DIY Battery UPS for 80W Central Heating Pump 1

[gwolf2]

I am trying to source a 230v UPS from 12v battery backup to keep a gas central heating system running in the event of a power cut. I need about 80W for 10-20 hours, maybe another 50W for minimal lighting.

I just can't find a small system to do this - it seems on net searches to be either very small capacity systems for computers or whole house generators for thousands of pounds. Maybe I'm using the wrong search words?

I have started to design one myself using batteries, an inverter, charger etc. Not my field but not that hard either, clearly there are some serious switching issues. I think I need two 75 Ah leisure batteries, a maintenance/conditioning type battery charger, a 150W inverter (probably pure sine because I need to drive a motor, but maybe a cheap one will do too, system control is analogue clockwork not microprocessor). Total cost around £270/$400.

Any advice would be greatly appreciated especially circuit diagrams relating to managing the switch to/from backup safely. Better still any commercial systems which I have missed in my searches.

gwolf

 

[IRstuff]

This:

http://www.upsforless.com/apcsua1500ref.aspx

should run about 3 hrs at 80W:

http://www.apc.com/products/resource/include/techspec_index.cfm?base_sku=SUA1500JB&total_watts=50

TTFN

FAQ731-376

 

[gwolf2]

IRstuff - these are way over the top on power and way under on duration, probably cost a fortune too.

I bumped into an old friend of mine in the mean time who suggested that I buy a 150W+ used APS UPS off Ebay because it was awash with them because of mandatory 2-3 year replacements for ISO*whatever companies and the associated environmental disposal costs. Then wire in two leisure batteries in parallel with or as a replacement for the existing battery.

Cheap as chips, add on as many lead-acid batteries as you want, all of the switching and battery charging in one box.

Any advice on this?

gwolf

 

[sibeen]

The charger in the small UPS will not charge larger batteries sufficiently.

A 100 A/Hr battery will give you about 100 watts output for 10 to 12 hours. The charger in the APC will give out approximately 1 amp. Charging a battery at the 1% rate is not going to do great things for its life/

 

[IRstuff]

Not sure what part you're reading, but the second link's graph shows about 3 hr life at about 100 watts. Since you still have another conversion efficiency, it just gets you into what you posted in your OP.

TTFN

FAQ731-376

 

[gwolf2]

IRstuff > "3 hr life at about 100 watts"

My first post> "I need about 80W for 10-20 hours"

I am not sure what you are reading?

sibeen - OK, interesting.

 

[DanEE]

......to keep a gas central heating system running in the event of a power cut. I need about 80W for 10-20 hours.

Does this central heating system not have a blower motor? typically 1/4 to 1/3 hp? I could see 80W for the control system but...

A larger used UPS would be more appropriately designed to handle the batteries you need for extended run time and cooling wise, would be designed to run forever on batteries.. From what I have seen, by the time you get to a 1.4kva or larger UPS the designs are more robust.. Looking for a forced air cooled unit would be another sign of a unit good for your application. Or any unit that was designed for use with external battery cabinets such as the APC Matrix 3000 mentioned below.

Some of the smaller UPS in the range you are looking at will shut down in approximately 10-15 minutes by the micro controller, irregardless of how much battery you put on it.. Reason being the SCRS (which are the heart of the inverter) do not have adequate heat sinking/cooling for extended run time.

Some just use an aluminum slab heat sink to absorb the heat and don't dissipate the heat at the rate generated.

I do have a little APC 450VA unit that years ago I modified for extended run time, including adding a muffin fan inside for forced air cooling. This unit did have legitimate heat sinks on the SCRs and does not do another little dirty trick that some cheap UPS units do as described below.

In one low end UPS I looked at, the batteries were in the circuit in such a manner the 12 volt battery had charging voltage across the battery, BUT the entire battery was floated off of ground by about 50 volts. The spikes across the battery was enormous when the inverter was running. You could run into some problems if you were to try to use an external charger, with a unit like this.

I use a BEST(now Eaton Powerware) FERRUPS 1.4kva UPS running a standby branch circuit. btw that was the largest FERRUPS that would run on 12 volts.. They go to higher battery voltage on the next larger unit (1.8kva) and up.

You probably could also find something like a APC Matrix 3000 reasonably cheap. I've seen a few on the internet for $300-400 US. That unit will use 4 12v batteries in series to make up the 48volts it needs. It is designed to run an extended period and also will have enough charger to handle the battery AH you are looking at for the run time you want.

Alpha Technologies also makes good (not cheap, corner cutting) UPS.

 

[gwolf2]

DanEE,

Thanks very much for all those tips, this is very helpful. When my friend said "you just have to........" I wondered if there might be a little more to it.

I can confirm no blowers to power, it's a gas fired hot water central heating system - all it needs is 80W for the circulation pump and some clockwork-like timers - I've measured it.

gwolf

 

[DanEE]

Another thought that led me down to the path to a larger UPS... You are likely to find over time other things you would like to have on back up power.. i.e. computers, internet modems, TV, radios, emergency lighting in critical places.. Not to mention a good asset in the overall plan for providing lightning protection for high value electronics.

Communications becomes important when the power drops, as it's natural to want to keep posted on outside conditions. We have had ice storms and one hurricane that meant no commercial power for a week.

The little APC 450VA was enough to run temporary small fluorescent lights on power cords..

When the opportunity came up to get a larger UPS, I installed a standby power branch circuit(backup power for convenience)as opposed to emergency power (much more rigorous requirements) as defined by NEC Article 702. Article 700.12E explains how to power such a circuit through cord and plug connected UPS.

Batteries will be a considerable cost. I am fortunate in that we have IT center customers who need their batteries replaced every 3 years even though what we install have a 10 year design life. These are large UPS systems typically using 1 or 2 strings of 12 volt 80AH AGM VRLA batteries, 20, or 24 batteries per string.

 

[fsck]

The thing to remember is there's a significant loss just keeping the UPS running, even at no load. Calculate accordingly.

I've used older SMARTUPS UPS's with large batteries. It was a ~1KVA unit originally with 4AH of batteries; I had two 65AH 13.8v deep-cycle batteries I used.

It would take days to recharge fully, but that was no problem in this case. [It also lasted 24+ hrs at my light load..]

Be sure and fuse the short jumper between the two batteries so as to avoid fires.

 

[IRstuff]

Sorry, my bad, I was blocking the zeros for some reason.


TTFN

FAQ731-376

 

[gwolf2]

Thanks for all your comments.

I have a contact who may be able to get hold of an old 400W UPS given time. We can try it out with to check for automatic timeouts and charging rates.

Failing that I will revert to my initial plan of buying an inverter, charger and batteries. I have a basic circuit which ensures that it is impossible for the mains AC and inverter to clash - it's a standby type circuit not UPS but that is quite sufficient. I may risk some used batteries because the NUMAX charger I have in mind has a fairly good reputation for rescuing old batteries.

All for about £270 - or about $50 after the money markets open on Monday :-(

gwolf.

 

[fsck]

There is another approach.... have you looked for a 12/24vdc pump?