How To Calculate How Long Your RV Battery Will Run The RV Furnace
When calculating how long your RV battery will run the furnace there are few factors that come into play but the main thing you need to figure out is how much power the fan is using and how many amp-hours your RV batteries hold.
Once you know both of these things it’s just a matter of taking the amp hours your battery has and dividing it by the amps the RV furnace uses.
See Also: What Will The RV Battery Run?
But there is a catch.
You can take the total amp hours in a brand new battery and divide it by amp usage of your RV furnace and you’re going to get a pretty large number.
That number isn’t going to be 100% accurate because it’s not just your furnace that will be drawing power from the RV battery and the temperature outside will also come into play as well as what kind of condition your RV battery is in.
The RV Battery & Temperature
A general rule of thumb for most kinds of deep-cycle lead-acid (flooded, AGM, or GEL) RV batteries is you don’t want to take them below a 50% discharge especially in cold weather.
Any kind of lead-acid battery can freeze at 32°F (0°C) when it’s below a 20% charge. In cold weather, batteries will lose charge faster as well so even if you went to sleep with a 100% charged battery by the time you wake up in the morning it will have lost some charge.
Lithium-ion batteries are quite a bit better and can be discharged to as low as 20% before taking any damage.
Taking all of this into account when you calculate how long your RV batteries can run the furnace you should be taking the number of amp hours your battery holds and cutting it in half.
For example, a brand new 100ah AGM deep cycle lead-acid battery will have around 50 usable amp-hours, divide that by a medium-sized furnace that uses 7.6 amps and you can technically run your RV furnace for 6.5 hours in moderately cold weather without damaging your battery.
That being said most RV batteries no matter what kind will take some damage over time and start to lose their ability to hold as much charge as when they were brand new.
If you have old batteries that have been charged and discharged a lot then you will probably have less usable amp-hours.
When calculating how long older batteries can run an RV furnace you may want to knock one or two hours off of your final calculation just to be safe.
Other RV Appliances Using Battery Power
Lots of appliances use the 12V battery power in an RV and some may be drawing more power than you think. The lights and water pump are the obvious ones but using them once or twice at night isn’t going to take too much power from your batteries.
One appliance people often overlook is the RV fridge. Even when it’s in LP (propane) gas mode it’s still drawing power from the battery to keep the sensors and the electronic ignition running.
A regular 6 cubic RV fridge will use around 0.8 amps every hour. If you fully charge your battery and let it sit for 6 hours before turning the RV furnace on your RV fridge may have taken around 5 amps from the RV battery.
If you use the same calculation used above subtracting the power used by the fridge first, it knocks a half-hour off of your RV furnace run time.
Granted it’s not a ton of power loss and it may be a little unnecessary to add the RV fridge into your calculations but I thought it was worth mentioning.
How To Estimate The Amp Hours Of An RV Battery
If you don’t already know how many amp hours your RV battery holds you can get a good estimate using the “reserve capacity” number which is usually listed on the top of the battery.
There are a few different opinions on the best way to calculate a battery’s amp hours using the RC number. Some say you should divide by 2.4 other say divide by 2 and add 16, and others say just divide by 2.
For my battery, the most accurate way seems to by dividing by 2. Note that this is just an estimate and not an exact calculation for finding the AH rating. The best thing to do would be to look up the amp hour rating online or on the specific battery. But if you have no way of finding that information this is just a way to get an idea of what the amp hours are.
Take the RC number and divide it by 2
I’ll show you using my old RV battery as an example. You can see the RC (reserve capacity) number is 120 in the picture below.
I take 120 and divide it by 2 and end up with 60. This battery has 60ah total, it’s wired in parallel with another one of the same kind so that means the RV has 120ah of battery power but only 60ah of that is technically safe to use.
How Many Watts/Amps Does The RV Furnace Use?
Different sizes of RVs, travel trailers, 5th-wheels, and truck campers use different furnaces.
We RV full-time in a 32-foot travel trailer. The furnace only has 3 furnace ducts it needs to push hot air through so it’s not as large as what would be found in a 5th-wheel.
When I pulled out the manual for our Atwood RV furnace to see how many amps and watts it uses I found information for not only my model of RV furnace but 6 others as well.
We have the 8525-IV Atwood furnace in our trailer, according to the information on the manual the fan uses 7.6 amps which is 91 watts per hour.
Tables For Most RV Furnace Power Usage
Atwood RV Furnace Power Usage
|Model #||8516-IV or LD||8520-IV or LD||8525-IV or LD||8531-IV or LD||8535-IV or LD||1522 Low||1522 High||2334 Low||2334 High|
Dometic RV Furnace Power Usage
Suburban SF-Q & SF-FQ Series RV Furnaces
Suburban NT-SQ & NT-SEQ
Suburban NT Series
Suburban Park (P) Model
Suburban SH Series
If your model of RV furnace is not found in these tables you can look at your RV furnace manual to find the power usage, google it, or leave a comment below and I’ll help you figure it out.
Final Formula For Calculating How Long An RV Battery Can Run A Furnace
Battery Amp Hours Divided By 2 (AH/2) = Total Safe Battery Amp Hours (unless you are using lithium-ion batteries then you can multiply the amp hours by 0.8 instead)
Total Safe Battery Amp Hours Divided By The Number Of Amps Your Furnace Uses (see table above for amp usage by RV furnace model) = Total Hours The Battery Can Run The Furnace
If you have old batteries and it’s very cold outside you may want to take off one or two hours from your final number just to be safe.
Running an RV battery too low in cold temperatures could cause it to freeze which can damage a battery permanently.
See Also: When To Charge A Deep Cycle Battery
Conclusion & RV Furnace Run Time Calculation Example
As an example, I’ll use my RV batteries and furnace.
Earlier I found that RV batteries were around 60 ah each and they are connected in parallel which gives me 120 ah in total. They are regular lead acid batteries, which means only about 60 ah can be used safely before causing damage.
I found the safe number of amp hours I could use by taking the total amp hours (120) and dividing them by 2 (120/2 = 60).
Now I need to divide the amp hours I have by the amp that the furnace will use. My furnace manual said the fan would use 7.6 amps of power every hour.
60/7.6 = 7.9
That means in a perfect world our RV batteries would be able to run the RV furnace for around 8 hours.
8 hours isn’t exactly accurate in the real world though.
If you include factors like other RV accessories using power, cold temperatures reducing amp hour capacity, the age of the batteries, and the battery not being fully charged by the time you wake up in the morning the average time the RV batteries can actually run the RV furnace before being damaged might even be half what was calculated.
My suggestion would be to half the hours that were calculated just to be on the safe side. It might seem like overkill and after using your furnace for a couple of days you might find that the run time is either better or worse than what was calculated.
In the end, it’s better to have your RV batteries more charged than not, especially in cold weather.
How We Run Our RV Furnace While Boondocking In Cold Weather
Most people use a generator when boondocking/dry camping in cold weather to run the RV furnace in the cold morning hours or even all night.
This works well if you have a lot of fuel and you don’t mind the generator noise but most campers don’t appreciate the RVer who runs a noisy generator early in the morning or all night long.
Another option and the way we run our RV furnace on cold mornings is by using a portable power station that sits inside our travel trailer on the top bunk.
We use a 30 amp to 15 amp adapter to connect a long extension cord to the 30 amp power cable on the outside of the travel trailer. We use one of the outer storage doors to get the extension cord inside the trailer and to the power station that sits on the bunk.
A portable power station is basically a lithium battery with an inverter built in. The size of power station we had included two 15 amp outlets we could plug the 15 amp extension cord into.
When plugged in, our travel trailer was basically plugged into shore power. It was a lot like plugging the RV into a wall outlet on the outside of a home.
Our portable power station when fully charged is able to run the RV furnace fan easily for 6 hours and also charge the RV batteries at the same time. The power station has an LED display that tells us the battery percentage so we knew exactly how long we can run our RV furnace before needing to recharge the power station with solar panels.
Our set up has worked well for us for a couple of years now. Even though installing a large RV battery bank and an inverter might be the better way to go for full-timers, our simple extension cord and portable power station set up has worked very well and it was super easy to install.
I highly recommend this easy solution to weekend campers and full-time RVers who maybe don’t have the budget or the know how to install an inverter and a large RV battery bank.
Have any questions about calculating how long an RV battery can run a furnace? Leave a comment below.