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 it 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.
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 calculate it using the “reserve capacity” number which is usually listed on the top of the battery. Take the RC number and divide it by 2.4
I’ll show you with this RV battery as an example. You can see the RC (reserve capacity) number is 120 below.
Then I take 120 and divide it by 2.4 and end up with 50. This battery has 50ah total, it’s wired in parallel with another one of the same kind so that means the RV has 100ah of battery power but only 50ah 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 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 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.
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 = 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.
Have any questions about calculating how long an RV battery can run a furnace? Leave a comment below.