diy solar

diy solar

12v heating pad?

Sojourner1

Itinerant
Joined
Sep 30, 2019
Messages
761
Location
USA
As a fulltimer living off lfp/ solar not being able to charge the batteries could be an issue as I recently found out.
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I'm thinking here of adding a 12v heating pad under my battery bank in the rv. This will be a backup for if it really gets cold or one of the heating sources for the compartment goes south. We all know that lfp batteries shouldn't be charged below 32°f and with my batteries the EMS will shut charging down if any of the 20 prismatic cells do drop below 32°f, after 44 months we were in extreme cold and couldn't move from it like we normally would do.

I have no experience with this pad maybe someone else does or installed something similar. It has a built-in thermostat that is set to turn itself on at 45°F( + - 5 degrees) and turn itself off at 68°F( + - 5 degrees). As long as the on/ off switch is in the on position.

This is what I'm thinking of using...
Facon 12v heating pad 7.5" x 25"

These are sitting directly on the metal floor of the compartment and the furthest cells (17-20) can be 4°f colder than other cells (I can see all individual cell temps/ voltages). I can raise the batteries about 1 inch. My thought is to pull the batteries out of the enclouser their in (keeps them moving) putting 1/4" piece of plywood on the bottom (maybe a thin piece of solid foam insulation under it), piece of felt on the plywood top, the heating pad normally would get stuck to a plastic holding tank/ water tank I was thinking of getting a piece of aluminum and sticking the pad to the bottom of it to tansfer the heat better, then putting a piece of felt on the top of aluminum with the batteries on top of that.

Looking like this...
Batteries
Felt
Aluminum sheet
12v Heating pad
Felt
1/4" plywood
(Maybe) Thin foam insulation
Metal compartment floor

The heating pad can be connected to the 12v fuse panel with a switch added inline to turn it on/ off if needed. Power consumption should be roughly 6a which would be a little more than 1% an hour of my battery bank, I'm thinking it probably wouldn't need to be used more than an hour or so periodically.

Using 12v instead of 120v saves the lose of converting from the inverter. The only thing that I'm thinking about is with the built in temp on the heating pad will it be turning itself off because it will be within the sandwich under the batteries and not in an open air space of holding tanks?

Opinions? Critique? Thoughts?
 
I have two of those hooked up to a digital thermostat. I don't think it needs to be ad warm as the internal one is set at. One of the internal thermostats failed. Company replaced it. I'm going to cut out the internal one and just use the external one.
You can get the exact same thing on eBay for less.
I sat my batteries on an aluminum plate with the pad underneath. I put wood slats around the pad so the weight of the batteries wouldn't be directly on the pad.
 
never seen the falcon heating pad, So not sure...but the main thing is the heatpad be built out off flat traces, if you can feel the wire in the pad snaking back and fourth I don't think you want to be putting a ton of weight on that style.

You'd be surprised how much heat even 30 watts can give off inside a small ice chest or insulated box.
 
@somewherethe, price I'm not worried about, in the big scheme of things $40 isn't that much. I don't trust or do ebay.

@Picasso, I've never seen one of these heating pads in person that's I'm wondering about the placement of it.

I'm not even worried about the heat generated from them other than if it will stay on for a period of an hour or so without having it turn itself off being sandwiched in the design I'm thinking about. As I said all cells can be seen/ monitored on a display panel in live time readings. Once the cells warm up it would as simple as turning the heating pad off.
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Maybe I don't even need the heating pad after the plywood and foam insulation is added raising the batteries off the cold steel floor.

I tapped into the heat duct run from the furnace as can be seen in the original post, all that's needed is to remove the cover on the end of the drop down hose for more heat to get pushed into the compartment instead of just radiating off of hose. The compartment has already been insulated fairly well, maybe the entire floor in the front compartment should have a piece of foam/ plywood on it just as a layer of insulation.:unsure:

I would use this heater placed in the compartment plugged into the generator, but only as last resort.

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After thinking about this I decided to not go with a heating pad instead I purchased 1/2" thick rubber pad, cut it down to fit snuggly in the bottom of the battery box to get the batteries off the metal floor. I believe that should help some, along with the heat duct coming down towards the batteries. I'll also fit the remaining rubber to cover the entire floor of the front compartment as a barrier.
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Misc pics while doing this.
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I think you made a good decision. Everything you described was pointing to the metal floor causing all the problems. Fixing that with a simple passive solution is far superior to adding the complexity of a heat pad and it's controls. The only thing I might have done different would be to add more insulation (double layer of rubber?). Keep us posted on the results.

Side question: What kind of cells are those?
 
I was thinking thicker but didn't want to raise them to much with clearance near the solar charge controller and negative cable going into the housing of the EMS/ CPU of the batteries.

Batteries are from Elite Power Solutions, GBS cells.
 
An update... down to 25f my added heat source and rubber mat does ok keeping the cells at a temp of 35-39f.

We're having a cold spell of single digits tonight but yesterday was a high of 32f and got down to 14f outside. This morning at 5am compartment was holding at 47f but the cells were 34-35f and was thinking great but at 9am they dropped another couple degrees and one went to 31f.

I was prepared incase this happened with my little heater to give the compartment and the cells some quick heat. Within 30 minutes compartment 70f and cells at 40f and rising.

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So no more screwing around, when I get to my winter boondocking in AZ I'll get the 12v heating pad and add it under the cells. I'll update with pictures when I get this down.
 
With compartment holding higher temp vs cells sounds like cold soaking past your rubber mat. worth it to lift and try to gets a little more R value from the ground? aka little more insulation.
 
Yes, the metal floor of the compartment is just a large heat sink.

Plan "b" of the small heater is working as a bandaid for now but rather have something running in the backyard that will work if I'm not around.

I like the heating pads that @HRTKD ordered, I think they can build to the size of my enclosure the batteries are held in.

Wire it to its own fuse in the 12v panel (power consumption will be counted this way) and have it's own on/ off switch for when I want it to get power besides its own temperature range for the heat pad.

There are some empty slots to add it to the 12v fuse panel.
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Obviously it depends on your C rates, and how much hot weather you see, but putting some 1/2" XPS foam around the batteries (especially the floor, if thats a major conduction path) can help with heating. Obviously you need the heating pad. I have the smaller one in my underbody pack, and its been good down to 20F so far (1/2" XPS insulation, and 5 exterior sides). Its only been in service a few months though.

For safeties sake, I would be more conservative than 32F, and aim for 38F as your minimum charge target. Though low C charge rates (0.05) are typically fine even a bit below freezing.

Some ballpark math indicates that if you insulate that pack with 1/2-3/4" XPS all around, then a 60W heating pad should be good below zero. Though at that ambient level it will run nearly 100% duty cycle.
 
Heat is never an issue, in 4.5 years I've only seen a cell at 80f and that has only been twice. 45-75f is the normal cell temp range depending on season and location.

Usage C rates are minimal...
Humidifier, misc. .02c (this is a baseline 24 hour countinous load)
Coffee brewing .17c
Microwave .3c

Even charging is a low C rate, .12c is max with solar, .16c is max the panels can produce but there are always loads. If the generator is needed for a boost charge then .2c (100a) is what I'll set the charger to.
With early morning winter solar charging when cell temps might be low .04c would be about average.

The goal to fulltiming in an rv is to run from extreme cold weather but in the past 2 years I've had to be in it for a short duration before I could move.

The moderate heating of compartment itself is working out pretty good, but the metal floor is just a heat sucker from the cells. When the nighttime ambient drops to 25f and gets up to 45f +/- the cells will hold at a low 38f +/- and then warm up some during the day with use and charging which is good for my use pattern.

I believe that I'll use some sort of insulation under the rubber mat along with a heating pad. I'm still thinking about the heating pad though, if the heating pad is sandwiched under the batteries will the auto on and off temp range be affected? Might be one of those trial and error installations.
 
Consider:

The ambient temperature in the compartment is generally warm enough.

The metal floor below batteries is the “source” of the cold.

Placing a heating pad below the battery will eliminate the only face of six that is causing the low temperature problems.

Being able to sense the temperature at the top of the cells is unnecessary in your situation, the top is ambient air which is not the cold source. Therefore the thermostat being below batteries should not pose an issue.

Hope this helps.
 
The heating pads with integral thermostat are typically fine, even with some insulation on the side opposite the battery. They may cycle a bit more, but that's not a big deal.

If the bottom of the compartment isn't insulation well, I would considering just putting down some XPS foam. That will make your current heater approach effective. It sounds like the top of the compartment is getting up to 45F or higher, but the bottom, where the cells are, is colder.
 
The front compartment of a 5th wheel is not insulated at all because it is a metal floor.

In this image I've circled the bottom of the floor.

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I'm thinking about adding a thin layer of foam board (glued to bottom), then fiber tape over it to protect it from the weather traveling down the road or maybe this on top of it. https://www.amazon.com/dp/B07FYLM47D/ref=cm_sw_r_cp_apa_i_fJyOFbSEMKSZZ

I have some time now that we're moving south away from the extreme cold weather. I have to think something over the outside floor is better than what I have now, nothing. :unsure:
 
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Can you put the insulation on the inside? You can even cut it into smaller sections to make install easier. XPS foam won't absorb water, but it needs to be painted to prevent UV damage.
 
I'm going to insulate the entire inside floor with foam board and put the rubber mat on top of it, along with pulling the batteries out again and adding the foam board under the rubber mat already in place.

I think I'm still doing something with the metal floor outside.

Easiest solution is to get out of the cold weather and use my 2017-2018 battle plan, if it's to cold for shorts and flip flops I don't need to be there. The good old days. :cool:;):cool:
 
A little update on the front compartment. I believe the metal floor is working like a large heat sink in the below freezing weather and just won't hold any heat during the winter and sucks out any warmth in the compartment and cells. So, I'm still staying in my passive approach and will give it one more try.:unsure:
Bottom layer is Reflectix, middle layer is 1" R-Matte plus-3 (insulation board composed of a closed-cell polyiso foam core bonded to reinforced aluminum facers) R6 rating. Top layer is 1/2" thick rubber mat from Tractor Supply.
Looking like this...
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Also added the foam board in the 2 sections on the wall that I didn't get it the last time.
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I still have to pull the batteries out one more time to add the Reflectix and foamboard along with the rubber mat under the batteries.

We'll see what happens in the next cold spell but that might not be till next winter. ;)
 
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