Using an old APC UPS as a power inverter for a home sump pump backup system? - Page 1
Forum section: electronics / beginners

author: wblackledg
author_rank: Regular Contributor
timestamp: March 29, 2015, 02:27:12 pm
content: I have this old UPS.I wondering if swapping the battery out for a large 100 Amp Hour Deep cycle marine battery, and using it for a 120VAC backup 1/4HP sump pump (3 amps). This backup would only be used if the power went out, but would have to dependable in that case.One advantage is the wife is after me to throw away all of my unused equipment and other stuff (like this UPS). If I could show her an example of saving $100+ by not pitching out stuff, then it might calm her down.I'm also looking at a 12VDC system. Which has its own advantages and problems. Also a water powered backup.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 29, 2015, 02:30:21 pm
content: Bear in mind a UPS with a small battery may not be designed for long-term operation from the thermal point of view - probably OK if run below full load

author: Ian.M
author_rank: Super Contributor
timestamp: March 29, 2015, 02:41:38 pm
content: Are you sure the UPS used a single 12V battery and not a series string at 24V or 48V? The economics of reusing an old UPS rapidly become unfavourable if it needs two or worse four batteries.Also, is the battery circuit fully isolated from the mains input? If not, its unsafe to extend the battery wiring outside the case.

author: wblackledg
author_rank: Regular Contributor
timestamp: March 29, 2015, 02:47:14 pm
content: Quote from: mikeselectricstuff on March 29, 2015, 02:30:21 pmthats why I wanted to replace the bad battery with large deep cycle marine battery with 100+ amp hours.so that would work?'thanks.

author: wblackledg
author_rank: Regular Contributor
timestamp: March 29, 2015, 02:57:57 pm
content: Quote from: Ian.M on March 29, 2015, 02:41:38 pmIt is a 12VDC system. the battery isn't the problem since I'll replace it with a deep cycle marine battery which has to be bought in any case.However the point you made about the circuit being isolated from the mains input is interesting. I don't know if it is. Is there an easy way to check?thanks.

author: wblackledg
author_rank: Regular Contributor
timestamp: March 29, 2015, 03:00:37 pm
content: let me also state that this back up pump would only be running about 45 seconds every 20 minutes. (during the worst rainy days ).thanks.

author: Ian.M
author_rank: Super Contributor
timestamp: March 29, 2015, 03:08:58 pm
content: Sure. Disconnect battery, disconnect mains input and measure resistance from each mains input terminal (except ground) to battery terminals. If you get any reading less than several megaohms, its not isolated.If battery -ve has continuity to ground, its a fair bet that its safe to extend the wiring. You will also need to increase the wire gauge to keep the wiring resistance similar, and you should fit a suitable fuse at the +ve battery terminal.Be aware that a sufficiently smart UPS may error out if it load tests the battery and it has too much capacity, or if there has been a long outage and when power is restored, the battery doesn't fully recharge in the time it expects, again due to too much capacity, so you may need to restrict your choice of battery.A separate backup 12V pump, a battery and a dumb float charger is likely to be more reliable than repurposing an old UPS.

author: wblackledg
author_rank: Regular Contributor
timestamp: March 29, 2015, 03:27:04 pm
content: Quote from: Ian.M on March 29, 2015, 03:08:58 pmThis a very good point. I was leaning towards this option last night. I found some cheaper float chargers that people online complained that the charger would drain the battery after only a few hours of it being unplugged which defeats the purpose of what I'm trying to do.I did find more expensive that looked like it would work (this one for example)http://www.amazon.com/BatteryMINDer-Charger-Maintainer-Desulfater-Model/dp/B005EKY1EM/ref=sr_1_2?s=automotive&ie=UTF8&qid=1427642687&sr=1-2&keywords=battery+charger+maintainerplease let me know if you believe this is would work on that deep cycle marine battery or maybe two in parallel?thanks.

author: Ian.M
author_rank: Super Contributor
timestamp: March 29, 2015, 06:50:11 pm
content: The solution for chargers that drain the battery during a power cut is to add a relay with an AC mains voltage coil between the charger and the battery - if the power goes off, the charger is disconnected.

author: wblackledg
author_rank: Regular Contributor
timestamp: March 29, 2015, 07:32:34 pm
content: Quote from: Ian.M on March 29, 2015, 06:50:11 pmthat sounds a lot cheaper than blowing another $50 on a charger that doesn't drain the battery.thanks!

author: Seekonk
author_rank: Super Contributor
timestamp: March 29, 2015, 09:21:24 pm
content: And have a 3W continuous load on the house power all the time. How about just a diode? I don't think that draining the battery story is really true.

author: wblackledg
author_rank: Regular Contributor
timestamp: March 29, 2015, 09:48:59 pm
content: Quote from: Seekonk on March 29, 2015, 09:21:24 pmon amazon multiple people griping about various low cost schumacher chargers draining their batteries if they become unplugged. I have no idea if its true.I found this, and i think if I'm reading the chinglish correctly, it will work for me to drive the 12v pump. I can set one sensor near the top of the sump (above the primary pumps float) and one near the bottom. Once it kicks on it will stay on until the water falls below the bottom sensor.http://www.ebay.com/itm/Liquid-Level-Controller-Sensor-Module-Water-Level-Detection-Sensor-Low-pressure-/171735876085?pt=LH_DefaultDomain_0&hash=item27fc4181f5

author: Ian.M
author_rank: Super Contributor
timestamp: March 29, 2015, 10:17:15 pm
content: I wouldn't use a diode - the float charger doesn't know about its voltage drop and the battery will be perpetually undercharged. It will fail prematurely and wont deliver its rated capacity. Exception: if the charger has an adjustable float voltage, you should be able to compensate for the Vf drop of a high current Schottky diode.Chargers designed to support multiple batteries with a single -ve common usually have built-in isolation diodes and are already compensated for their Vf drop.