My Pickup Truck has a Roofnest Falcon Roof Top Tent installed above the truck cap. Even though I might be camping far off the beat’n path, I still need access to electricity for charging devices, powering a portable refrigerator/freezer, running a diesel heater, and more.

To accomplish this, I added a solar panel to the top of the RTT (Roof Top Tent) and a charge controller to a 100amp hour battery.

Originally this battery was a very heavy (65 pounds!) AGM type battery. Since I sometimes tent camp away from the truck, or use the battery in my fishing boat for the trolling motor, this battery needs to be portable. Hauling a 65# battery through the woods or down the dock is no joke!

Recently I changed the battery to a Lithium with its many inherent advantages:

It weighs under 25 pounds

It charges up much faster once the sun comes out

The voltage stays constant throughout the discharge cycle

Nearly twice as many amp hours as the same size AGM battery

But there is one big disadvantage:

You can’t charge a lithium battery if the battery is at or below freezing…..

I camp a lot in the high desert of New Mexico and Arizona in the winter. During the night it can get down into the 20s and sometimes it can stay below freezing for days at a time. If the battery is frozen, I can discharge it until depleted, but then I can’t recharge it unless I warm it up somehow.

The battery I bought was quite inexpensive

At the time of this writing (11/22) it is around $350. This is a huge price drop from a few years ago when a 100ah lithium battery was $1000 or more!

To deal with this freezing issue, lithium battery manufactures are now offering batteries with self heaters built in. But they are near twice what I paid for my Lithium battery.

So how am I going to keep my battery from freezing.

First thing is some type of insulation to store whatever heat the battery might have or is putting out while being used.

I decided to simply put it in a “cooler” like you’d put food or drinks in. A quick search on Amazon and I found a soft cooler that fits this Group 31 size battery perfectly! And the straps and shoulder strap the cooler is supplied with make transporting the battery a snap!

A MOYAQI Large Soft Side Picnic Cooler

As a heating element I decided to use a RV water tank heating pad. These pads are adhered to the bottom of RV water tanks to keep the water from freezing in cold weather.

The one I chose is made by Falcon (no relation to the RTT)…. #CW-ST725, 7.25″ X 25″… Runs off 12V and draws 78 watts

I didn’t want to adhere it to the battery, so the first thing I did was remove the paper backing

And layed down a layer of aluminum foil to the adhesive

These tank warming pad have a built in thermostat that turns the heater on at 45 degrees F and shut it off once it reaches 68 degrees F…..This delta is much too large for heating a smaller battery to above freezing.

The built in thermostat needed to be found and removed.

Feeling around on the back side of the pad, where the power wires enter, you will feel a small metal device, about 3/4″ X 1/2″ X 1/4″…..You need to be careful and cut it out with an exacto knife….

Once you have it exposed, cut the two wires feeding it, solder them together and insulate with some heat shrink tubing and a piece of duct tape.

Now that the pad is modified you can simply place it around the battery that is in the cooler

I decided not to put it under the battery. My concern was the battery bouncing around on top of the pad might not be health for the pad in the long run. The 25″ pad I bought covers nearly three sides of the battery. Should supply plenty of heat.

I added an LED light on the side of the cooler to tell at a glance if the heater was on or off

Now because I want the battery to be portable, it get a little bit more complicated.

ANDERSON PowerPole quick disconnect plugs were employed to allow quickly removing the battery from the truck.

I actually experimented with three Temperature Control Units before I settled on the one I used. The other two were VERY difficult to program and only read in Celsius……Too confusing!

I finally found a very well made, easily programmed and “F” readout

BAYITE DC 12V Fahrenheit Digital Temperature Controller ($18 in todays date 11/22)

This unit was located in the sidebox of the truck cap where all the other solar and power electronics live.

I set the parameters for the pad to heat up at 35F degrees and to shut off once the battery is at 40F degrees.

This video might explain some of this better than my words can:

Once the controller was installed, and it was just stuck down to the inverter with 3M VHB double sided tape (Very High Bond tape is amazing stuff BTW!), the wires for the temperature sender and the power for the heater pad were snaked down to the battery,

The battery “Cooler” was secured to the bed of the truck with a strap.

Power for the controller simply plugged into one of the jacks in the power panel in the truck cap.

To clarify, on the top of the battery is a small panel with USB connectors to charge devices when away from the truck

Also a cigarette lighter socket

And another Anderson PowerPole plug to attach other devices that use this type of connection

All are fused and there is a 50a circuit breaker to protect the battery and wires of the large Anderson Plug

And we’re done!

Now we just need to wait for it to get cold enough to test …(Actually, I tested it with the temperature sender in a glass of ice water and it worked great!)

Thanks for reading! Be sure to visit the full website to see many more articles on Solar, Camping, RTT, Travel Trailers and way more!

Wiring of BAYITE DC 12V Fahrenheit Digital Temperature Controller

UPDATE!!!…Took advantage of a Black Friday Sale and bought a second matching100w solar panel for the Roof Top Tent……Now 200w of solar to recharge battery faster….

Here are the videos on the battery heater install of both the RTT and the Travel Trailer



Battery Heater or Low Temperature Protection…Absolutely need low temp protection in an RV installation, or battery heaters. Sun comes up, solar panels start charging, batteries are at below freezing…damaged battery. This scenario happened just about every night in the high desert of New Mexico this past winter. Down in the single digits or teens overnight, in the 50-60s during the day. Water hose froze solid every night. If batteries are mounted outside, and you are not drawing them down sufficiently to cause internal heating, there could be an issue. I built a battery heater system to thwart this issue. Worked great. I’d rather heat the batteries so I can start charging immediately when the sun rises rather than waiting for batteries with low temp protection to heat up so they can then accept a charge. My new Dr Prepare batteries have low temp protection as a failsafe should the heating system fail.


Some are relying on the internal temperature control of a Solar Charge Controller…..But if the controller is in a heated space, it might be above freezing but the batteries could still be below 32F/0C. Be careful!

Share this Article:


  1. Gary Reply

    Very cool project. I am about to add a heater to my LI battery. I was not planning to remove the built in thermastat as the external unit would control the power to the pad it will set at 34 degrees and would not start the heating pad until it reached that temp. The remote controller would also shut the heater off at 38 or 40 degrees. I am not sure how the built in thermostat would interfere with the thermostat controller. Am I missing something?

  2. Robbi Reply

    I live in Massachusetts also, getting ready to install lithium batteries into my enclosed trailer and I’m very happy to run across your visor. Thanks for sharing

  3. Rob Duncan Reply

    I tried using Facon ST725 tank heating pads, mounted on .020 aluminum panels, tightly sandwiched between batteries. I found they were short-cycling, only remaining on for about a minute every ten minutes or longer, while the insulated battery compartment temperature dropped.

    I am using two Tesla S modules. I used heated antifreeze for first two years, but some particles appeared in the fluid and blocked the internal passages.

    I read your thread here about cutting out the internal thermostat, and using a stand alone controller. I am worried about pad damage while peeling the adhesive backed pads from the aluminum. But that would be my next step.

    Did you try using tank heaters with the internal thermostat, or have other ideas?

    • Roy Bertalotto

      No, I never tried them with the internal thermostat. I thought the delta between on and off was way too long. The way they are set up now it is working perfectly. The Renogy batteries have Blue Tooth and I can see average temperature….They are always at 40F +/- regardless of outside temperature. I mounted an LED on the outside of the camper to show when the heater is on. And I can monitor the light with my RING security camera. Then I can look at the camera’s recording from the night and watch the time stamp. I can see the heater comes on about every hour for about 10 minutes at around 30F……Will need it to get alot colder to really test it.

    • Ron

      Thought the internal battery heaters turned on at 45f and 30f would be too cold to charge the batteries so wondered about your temp settings ??


    Since the heating pads are all 12V and nearly all the thermostats are 12V, I’d just use a step down device to convert 24V to 12V to run the heating system.

  5. Rob Duncan Reply

    I already have a 70 amp 24 to 12V Victron converter that runs all coach 12V pumps, appliances, etc. However should the battery temperature reach the BMS-programmed thermal shutdown value, all loads are disabled – thus including same converter that powers the heating pads. Sadly the BMS action of disabling both the loads and the charging devices (e.g. solar) can not be modified. I am considering using your suggested 12V thermostat unit for testing while I continue my quest for a 24V thermostat. I will wire the Facon pads in series for the 24V application. Today’s project is to remove the three existing pads for a thermostat-ectomy, and complete wiring for a DC current logging module, with Bluetooth. It also can track temperature so I can get a good reading about what duty cycle vs. OAT vs on/off settings are.

    I plan to first try these three pads in series, thus each pad would be operating at only 8V, and each yield only 20 watts. I am doing this because 1) I am unsure if there is sufficient space for another pad between the two battery modules, 2) unsure of the total heat produced and if the batteries’ thin plastic covers can take it, and mostly 3) the 5 amp total load is well within the capability of proposed thermostat. I already have the 4th pad in hand, but if I wire as 2S2P for 60×4 total watts, the total current draw will be 10 amps and I will need to add another relay – likely a solid state. However if more wattage is needed, I am “almost” ready.

    Bottom line will be a self-contained heating system running at native 24V. If the batteries go below the low temperature limit, heating will still be applied. I can add another controller if a lower voltage limit is reached to curtail further battery depletion.

    OAT reached 9F early this morning. Perfect for testing, but not ready yet! an we switch to direct email conversation?


    Sounds like you are on top of it…Be sure to let us know how it works out. Good Luck!


    “I just found and ordered this.”

    I bought one of those…A real bear to program I found. But once I figured it out, works fine. I just like form factor much better on the one I’m using and very easy to program.

    • Rob Duncan

      Thanks Roy. I hope I can figure out the programming. Am I correct in assuming that setting heat mode “on” at 40 degrees, and “hysteresis” at 10 will attempt to keep between 40-50 degrees? I also liked the module form factor, but will add a cover, Amazon B07BQ42RBH. Getting pretty crowded in my “electronics bay”. See

      Ordered today six Aiyunni 8 x 16 high thermal conductivity pads on Amazon to buffer the four (1.5 pads per sheet) .020 aluminum sheets from the thin clear plastic outer walls of the Tesla battery modules. I plan to remove and inspect the heater sandwiches (Falco heater pad, + aluminum sheet + thermal conductive pad) for abrasion after the first couple hundred miles. Will be configured as: sandwich, battery, sandwich x 2, battery, sandwich. I tested that a Falco pad produces about 140F at the aluminum sheet surface – just hot enough you don’t want to leave your finger there. Instrumentation will allow tracking of my insulated enclosure battery heat and power use. I can always reduce hysteresis value, or remove one sandwich if too much power. Each Tesla battery module has two internal thermistors, of which I am only using one each right now.

      Single digit night temps here now, and hope they remain for later testing.

      Is there a way to subscribe to this forum?

    • Roy Bertalotto

      Am I correct in assuming that setting heat mode “on” at 40 degrees, and “hysteresis” at 10 will attempt to keep between 40-50 degrees?

      No , you would have hester turn on at 30 and shut off at 40

  8. Rob Duncan Reply

    Everything is functioning circuitwise, but having issue programming the thermostat. From what you said earlier, should I set temperature at 50 and hysteresis at 40 to maintain battery temperature between 40 to 50F?

Leave A Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.