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Old 01-12-2025, 09:21 AM   #15
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they sell adhesive heating pads that you can stick to the battery.
That will be all you really need. Buy one test it on a battery and see if you are comfortable with that. Cheaper and easier than running new ducts. They are used on car batteries, oil pans and anything else mechanical that needs to be kept warm. I use one on the hydraulic tank of my log splitter when it gets really cold out.

You really only need them warmed up to charge, they can discharge with no issues to -4
biggest issue I see is the pads are usually 120v but I believe still workable.
They still need power, and they will still take time to raise the temp of a cold battery. If you have power to run them all the time, you do not need the batteries.
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Old 01-12-2025, 09:23 AM   #16
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They still need power, and they will still take time to raise the temp of a cold battery. If you have power to run them all the time, you do not need the batteries.
they need to charge the batteries with power, so how is it an issue to start getting the batteries warmed prior? you have to think about?
If they are using solar, than divert it, if its a genset than start it up earlier.
Lil planning and it's perfectly doable.
plus passthru isn't going to be the same temp outside, it will get residually heated anyway.
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Old 01-12-2025, 09:45 AM   #17
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If using a generator to charge batteries you will only have power for charging. If you have to wait for them to heat up the genny will be needed for that time before changing can begin. My suggestion is that you should do what you can so the batteries do not get cold to begin with. If the trailer or RV has a heat source for comfort, try to use that to maintain battery temp just as is done for the water tanks and plumbing.
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Old 01-12-2025, 09:49 AM   #18
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If using a generator to charge batteries you will only have power for charging. If you have to wait for them to heat up the genny will be needed for that time before changing can begin. My suggestion is that you should do what you can so the batteries do not get cold to begin with. If the trailer or RV has a heat source for comfort, try to use that to maintain battery temp just as is done for the water tanks and plumbing.
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Old 01-12-2025, 12:42 PM   #19
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OP here:


Some interesting ideas and comments so far in this thread.


What I'm really looking for are real tried solutions to the issue of keeping lithium batteries warm in the pass through of a 19MKS (21MKS) is setup the same.


Stick on or wrap around heating pads for LA batteries works well because of the convective heating of the water inside the battery. In a lithium battery stick on heat pads are not a good design idea because they may easily overheat one area of the battery while leaving the far side or center cold. This may trick the BMS into thinking it is safe to charge and damage the battery. In worst case it could lead to thermal runaway and a fire.


I think it is important to never let a lithium battery drop below it's safe charge temperature if you plan on charging it. Maintaining a target temperature is much easier than raising it backup. This is my objective.


What I've done before and may do in this case to place 12v 25 watt heating pads on metal plates air gaped below the batteries in an insulated box. This prevents any direct spot heating by providing heat all around the battery.


In reality I would only need secondary heat with the internally heater BB batteries when the temps drops very low. The internal heaters will handle the needs down to about 13 F. So all that's really needed is raise the temperature of the space around the battery to something above 13 F and then let the internal system take over.


When I get my trailer in month or so, I'll place some recording thermometers in the pass through, outside and trailer's interior and compare temperatures and lag times are various winter temperatures.


I may find that the internally heated batteries in an insulated box will perform just fine down to my lower camping limits of about 0 F.


As said I would love to get some comments on real world experience.
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Old 01-12-2025, 12:57 PM   #20
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I may find that the internally heated batteries in an insulated box will perform just fine down to my lower camping limits of about 0 F...

I would certainly think so. I think you're overestimating the localized heating that would occur with typical low wattage heaters. I do agree that you want the batteries kept at an even temp, for balancing purposes if nothing else. And insulating them is the best way to do that. There's a significant amount of metal inside batteries, I think they will equalize temperature just fine.
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Old 01-12-2025, 05:11 PM   #21
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It is surprising how well the batteries equalize temperature, and the main reason is the lithium is super conductive. I ran a few tests with radiant heater facing battery, being concerned that if a heating pad is in same area as internal battery temperature sensor it might fool BMS and allow BMS to charge battery when most of battery was cold. BT showed what the BMS was sensing. If one uses heaters that stay below 150F when sandwiched between insulation (if you have it), heating pad and battery, localized surface heating should not be a problem--concern for ABS housing damage. Heating on two sides, likely on long face (facing ends of cells not flat face of one cell) is advised for better heat distribution.

I also did the above to find where in battery temperature sensor is located both which side and where on long side of battery face.

I would not put heater under battery because battery movement and being clamped down may shorten life of heaters and any sensors (avoid in heater anyway).

As other mentioned, it takes a very long time for battery to heat up or cool down due to the large thermal mass.

For rarely used or temporary insulation, aluminized bubble wrap bag and if you have access to food delivery box/ bag (Blue Apron), the battery can be put it there, depending on battery size and food box. I double the bag and am using group 24 batteries. My use is not much, but I also put another cardboard box over what was just mentioned that can be replaced when wear occurs. If more insulation is desired shove some flexible foam inside bags.
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Old 01-12-2025, 05:53 PM   #22
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Tim


"I would not put heater under battery because battery movement and being clamped down may shorten life of heaters and any sensors (avoid in heater anyway)."


I would never place anything directly over a wire or heating element, etc. I've seen in someone else's truck how that can cause a dash fire.


My proposal is to air gap the heating pad about 1/2" below the bottom of the battery with an aluminum plate in between for heat distribution. Basically warming the battery with warm air only. The battery would be supported by a frame on all four sides with a section open in the bottom of the supporting structure. Like a large rectangular washer. Warm air could also flow over the sides and top.
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Old 01-12-2025, 06:00 PM   #23
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If you are worried about the batteries in the pass thru, have you considered switching and putting the inverter in the pass thru and batteries under the bed? I put my batteries under the bed with the lynx distributor and cerbogx and put the inverter, and solar controller on the wall in the passthru in my 24RKS.
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Old 01-12-2025, 06:58 PM   #24
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If you are worried about the batteries in the pass thru, have you considered switching and putting the inverter in the pass thru and batteries under the bed? I put my batteries under the bed with the lynx distributor and cerbogx and put the inverter, and solar controller on the wall in the passthru in my 24RKS.

Yes, that was Plan A. The BB 100AH are too tall to fit under the dinette seat. The 19MKS has what ORV calls a wall bed, like a Murphy bed that folds down over the dinette seats instead of a sofa. Thus the height under there is little shorter.


I had looked at possible getting some larger format batteries like the LiTime 270 AHr. They would fit under the seats. But after emailing the company they do not approve their batteries to run things like power tongue jacks or room slides. Voids the warranty. Besides not very excited about shelling a couple thousand for new batteries.
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Old 01-12-2025, 07:29 PM   #25
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Tim

"I would not put heater under battery because battery movement and being clamped down may shorten life of heaters and any sensors (avoid in heater anyway)."

I would never place anything directly over a wire or heating element, etc. I've seen in someone else's truck how that can cause a dash fire.


My proposal is to air gap the heating pad about 1/2" below the bottom of the battery with an aluminum plate in between for heat distribution. Basically, warming the battery with warm air only. The battery would be supported by a frame on all four sides with a section open in the bottom of the supporting structure. Like a large rectangular washer. Warm air could also flow over the sides and top.
Good advice and a lot of wisdom--thanks very much! It is important to be comfortable with the safety of our campers!

Your setup is possibly safer than mine. Heaters right on battery BB cells raises questions (25W will heat much faster than my design--a benefit) but, it is done by an excellent manufacture. Your above heaters will need higher temperatures and more energy to be effective because convective heat transfer is not as efficient as conductive (more energy loss), which has its own risks.

My thoughts were not that far from yours (keep safe!)--What I have: maximum temperature possible with heaters stuck on--125F (verified through testing). As you know, I have maximum watts with four in series 6.25W, which makes 3.15W per battery and 1.6W per heater element. Then this series set has 1A fuse. Sensors very near heaters but not too close to significantly affect control. Using double temperature controller/switch for redundancy. Clearly, I am relying on very good insulation (messy) to greatly reduce temperatures (risk) to make system practically safe.

General comment: With the gap between battery cells and battery enclosure convective heat transfer within battery housing is good. Various tests were showing this.

I am not recommending anyone doing what I've done but throw out ideas for others to start with. See how much heating is needed while camping, then spend time to design and build.

rarebear--I just ran across one of your posts 2021, about heating when not camping in Santa Fe, so I have a little more idea about your needs. You likely have a very good idea of your heating needs but with a new camper, time to redo.

Added: I would not insulate BB HEATED (internal or external) batteries unless BB clearly says (document would be very valuable stating this) that is acceptable! It deviates from orginal design, unless BB has tested this.
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Old 01-12-2025, 10:29 PM   #26
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When I designed that system in 2021 I had an email exchange BB about insulating the batteries. My real question to them at the time was about if there would be any need for cooling in the summer. They said no issue. I do not have those emails anymore. They did not seem to have any issue about insulating them at the time.


Couple of details I have not disclosed here about that design back in 2021. I used an Inkbird thermostat for temperature control. Through some extensive bench testing I learned those 25 watt heating pads at 12v delivered very high temps, 120+ F. Too hot for me. So using a variable bench power supply I calculated that 5 volts kept the high temps down to something in the 70s F, I think. Close to an ideal operating temperature. I used three pads. Added a buck transformer to reduce 12 v to 5 volts. Placed a master switch before the Inkbird. Then after the buck transformer I added a DPDT switch. On - Off - On. In the center position only one pad would be used. Another position added a second pad and the last switch position powered all three pads. I also had remote temperature monitoring inside the insulated battery box and ambient temperatures. Thus I could select how much heat to add in multiple steps. The battery box on that RV is located under the floor fully exposed to ambient temperatures.


My thinking right now is build the insulated battery box with the idea of adding heating pads. But not add the pads or support components until I get some real data for differences in ambient, pass through and battery box temperatures and the propagation delays in thermal transfer. All I need to do is keep the interior of that box above 13 F for the internal heaters to do everything.


I guess I was hoping someone else might have already done this work and have some data and experience to share.
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Old 01-12-2025, 11:24 PM   #27
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So I assume that you will be using the trailer and keeping the living space heated with the on board furnace? If so, I would just build an insulated box for the batteries in the pass through and cut out a rectangle in the pass through wall and cover with a louvered panel. It shouldn't take much heat to keep that insulated area above 40 F. Keep it simple.

On the other hand, if you plan to not keep the living space heated I could see the need for heating the batter box. It shouldn't take much to stay above 40F. If it's below freezing outside your tanks will freeze with no heat in the trailer. Getting the batteries up to temp after setting unused and already cold would be my only worry. The built in battery heaters should take care of that in a few hours I would think. Just thinking out loud I guess.

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Old 01-13-2025, 12:28 AM   #28
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Thanks Fred for the interesting update and useful information.

Good to hear that BB feels being insulated when hot was not a concern. They should know. To me, it would very much depend on how hard the batteries were working and how well batteries were insulated, but if not doing much work very little heat would be created. You saw my low watt figures to keep my batteries 35F when in a freezer at about 5F.

I made an incorrect statement about high thermal conductivity of lithium, which is far from the case--it is very low. It has very high thermal energy density, so takes a long time to heat and cool.

It sounds like you have good well thought out plan and plenty of related experience! Enjoy the build.
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