My Battery Heating System

[Rbertalotto]

I stay in the high desert in New Mexico for the winter months. At night it can get down into the 20s and by 9AM it will be in the 70s. I recently installed two 100ah Lithium batteries into an insulated battery box I fabricated out of diamond plate aluminum.
Concerned that the batteries would not come up to charging temp once the sun rises and the charging begins, I decided to heat the battery compartment.
I decided to use a Falcon 7.25" X 25" RV Tank Heating Pad ($40). This pad has a built in thermostat that turns on the heater at 45 degrees and turns it off at 68 degrees. I wanted more control over the heat cycle so I removed the built in thermostat and installed a programable temperature control unit.
The first chore was to find one that was easily programable and the read out was in Fahrenheit........I went through two other very inexpensive and extremely hard to program and only read in Celsius. Then I found a beautiful unit, Bayite DC 12V Fahrenheit 10A Relay control ($18).
The heat pad has a removable paper that covers an extremely agressive adhesive used to secure the pad to the water tanks on an RV. I removed this paper and applied a layer of aluminum foil. I did not want to stick the pad directly to the batteries.
The pad was installed against the side of the batteries. The battery box has 3/4" closed cell foam all the way around the batteries for insulation.
I then wired it up to the controller and added an LED light to the exterior of the storage compartment where the controller lives. At a glance I can see if the heater is on or off.
As it turned out, once I had it installed, here in New England we went through extreme warm weather. The nights never got below 60 degrees, until last night.
Last night the temp went down to 27 degrees.
I have the controller set to turn on at 40 degrees and turn off at 50 degrees.
This morning at 5AM the battery temperature (From the Renogy battery internal blue tooth) was 53 degrees and the battery box itself was 51 degrees.
But to keep the battery at 50 degrees throughout the night used 17% of my 200ah battery bank.

I need to do the math if it might not be better to install another heat pad to bring the temperature up faster and not have to be on as long.

So, in conclusion, I'm very pleased on how this system works. When actually camping, I will disconnect the solar array at nightfall and shut the battery warmer off until 5AM the next morning. No reason to use battery storage to keep the batteries warm when not charging.

Looking forward to New Mexico i8n a few weeks!

 

[BillC]

How did you remove the built in thermostat? I have believe I have the same heating element and think being able to have control over the on and off cycle would work better.

 

[Rbertalotto]

Check out this article I just finished writing. I talk about removing the thermostat... http://rvbprecision.com/rv-projects/lithium-battery-heating-project.html

 

[kbeefy]

Would you be able to set the heating element to only get power when your solar panels begin producing? Or maybe a timer so the heating pad kicks on 2 hours before sunrise? Then your batteries BMS would prevent them from charging until safe?

20 overnight is chilly, but if your battery box is sealed/insulated well enough your batteries may not get near that cold.

 

[Rbertalotto]

With a week of monitoring two systems, the travel trailer with 200ah of lithium and 420 watts of solar, and the roof top tent with 100ah and 120 watts of solar. Temperatures getting down into the mid 20s (F). Both systems are on for only a few minutes at a time. Both are keeping the batteries at between 35 and 40F all night. This proves to be the best way to assure you are never trying to charge a freezing battery.
Both systems are down between 94 and 96% at sunrise. Both systems are bact to 100% by 10am (Massachusetts latitude in the winter!)
Adding timers, controllers from the solar charger and other devices just complicates things and adds another point of potential failure.
So Far....So Good!

 

[SolarInMaine]

I realize this is an old post but want to share an easy, energy efficient way to keep warm lithium batteries when temps drop below the charging thresholds of the batteries. I stumbled upon this idea while building what is essentially an extremely large version of those little battery/inverter combos such as the Ecoflow River. Ive installed five 200ah Weize lifePo4 batteries connected in parallel, in a four drawer filing cabnet(a battery in each drawer and one in an insullated box attached to the top), bolted two BT, 50amp charge controllers to the sides of the cabinet and a 2500 watt PSW inverter to the back. Except for where cables loop out from the battery in one drawer to the battery in the next drawer through neatly drilled and rubber grometted holes in the front of each drawer, most of the wiring is hidden neatly inside the cabinet. My purpose in building this rather large "solar generator" was to get my home built solar power system out of the basement of my motorhome and thus evict the wire & cable spagetti monster that had taken up residence there.
Anyway, more to the point, since I moved my lithium batteries from the basement of my RV where temps never drop much below 50f even on the coldest nights, to a filing cabinet external to the RV where the batteries are very likely to experience severe cold, I had to come up with a way to keep the batteries warm when needed. The solution that I stumbled upon uses "Quietwarmth underfloor radiant heating pads"; Affordable, flexible, paper thin heating pads normally meant to be installed under hardwood/tile floors. They come in 120v and 240v versions and a wide variety of user customizable sizes. These strips can be wired to plug directly into an outlet or used with a thermostat. Since they max out at 85f regardless of size, most people simply connect them to power via a simple switch or an Alexa monitored outlet. Initially, I purchased a 5'x1.5' strip to install under under the carpet, in front of my RV's couch. I plugged it into my inverter through an Alexa actuated outlet and can turn it off and on by voice. It didnt take me long to realize that such a strip could be the answer to my LifeP04 battery heating needs as well. So I purchased a second 120v QuietWarmth heating strip from Ebay for cheap because it was used. Sized by the manufacturer to fit between standard floor joists, this strip measured 5'x16" and uses 48watts to maintain 85f. I lined the inside of my solar generator file cabinet with a 1/4" layer of foam sheeting and installed the 48watt heating strip on top of that. A programable thermostatic switch turns the strip on at 40f & off at 48f. The strip is powered by the very batteries it is meant to protect. Using 48watts, the strip pulls just 4amps per hour from my 1000ah bank of lithium batteries. Thats just 4% of the capacity of my batteries if the strip is in constant operation throughout a 10hr period on a particularly cold night. That the strip would be "on" for so many hours straight is highly unlikely but I like to figure worst case scenarios when it comes to the design of my system. 4amps per hour would be a big draw on battery banks of lesser capacity but a great thing about these heating strips is that they can be cut down into smaller sizes and the smaller they are, the less power they use. They can also be cut into multiple pieces and each piece can be used on its own as a viable heating strip which means that someone can buy a 5' strip and cut it up to heat several differently located batteries. If youve read this very long winded post up unto this point, thank you! I hope something Ive said proves helpful. Im probably not the 1st to come up with the idea for using QuietWarmth strips as battery warmers but Im the 1st person that I know of who has!

PS: RV tank heating pads are rated by the manufacturer to draw around 50 to 60 watts-which isnt too much more than my 48watt 5'x16" Quietwarmth heating strip but I could reduce the draw of my QW strip by half or more simply by cutting it smaller. RV tank heating pads cant really be cut into different sizes, period. Or into multiple, usable pieces. Neither do they cover as much surface area(I could literally protect all 5 of my 200ah lithiums from cold AND all 6 of my semi-retired AGMs by rolling out one 5 foot Quietwarmth heating strip beneath them in the basement of my motorhome).

PPS: My experience with Quietwarmth heating strips has got me wondering about ways they might be used to melt snow off solar panels. The strips are so thin and light weight with heat production that can be easily regulated by voltage input. If only they made them in 12v or 24v.