UPS with auto battery

[lzr1621]

Hi,
Anyone has any ideas about pitfalls on using a regular UPS for desktop PC with an external auto battery?
I am going to make a backup power source for my natural gas boiler (steam heating). So, I plan to use a conventional UPS (such as APC's ES350) and wire an auto battery parallel to UPS's battery for more capacity.
Could UPS's charger have any problem with auto battery (I presume they have a specific charging algorithm)? Any problems paralleling these different types of batteries?
Thanks,
Lazar

 

[rawelk]

I've never worked with residential natural gas furnaces, but imagine there is a combustion blower motor.

Before doing anything else you probably want to find out how many amps the boiler load draws (start-up, steady-state running, and idling current). If motor starting current is more than the UPS can deliver - almost certainly the case here - then you'll have a problem.

My guess is you'd have a problem just running the load, because 350VA at 120V is about 2.9 amps into a resistive load, and once you figure for the typically low power factors for single phase AC motors in the likely power range you have at best 2 amps, and probably lower.

The model UPS you mentioned is quite small for this type of service, and is rated 350VA, 200 watts full load for 2.2 minutes (50% load for 9.4 minutes) using a single RBC35 battery rated for 3 amp hours/16 hour recharge time spec.

It isn't a good idea to parallel a 3 amp hour sealed lead acid battery with a car battery capable of sourcing several hundred amps. Their internal resistances will almost certainly be different, and, unless you add fusing or other circuit protection in each battery leg then a shorted cell could cause a bad hair day.

For instance, if a cell in the UPS battery shorts then the car battery will drive current into it, and could cause the UPS battery to blow up. If the UPS can handle the load then you'd be better off to take the UPS battery out of the picture, and use the car battery exclusively.

I think you'll end up using a larger UPS, or, better yet, putting together an inverter, battery, and charger sized for the application.

Remember to place the battery away from the furnace in a well ventilated location - open flame and hydrogen gas don't get along well (or get along all too well, depending upon your point of view).

 

[OperaHouse]

The biggest problem I see is that the semiconductors will overheat if run for longer than the normal battery operates. Open the case up and add a little 12V computer fan and should be ok. Car batteries can only take a heavy discharge about a dozen times before the plates are damaged. Jf buying new use a deep cycle battery.

 

[danw2]

I did it with 2 different APS brand power supplies, 250VA models for PCs (1999 era)

I bought a deep cycle, lead-acid 12V battery for each one. The batteries were the same physical size as an automotive battery. I stuck each in an RV plastic battery case.

I threw out the internal batteries in the UPS and ran heavy cables over to the RV battery.

The UPS charger charged the batteries OK, but the batteries consumed lots of water. I had to check them monthy and add water to the cells.

One time the batteries supplied power for a CRT monitor and PC for well over 3 hours. The UPS charged the batteries OK after the discharge.

The batteries lasted about 5, maybe 6 years.

Dan

 

[itsmoked]

rawelk; Hit all the facts.

Parelleling.. A big NO!

Switching entirely to a wet cell from a gel cell.. A big NO!
Proven by danw2's result. The charging voltages for both types of battery are very different.

He also had a 'little' luck because he used old UPi. Newer ones are designed border-line. They are also "smarter" for safety. They will do things like fault because they've measured the energy they've put into charging a battery and will trip out while trying to charge a larger battery as it looks like a failed small battery.

I also don't see how you can run a blower motor and the house circulation blower on ~250VA

What exactly does your furnace draw?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[lzr1621]

People,
Thank you all for your input.
A little clarification: I has steam heating running on natural gas, so no heavy motors or blowers. The power is needed only to operate a few valves that turn the boiler on and off. Overall power may be under 20W. The goal is survive a possible long term winter blackout.
Deep cycling battery of course is better, but I prefer maintenance free one.
Possible tripping of UPS's charger protection by car battery is what I was afraid of. However, the system is not going to be standby, i.e. I am not going to keep car battery always connected. I will just periodically charge it by a 12V lab supply and would connect it to UPS only when I need power backup (but then maybe indeed I need just an inverter rather then UPS).
Decoupling fuses between paralleled batteries or using only one battery at a time was a valuable suggestion as well.

 

[VEBill]

I hope the valves and control system are designed to be perfectly fail-safe as your UPS power slowly fades away at 3am some dark and stormy night. Be very careful about failure modes as the battery gets low.

My coworker rigged up some solar panels and some huge deep cycle batteries and a 3kw inverter to provide backup power for his water pump. He eventually got bored of waiting for a power failure, so he rigged it to run some of the commonly-used lights (CFLs) in his house. By finding local industrial suppliers, he was able to get the huge batteries for reasonably affordable prices, much cheaper than the same amount of storage built from individual smaller retail batteries.

If anyone doubts the existence of infinity, the payback period for my coworker's solar panel system was about three of them.

 

[lzr1621]

VE1BLL,

A UPS or an inverter usually have a low battery shutdown. They are supposed to regulate the AC voltage down to this threshold. I can experiment first with a lab supply.
However, I just realized another thing. My furnance uses a 60Hz transformer that converts 120VAC into 24VAC for the valves. While UPS provides pure sinewave, inexpensive inverters are usually modified sinewave. I wonder how will transformer work with a modified sinewave, which is basically rectangular pulses with dead time? I suspect it may overheat by high frequency harmonics present in rectangular waveform. Anyone happened to use a 60Hz transformer with a modified sinewave inverter?

 

[itsmoked]

lzr1621; Here's my suggestion. Buy a "large" Deep cycle battery. Maybe two 6 volt'ers for the biggest bang for the buck.

Then get a nice little wall wart charger that has cut-off ability so you don't have to worry about water loss.

Pick up an inverter that will do the job.

Leave the little charger on the batteries forever.

If the power goes off fire up your inverter and you will have, probably, a few days of heat.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rawelk]

lzr1621,

Most of the things called UPS - Uninterruptible Power Supplies - are actually fast-changeover SPS - Standby Power Supplies - especially in the lower ratings range.

They feed through the AC line supply until a low line voltage is detected, then turn on an inverter and switch it to feed the load in anywhere from 4 ms to 20 ms or so.

As you've noted, inverter output could be anything from 50% duty cycle square wave to a modified "stepped" square to a quasi-sinewave (usually a PWM waveform)... and cost goes from cheap to expensive in this progression.

Full-conversion (online) UPS don't have this switchover mechanism, and the load is always fed from the inverter (which may have any of the above waveforms, as well as fairly pure sine). I haven't seen one in a while, but fondly remember an early-80's vintage Sola 3KVA unit used to keep up our IBM 3270 communication controller - built like a tank, and just about as heavy

My guess is pure sinewave UPS aren't available in smaller sizes, and probably start in the 700VA/1KVA range (at $400-$500 and up).

If you already have an APC ES350 to experiment with you could hook it up, and see if the control system operates properly, and whether the transformer and/or solenoid coils tend to overheat when fed from it's output. At only 20VA load you might get on the order of 45 minutes of operation using it's battery.

OperaHouse makes a valid point - these smaller "brick" SPS are rated pretty close to thermal design limits, and aren't expected to operate for much longer than the OEM's recommended battery would keep it running (although, if only using 20 VA you might have enough headroom).

This SLA battery is rated for about 13.6 to 13.8 VDC float charge, and 14.6 to 14.9 VDC cycle charging, so I'd expect to see a regular lead-antimony battery (which usually runs in the 12.6 to 13.4 VDC float range, and 13.8 to 14.1 VDC cycle charging range) doing just about what danw2 observed - going through a lot of water due to the high charge rate.

A couple of years ago the battery in my Saturn started to go, and I measured 14.5 VDC when charging. Thought this was high, but learned it is normal for calcium alloyed plates in the Delco Freedom series batteries.

http://www.landiss.com/battery.htm

I'm thinking this would fit better with the UPS battery charger's output, but I've read that batteries using calcium plate technology are particularly unforgiving of deep discharge cycle operation - so probably not a solution you'd want to put money into.

Idle thought - does anything need to be powered from 120 VAC or 24 VAC (such as a proved flame controller)? Was thinking one way to do this is change from 24 VAC coils to 12 VDC coils, and power everything from a deep cycle 12V battery with a float charger across it. Con - you'd need to roll your own low battery voltage interlock circuit ...

 

[VEBill]

Another option to consider is a generator. The low power 1200 watt versions (unfortunately two-stroke engines with gas-oil mixture) are only $150 or so. 1200 watts would be enough to keep your gas-fired heat on, freezer frozen, fridge cool, and some lights on.

All for about the price of ONE deep cycle battery.

 

[OperaHouse]

Don't think the square waves would cause a problem. Newer gas furnaces use electrical conduction of the flame to sense it is still lit. Many inverters I've seen put out voltage on the low side and with square waves the peak high voltage of the flame detect may be lower and cause an occasional problem.

 

[lzr1621]

Thank again to all who responded. Having read the posts here and in other forums I realized UPS with an external battery would not be a good choice: it may shut down too early thinking the battery is discharged.
As itsmoked suggested, a deap cycle battery with inverter purhaps is the way to go.
I did find low power sinewave inverters in <$150 range. By the way, I looked APC ES350 output with a scope- it produces quite clean sinusoida as well.

Using DC as rawelk suggested, is an interested idea too. I could connect two batteries in series for 24VDC without messing with valve's coils. The problem is my furnance is old and I have no idea about the valves' operating voltage range. I guess, I'd rather work with a stabilized AC from an inverter.
A gas generator per VE1BLL is an option to consider too. The concerns are such generator burns fuel even when it idles, so it may not give me much run time per tankfull, shelf life of gasoline is short and gas pumps may not work during a blackout.

 

[waross]

For your application I would investigate the newer inverter generators.
These generators produce DC which is then inverted. The engines are able to idle down under light load for considerable fuel savings. The ones that I have seen are 4 cycle.
Try your local RV/Motor home dealer or parts supplier.
respectfully

 

[itsmoked]

With the deep cycle battery on a charger scheme, if it ever actually runs down, you could jumper it with a car and recharge it. Not very gasoline efficiency-wise but it's an option. But emergency planing is all about options.

Here is a suggestion with an inverter generator. Buy one that is rated at least 1000W then you can run your refrigerator every few hours. Many have 12VDC outputs for charging batteries. So every few hours throw a charge on the deepcycle battery while re-chillin your refrigerator.

Yes gasoline sucks for storage and handling. It also renders carburetor's gummed up and non-functional. So! Get a propane conversion kit. Then you have a few propane bottles and you can cook, charge your furnace battery, and keep your food from spoiling, and even run a light or two.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Just a point of interest, years ago when I used to work for Sola selling large UPS systems, the batteries WERE automotive batteries. The problem was, outgassing! That's why everyone went to gel cells.

But why are you considering gasoline as a backup, when apparently you already have natural gas for your furnace? Why not use the natural gas as your backup power?

 

[itsmoked]

Ah! Nice point. You can actually get the kits to be multi-fuel. So you could run the generator on propane during um..dire emergencies and natural gas during regular run-of-the-mill emergencies.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[OperaHouse]

I could connect two batteries in series for 24VDC without messing with valve's coils.

AC coils can often run on DC, but the voltage needs to be lower to prevent overheating. A resistor in series can work but requires a parallel cap on the resistor 0f several thousand uF for required pull in current. If triac operated instead of relay it will latch on.

 

[ScottyUK]

Keith,

I am worried that you have so many home emergencies that you have the need to split them in to categories!

The inverter generators are excellent.


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If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

This is Cal lee four nee ah! We lost power for 3 days when an earthquake made the local powerplant switch yard look like a house of cards. 230kV breakers lying on their sides...

Then more recently a mudslide took out the gas transmission line. PG&E had to relight 50k water heaters.. It was like passover, or the Black Death. The service trucks roamed the streets and when they lit a heater they'd go out in the street and chalk an X in front of your house. LOL

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com