops.

Fortunately, work was also discarding an expansion battery for another brand of UPS that was giving them trouble. I was able to salvage a couple 17Ah cells from that and while they were a little low initially they charged right up and held the charge for a couple of days. I stuck some 3M Command adhesive between them and bolted them together as a battery pack and stuck them into the APC and it started just fine and will carry a light load with no problem.

Next will be load testing the UPS to see if the batteries are performing at a reasonable level. If so I'll set this UPS up as the backup for my house circuit. It has more than double the capacity and doesn't need any hacking to be useful.
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