Testing Forced Air Furnace Electric Draw

[bajajoaquin]

I recently got a Lance camper for my truck, and it has a forced-air furnace. I recall from my past campers (large Class-A campers) that the fan would sometimes draw down a weak battery bank over night.

Rather than get my family out to see the snow this winter, I'd like to test it out. It's supposed to be 45 degrees here over night on Wednesday. I thought I'd just set the thermostat for 85 degrees and let it run over night. That would be a 40-degree differential, and would keep my camper warm enough on a night that dips into the 20s.

Any flaw with that logic?

(Yes, I know, I should get a control panel, test the batteries, etc. But I'm not likely to get that done in the next couple of weeks, so I'm looking for an empirical test.)

 

[Mwilliamshs]

Sounds like a solid plan. It's probably a Atwood 7900-series furnace. I also have a Lance and that's what's in it. If so, amp draw will either be 1.8 or 3.4. 1.8 is the direct vent only model so if you have any ducted vents at all, it'll be 3.4. Post your findings please

 

[JaSAn]

You didn't say what you are measuring:

Amp-hours/watts used?
Battery voltage before and after?
Battery % used?
Change in entropy of the universe?​

jim

 

[maktruk]

How many amp hours in your battery, to start. That's your initial limiting factor.

A nice flooded group 31 deep cycle will run that furnace a lot longer than a starting battery, for example.

 

[bajajoaquin]

You didn't say what you are measuring:

Amp-hours/watts used?
Battery voltage before and after?
Battery % used?
Change in entropy of the universe?​

jim

How many amp hours in your battery, to start. That's your initial limiting factor.

A nice flooded group 31 deep cycle will run that furnace a lot longer than a starting battery, for example.

I am simply testing to see if the battery that my solar panel charges during the day with no load will last the night with the furnace running a 40 degree temperature difference. If it makes it to morning, I can run a gen set if I were actually camping.

The battery is a group 27 interstate "deep cycle " that came with it. I'm not changing batteries at this point.

 

[Ziv]

Battery temp makes a difference in available voltage. Is the battery in an uninsulated battery box?
It's a lot tougher to go from 25 to 65 deg than 45-85. Also, what is your planned limit for depth of discharge? Huge difference in battery life if DOD is 11.2 vs 12v.

I have an uninsulated solar shed at home. On a 65deg night, open cell voltage is usually 25.5v. On a night like tonight, which is very cold, open cell voltage is 24.7. My planned depth of discharge is 24v… not a lot of overhead on a cold night.

 

[bajajoaquin]

Battery temp makes a difference in available voltage. Is the battery in an uninsulated battery box?
It's a lot tougher to go from 25 to 65 deg than 45-85. Also, what is your planned limit for depth of discharge? Huge difference in battery life if DOD is 11.2 vs 12v.

I have an uninsulated solar shed at home. On a 65deg night, open cell voltage is usually 25.5v. On a night like tonight, which is very cold, open cell voltage is 24.7. My planned depth of discharge is 24v… not a lot of overhead on a cold night.

Excellent feedback, thanks. I hadn't considered battery temperature or depth of discharge. I think that 12v is still a pretty deep discharge. I'll take a look at it more.

 

[dwh]

Taking a deep cycle battery down to 10.5v can be done 300 - 500 times, depending on who made the battery.

And while it's true that's a big difference from the 3,000 cycles that can be expected from a battery that never goes below 50%...that'd still be 150 - 250 weekends.

I think the 50% percent rule matters more to full timers, or those with big expensive battery banks than it does to those of us with a single house battery.

With the camper van I have now with a single battery, I just ignore that rule and plan on replacing the house battery every couple of years.

Doubles my available energy budget. But I also carry, and regularly use, a small generator for recharging.