OWB battery backup...solved!

In 2013, we went a total of about 10 days without electricity, 5 of those days were in a row when the temps were near zero. I have a generator, but someone is not always around to keep it running. So i bought some insurance in the way of a computer UPS (battery backup)

This is a used one i bought for $25.



First, in anticipation of this project, i purchased a pair of grundfos alpha 2 circulators. It was an easy decision as they will pay for themselves in less than 2 years with the electricity they save.



Then i condensed all electrical devices related to the boiler from 4 down to a single circuit and purchased a pair of deep cycle batteries:



After connecting the new batteries to the UPS, i plugged everything in and took some measurements. This is a photo of the kill-o-watt meter plugged in with everything running, except the LED porch light.

Less than 35 watts! I find that to be almost magical.



In the coming days i will try to do some performance testing to see how long the system will run on battery power.

I made one modification to the UPS unit. I carefully took it apart and disabled the audible alarm. This alarm goes off several times a minute when running on battery power. While a useful feature for its intended purpose, it is much less useful for my application.

I think the internal battery charger is NOT sufficient to charge the large battery bank. It is rated to charge the original 7ah battery in 24 hours. The bank of batteries is rated 15 times larger. i used a fancy computer controlled 40 amp charger to fully charge the battery bank and i hope the internal charger will maintain a full charge.

I shut off grid power to the UPS about 2 hours ago.

Battery voltage. The test began shortly before 0740

0740 13.0 volts
0906 12.6 volts

I want to conclude the test at about 12.1 volts as this is approximately 50% battery discharge.

so far, the UPS is staying cool to the touch.

I did have a much more fancy 1500 watt "smart" ups for this project. It has a built in cooling fan and produces pure sine wave. Unfortunately my 2 year old ruined it before i ever had a change to use it. I think this type, with the built in cooling fan is probably much better able to handle larger loads for long periods of time, and likely has a larger internal battery charging circuit.

I am curious to see how well the smaller less fancy unit performs.

A UPS with junk batteries inside is near free on craigslist.

maple1 said:

Curious to see how it performs bringing the batteries back up after your test.

Or will you do that with a charger?

Even if you do, and only use the ups for the switchover on outage, and maintaining, it would still prove quite useful.

Click to expand...

I plan to do that with a separate charger.


5 hours into the test, the meter is registering 12.5 volts. According to a chart i'm looking at, that voltage means the battery is still about 75% "full"

DaveBP said:

In order to judge the charge state of the battery the voltage must be measured with the battery at rest (no current going out to loads) for at least a few minutes and the temperature of the battery makes a difference, too. I'm pointing this out for the sake of those not familiar with assessing batteries with a voltmeter.

Click to expand...

thanks for that info. In my case, these readings were all taken with the electrical load still on the batteries.

on a separate note, Unfortunately i do not have a good meter for measuring DC current. It would be interesting to know how much useful energy i am putting into the inverter vs how much i am getting out.

I last checked voltage about 1730. 10 hours in voltage is still 12.5.

About 14 hours in and voltage has dropped to 12.4.

I chose to end the test. It's supposed to be near zero tonight and I don't want to risk having the unit shut off while I'm sleeping.

I consider anything over 12 hours a success.

I've read that the batteries actually increase their capacity after being cycled a few times. Perhaps I should hook up some other critical infrastructure items to the ups, like some interior lights.