2 questions: Wattage for a backup power supply, and modified sine waves.

[Jeff7]

Yes, a semi-technical question in OT, but there really weren't any other categories where it seemed to fit.


I'm working on a backup power supply for a wood pellet stove. Two final questions:

My Kill-A-Watt gives me this:
in 263 minutes, power usage was 0.40KW/H. So that's about 91.255W, correct?

I want it to run for 24 hours - about 2190 watt-hours.

So if I get two 12V, 105A lead acid batteries, stick them in series, that'll provide an absolute maximum of 2520 watt-hours, correct? Granted, this will be close to a full discharge, so it might not be too great for them, but these will not be getting constant use anyway - only when the power is out for a long time in the winter. Will a deep discharge really be that damaging? Or should I look for a slightly higher capacity battery?


And the last section is on the quality of the inverter. This is the kind of motor that the augers use. The motors are rated at 1RPM. The blowers (1 combustion, 1 convection) both use motors of similar construction, but they are of variable speeds.
Can these use a modified sine wave, or should they have a real sine wave inverter?



Second option
We've also got a gas generator. Someone I just talked to suggested a sound dampener for it. I might just invest the money for this project into that, and a larger gas tank for the generator. So this lovely battery backup project might die right here.

But even so, was my math at least correct? 😀


Edit#3: I'm now looking at these mufflers.
The decibel reductions don't seem to impressive though:

Up to one-half the exhaust noise level of any other spark arrestor or compact muffler (2 - 4 dbA quieter). Optional Resonator available for additional 4 to 6 dbA reduction.

I wonder about a wooden enclosure with some kind of powerful ventilation. Two of them should do the job - adds about 70W to the power consumption. Only problem though would be weatherproofing the things, but I imagine that could be solved easily enough.

That, with some flame-retardant sound absorber lining it might actually do more than those little metal mufflers.
Thoughts on this?

 

[woowoo]

Use "Deep cycle" batteries
I dont think moters will care if it's not a pure sine wave.
They just play "Chase the phase"

 

[Jeff7]

Originally posted by: woowoo
Use "Deep cycle" batteries
I dont think moters will care if it's not a pure sine wave.
They just play "Chase the phase"

I had something like this in mind.

 

[Bassyhead]

Three things:

- Make sure you don't get your units mixed up. Watt is a unit of power. Watt seconds (or hour or any unit of time) is a unit of energy.

- Assuming your calculations are correct, youre probably going to need a higher capacity power source (if youre using DC-AC inverters, I'm assuming). DC-AC inverters are not the most efficient devices. It probably varies from inverter to inverter.

- The motors should work fine if they're not running on pure sine waves

 

[Analog]

Originally posted by: Jeff7
Yes, a semi-technical question in OT, but there really weren't any other categories where it seemed to fit.


I'm working on a backup power supply for a wood pellet stove. Two final questions:

My Kill-A-Watt gives me this:
in 263 minutes, power usage was 0.40KW/H. So that's about 91.255W, correct?

I want it to run for 24 hours - about 2190 watt-hours.

So if I get two 12V, 105A lead acid batteries, stick them in series, that'll provide an absolute maximum of 2520 watt-hours, correct? Granted, this will be close to a full discharge, so it might not be too great for them, but these will not be getting constant use anyway - only when the power is out for a long time in the winter. Will a deep discharge really be that damaging? Or should I look for a slightly higher capacity battery?


And the last section is on the quality of the inverter. This is the kind of motor that the augers use. The motors are rated at 1RPM. The blowers (1 combustion, 1 convection) both use motors of similar construction, but they are of variable speeds.
Can these use a modified sine wave, or should they have a real sine wave inverter?



Second option
We've also got a gas generator. Someone I just talked to suggested a sound dampener for it. I might just invest the money for this project into that, and a larger gas tank for the generator. So this lovely battery backup project might die right here.

But even so, was my math at least correct? 😀

in 263 minutes @ 400W/hr, I get 1753 watts...

And you don't want to connect the batteries in series, as that will give you 24VDC. I think you meant parallel.

As far as a modified sine vs. sine, it depends on the motor. Most windings will get hotter with the added harmonics of a squared off wave, which will reduce their life. Will it still work? Yes, but at a price.

HTH

 

[Jeff7]

And you don't want to connect the batteries in series, as that will give you 24VDC. I think you meant parallel.

Yes, 24V. However, if they are connected in series, they can't be charged that way. I think that charging batteries in parallel is bad - something to do with one of them sucking up all the charging current. Once it's charged though, it'll overcharge while the other one charges.

in 263 minutes @ 400W/hr, I get 1753 watts...

The 400W/H was cumulative. The readout the Kill-A-Watt gives for the KW/H is the cumulative power used from the time the Kill-A-Watt is plugged in.

 

[woowoo]

Use the generator.....

 

[flot]

Interesting project... one other alternative, could you retrofit the stove (possibly a manufacturer option?) to run off of DC instead? Then you'd *always* be running off 12-24vdc via a battery bank and charger, and when the power went out / came back on, there would be zero interruption, it's just that the batteries would/wouldn't charge.

I was looking at doing something similar for my saltwater aquarium, it has pumps etc that need to be running (and we're talking about a potential loss of hundreds of dollars if things die, so worth saving) and as it turned out, switching everything to 12v was a pretty viable option. Plus then you never have to worry if something won't work when the power goes out, as it will be well tested and running off batteries in the "normal" state.

Otherwise your math etc looks good to me. Two suggestions - for maximum power for cheap, marine deep cycle batteries would be the way to go. However, if you're storing these indoors, which I assume you are, then I would look towards gel cell deep cycles (optima red top?) instead - that way you don't have to worry about venting, explosions, etc. It sounds like the whole thing could set you back $400-$500, but then again, seems like it would be worth it if you're talking about keeping your heat running for an extra day!

As for the generator - your best bet may be a combination of the two, maybe rig something up that will run your heater for 8-12 hours on its own, which gives you plenty of time to get the generator going etc.

Originally posted by: Jeff7
I'm working on a backup power supply for a wood pellet stove. Two final questions:

My Kill-A-Watt gives me this:
in 263 minutes, power usage was 0.40KW/H. So that's about 91.255W, correct?

I want it to run for 24 hours - about 2190 watt-hours.

So if I get two 12V, 105A lead acid batteries, stick them in series, that'll provide an absolute maximum of 2520 watt-hours, correct? Granted, this will be close to a full discharge, so it might not be too great for them, but these will not be getting constant use anyway - only when the power is out for a long time in the winter. Will a deep discharge really be that damaging? Or should I look for a slightly higher capacity battery?

But even so, was my math at least correct? 😀

 

[Jeff7]

I'm now looking at these mufflers.
The decibel reductions don't seem to impressive though:

Up to one-half the exhaust noise level of any other spark arrestor or compact muffler (2 - 4 dbA quieter). Optional Resonator available for additional 4 to 6 dbA reduction.

I wonder about a wooden enclosure with some kind of powerful ventilation. Two of them should do the job - adds about 70W to the power consumption. Only problem though would be weatherproofing the things, but I imagine that could be solved easily enough.

That, with some flame-retardant sound absorber lining it might actually do more than those little metal mufflers.
Thoughts?