Battery power for an Atwood Furnace

[Mac B]

I have an Atwood furnace in my Burro, as best as I know it's a 12K btu unit (but possibly larger, like 18K)

I am wanting enough battery power to run it for a number of hours at a time for dry camping. As my experience has been so far, if it's cold enough to need a furnace, it'll have to run more than just a few minutes and I don't want the battery to wimper out on me in the middle of a cold night! So I am thinking about adding another battery, and getting a generator to charge the batteries. I would hope that two batteries should be enough. For a generator, I'd love to get a honda 2000 but I don't want to over kill it (especially since they are over $1,000.00)

Who can sign the 'been there done that' log, and can give me some good advise, as to battery power and ample generator?

[Raya]

Mac,

How many amps does your furnace fan draw? Knowing that would help to calculate what size battery bank you'd want (assuming you don't want to run the generator in the middle of the night).

And how many days/nights would you want to be able to run the furnace between battery recharges?

I know you'll have to guess at how much of the time the furnace is running in a given 24-hour period, but knowing how many amps fan draws is a place to start the calculations. It makes more difference to the electricty side of things than how many btus the furnace puts out.

Raya

[Mac B]

Honeywell HW2000i 2000-Watt Portable Inverter Generator
Sear, $699 (those bandits)
Wallyworld $549.00

[Mac B]

I didn't see your post before I added my 2nd one

I figure that battery recharge during the day would be ok...

as far as the amps, I will have to call and ask the RV company

Mac,

How many amps does your furnace fan draw? Knowing that would help to calculate what size battery bank you'd want (assuming you don't want to run the generator in the middle of the night).

And how many days/nights would you want to be able to run the furnace between battery recharges?

I know you'll have to guess at how much of the time the furnace is running in a given 24-hour period, but knowing how many amps fan draws is a place to start the calculations. It makes more difference to the electricty side of things than how many btus the furnace puts out.

Raya

[Raya]

Mac,

I think it might be on the fan, and/or listed in the manual. Do you have the model number for the furnace?

Okay, I asked about how long you'd want the furnace to run because I thought there were some campgrounds where generators weren't allowed. But maybe that's only at night, so you could count on charging every day

Once you have the fan draw, then we can calculate for worst case scenario and work back from there.

Raya

PS: Another thought, if you are thinking about setting up a battery bank and generator just for the furnace fan, is one of the Wave catalytic heaters. I think the 6 is the larger one. They run on propane only and hence use no electricity.

It just seems a shame to use propane, electricity and batteries, plus gasoline for an internal combustion engine in a generator, just to run a furnace fan! Which is not to say that you shouldn't do it, but just to share ideas.

[Yvon Chayer]

In my Bigfoot i have the same furnace as you and for example at the tankgiving in Canada we have a snow when we camped for the last time this year
I have just one optima battery ( 4 years old ) in my Bigfoot and after one total day of heating the battery was half full
I have an electronic thermostat on the furnace , the furnace dont run longtime but run often
I run the generator an average of one hour per day to recharge the battery via my converter ( intellilli-power)and i am ready for the next cold day

PS I love my honda 2000i i can run small window ac in hot day with no problem but i use the ac small amount i prefer the maxair fan in hot day

Yvon Chayer

[BobB]

We have an Atwood heater and a group 27 battery in our Bigfoot. We have never run the battery down camping 3 or four days in the same spot without shore power. The same was true of our Casita/Suburban set-up. We have a good charging system in our Jeep, so we always start with a fully charged trailer battery when we arrive at the campsite. If you are really concerned about loss of power, a spare battery is a lot cheaper than a Honda Eu2000i. If you decide to use a second battery in parallel, the 2 batteries should be matched, including age. Otherwise, you would just changeover to the fresh battery when the first becomes depleted.

[Marty Smiltneek]

The best you can do is a good estimate because of all the variables in the system. The whole heating system includes the camper it is mounted in.

After a bit of research - - - The atwood 8012 draws 1.8 AMPS [at] 12 volts and the more common 7912 through 7919 series draws 3.4 AMPS [at] 12 Volts.
I would assume you have the 7912-II that draws 3.4 Amps per continuous hour of run time.

If the furnace runs twice an hour for 10 minuits each time would be a total useage of about 1.2 amp hours. Divide the total amp hour usage into your battery rateing of say 24 amp hours. The result would be a run time of 1 full day.

recap:
you need to know the amp hour draw of your furnace. (this is the actual measured current draw for a full hour of running)
The amount of time it actually runs. (Things that effect this are: how cold is it outside. how often is the door open, how good is the insulation, is the roof vent open all the way, window seals etc. etc. and on and on)
The amount of overhead the system thermostat uses (mostly 0 if passive or AA battery sourced).
The size of your battery in amp hours total capacity, (Don't forget that the battery looses capacity with age).
I think that any calculation is a good guess at best. You will need actual experience to find the answer and that will change with wind, temperature etc. Good luck with that!

Marty Smiltneek

[Marty Smiltneek]

Oh, yeah - - - I forgot that the capacity of the battery is diminished by cold temperatures I think that the battery manufacturer has a chart reflecting the effect of temperature.
Also if you fully discharge the battery it can freeze and crack in really cold temps.
Marty

[Raya]

Another good idea is to only plan to use about 50% of your battery's rated amp hours. You can draw it down further (say, 70%), but the more often you draw it down further, the shorter the life of the battery. So I would plan to only use 1/2 the amp hours in a given battery (or batteries) when making my calculations. Then plan to re-charge before drawing down to 50% again, etc.

Raya

[Mac B]

thanks for all the replies... yes, it does seem a shame to have to use battery power, propane AND gasoline for a generator just to run a fan motor!

I do have a series 27 battery.

I downloaded the Atwood furnace manuel... so I see where it draws 3.4 amps. I can't say I understand all these calculations though.

[Mac B]

ps.. I am asking all these questions because I took a trip to Yosemite and in one night, still hooked up to my tow vehicle, useing the heater, I depleted my battery..... my vehicle would barely crank over. And I am planning a trip (in June) to Alaska. I want to be prepared!

[Raya]

Okay, well, if the manual says it draws 3.4 amps, that means that it draws that much each hour that you have it running. In other words, 3.4 amps per hour.

Let's say your Group 27 battery has a capacity of 100 amp-hours (it should say on it, but this might be close). For starters, it's good to not drain your battery more than about 50% on a regular basis, for longer overall life. So say you have 50 amp-hours to work with (of course you can draw the battery down more if you have to, it's just not the healthiest over the long term).

Now, if you run your furnace only at night, and "night" is 9 hours, and it runs....oh, I'm just guessing... but say 30% of the time, that's about 3 hours of fan running. 3 hours x 3.4 amp-hours = 10.2 amp-hours. So, if that's all you have attached to that battery, then you will be down about 10 amp-hours for each night that you run the furnace. Then theoretically you would be able to run the furnace 5 nights and still only have taken about 50 amp-hours out of your battery, which is that 50% depleted level.

Now you have to put 50 amp-hours back in, one way or another. By electric charger, by internal-combustion generator, by solar panel, by wind generator, etc. (And of course you don't have to wait until this point to start charging.)

Does that make sense?

(The one thing that is really a guess is the part about the furnace running 3 hours in a 24 hour period. That may be low. Do you have a sense of how many hours it ran when you were in Yosemite? And what the temperature and wind speed were?)

Raya

PS: Edited to change amps to amp-hours.

[Mac B]

yes, that makes sense. I must have something wrong with my system if in one night it drained the battery. I doubt if it ran a whole 3 hours during the night. I may have to have the system checked to see if it is charging properly.

[Raya]

I don't know whether it might have some kind of ignition that draws power too, but that still does seem quick (3 hours).

I did guess at the amp hours available in your battery, but it shouldn't be off by that much.

And there is some life lost to being cold (if the battery is cold). How cold was it?

But yeah, that still does seem a bit off. Had you charged the battery by an independent charger, or were you using the connection with the tow vehicle? I've never charged with a tow vehicle, but I think I've read that they don't always put that much juice in (especially if you're running a fridge or something too).

It would be interesting to try it again (experimentally, not when you're counting on it) with a known good, fully charged battery.

Raya

[Gina D.]

Mac, you might consider installing a Cat heater. I have a WAVE 6 in mine, I use it in near freezing temps and it does the trick nicely.. no fan, no draw. Observe proper ventilation rules.. you will be more than stylin in the 14.

However, some folks don't like them because of the venting thing. If you are considering Alaska, against my usual advise of solar and low to no draw items... I think your generator idea is the best one. You will not be in a position to tuff it out in a a sleeping bag for a nite or two.. lack of heat could be a very serious matter for your health there.. not just an inconvenience and a quaint situation.

Certainly, a new battery is in your future. It will absolutely need to be in tip top shape for this trip. Don't risk it.. get the genny.

[ThomasE]

Hi Mac B, you said you vehicle barely started, are you saying you ran down your tow vehicle's battery by using the furnace? The tow vehicle shouldn't be supplying power to the trailer if the ignition is off, even if trailer is plugged into the tow vehicle.

[Mac B]

Hi Mac B, you said you vehicle barely started, are you saying you ran down your tow vehicle's battery by using the furnace? The tow vehicle shouldn't be supplying power to the trailer if the ignition is off, even if trailer is plugged into the tow vehicle.

Yes, it was connected, and in starting it, I felt like it was sluggish, and I assumed that the tow vehicle battery had been drained. Maybe I was mistaken. I will check with the RV place that did my wiring. Thanks for the clarification.

[Mac B]

I'll check it out. thanks.

Yes, heat is an important thing, not just for nights, but for just hanging around during a cold day, I don't want to feel like I am sitting in an icebox!

Mac, you might consider installing a Cat heater. I have a WAVE 6 in mine, I use it in near freezing temps and it does the trick nicely.. no fan, no draw. Observe proper ventilation rules.. you will be more than stylin in the 14.

However, some folks don't like them because of the venting thing. If you are considering Alaska, against my usual advise of solar and low to no draw items... I think your generator idea is the best one. You will not be in a position to tuff it out in a a sleeping bag for a nite or two.. lack of heat could be a very serious matter for your health there.. not just an inconvenience and a quaint situation.

Certainly, a new battery is in your future. It will absolutely need to be in tip top shape for this trip. Don't risk it.. get the genny.

[Raya]

I think I have read/heard that some tow vehicles' wiring will allow continuous discharge of the tow battery unless it is unplugged at the trailer/vehicle connector. Don't some people install a switch or cut off?

I'm not saying it should be that way, but I don't think it's super uncommon, either. So it's not out of the question that your vehicle battery was being drained. And car batteries are not typically maximized for deep cycling, like house batteries are.

Raya

Edited to add: It was brought to my attention that most vehicle manufacturers do put a relay in so that the trailer's battery will not be discharged when the ignition is in the "off" position, even if the trailer is hooked up. That said, I don't know whether Mac has "factory" trailer hook-up wiring in his tow vehicle. Also, perhaps I was not clear enough in saying that it should be wired to prevent this (in other words, I was not recommending leaving it so that the trailer could discharge the tow vehicle when connected but with the car ignition off). Sorry, I guess I'm not writing well today.