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Sounds like if needed, you could snorkel some PM engine floor heat to your Lithiums.
The boxes have no front side, so they are quite exposed to that air flow—except that the lower 6” are shielded by the step-down.

When you get a chance, I am wondering if you could post me the outside dimensions of your foot rests for under the swivel seats LxWxH?
None of these numbers are absolutes: They are 11.5” front-to-back (their width) and 11.75” tall including a 0.5” lid. They approximately line up with the seat base in the center, with a 15”-wide step between. The passenger box extends to the black base of B-pillar. The driver box extends to the wall—it does not have a "left side."

You should absolutely mock this up before assuming you will use it. My boxes are tall to accommodate the previous Group 31 AGM's, and could be uncomfortably tall with the lower factory swivels. If I were to ever be lucky enough to swap out for factory swivels, I would need to be able to transfer them to the original high base—which I would need anyway because my knees do not like low seating.
 

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Van #2 2021 EXT
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The boxes have no front side, so they are quite exposed to that air flow—except that the lower 6” are shielded by the step-down.



None of these numbers are absolutes: They are 11.5” front-to-back (their width) and 11.75” tall including a 0.5” lid. They approximately line up with the seat base in the center, with a 15”-wide step between. The passenger box extends to the black base of B-pillar. The driver box extends to the wall—it does not have a "left side."

You should absolutely mock this up before assuming you will use it. My boxes are tall to accommodate the previous Group 31 AGM's, and could be uncomfortably tall with the lower factory swivels. If I were to ever be lucky enough to swap out for factory swivels, I would need to be able to transfer them to the original high base—which I would need anyway because my knees do not like low seating.
Thanks You Very Much !!

As it gives me some reference dimensions & as I will not have batteries in that location. It will absolutely be a mockup & ergonomic test out situation before fabrication. 👍
 

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The low temp readings by GaryBIS & MsNomer are why I want to maintain a proper charging temp during down time w/o running an AC cord. Thinking an insulated battery box, 12v ,<5amp, thermostat, tank heater pad, and 200W solar may work for my 2 GC2s. An unknown is how much warming does a Li bank need to charge safely. What temp measurement (top, side, bottom,?) assures us that the battery is warm enough to charge? BMS low temp cut out is critical & I don't have it in my BBs :(
 

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2014, 138WB, High Roof, Gas, SW MT
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The low temp readings by GaryBIS & MsNomer are why I want to maintain a proper charging temp during down time w/o running an AC cord. Thinking an insulated battery box, 12v ,<5amp, thermostat, tank heater pad, and 200W solar may work for my 2 GC2s. An unknown is how much warming does a Li bank need to charge safely. What temp measurement (top, side, bottom,?) assures us that the battery is warm enough to charge? BMS low temp cut out is critical & I don't have it in my BBs :(
Good questions - I've been wondering about the same thing.

How much heat does it take to warm up an LFP battery?
Rough guess: Say 2 batteries at 30 lbs each, specific heat about 0.24 BTU/lb-F.
So, heat to warm up both batteries per degree F is about (30*2 lbs)(0.24 BTU/lb-F) = 14 BTU/F - that is, 14 BTU will warm the two 30 lb batteries 1 deg F if all of the heat goes into the batteries.

If the battery pack is cold soaked at 0F and you want the batteries to get up to 40F before trying charge, then the heat required is (14 BTU/F)(40F - 0F) = 560 BTU. This would be 164 watt-hrs, or all of the heat from a (say) 50 watt heat pad for (164 watt-hrs) / (50 watts) = 3.3 hrs. So, 3.3 hrs to warm up 60 lbs worth of batteries from 0F to 40F assuming all of the heat goes into the batteries. A 100 watt heat pad would cut this in half.

This is optimistic since it assumes all of the heat from the heat pad goes into the battery and none is lost to the surroundings. If the batteries sat on the heat pad, and the there was insulation under the heat pad and around the battery compartment, you might get most of the heat into the battery.
There is also the fact that you are adding heat to the outside of the battery pack, and it takes some time for it to be conducted into the center of the pack. Another reason to be conservative on the time allowed for the warm up.

It would be really nice to get the battery temperature sensor into the battery and place it between two of the cells with actual contact to the cells. But, hard to do on batteries with sealed cases. If you have two batteries and they are in contact with each other, then putting the temp sensor between and maybe taping between the two battery cases to prevent outside airflow between the two batteries - maybe that would be pretty good?

An actual test with a real battery would be really nice -- anyone want to lend me an SOK? :)

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Another thought: If you have an inside the van fresh water tank, and the Li batteries can be placed next to the tank and then insulation place around the outside of the tank and batteries, the thermal mass of the water could keep both the tank water and the batteries above freezing for quite a while.

The batteries have heat capacity of (60 lbs)(0.24 BTU/F) = 14 BTU
A 20 gallon water tank has a heat capacity of (167 lb)(1 BTU/F) = 167 BTU

Water is heavy and stores a lot of heat, so if insulated, it is very slow to cool down - days.
This also helps get around the problem that battery compartments tend to run quite a bit cooler than the van interior temp (as measured by MSNomer and me).
Just a thought - maybe someone can turn it into practice.

Gary
 

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@GaryBIS
You should use your flir camera and measure the heat generated by drawing amps from the battery :)

never mind, without access to cells it might not reveal anything useful
 

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The more I look at "self heating through discharge" the more I think using a heat pad would be more efficient use of the energy.
 

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2014, 138WB, High Roof, Gas, SW MT
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Hi,
Decided to do some heating cooling experiments with my Zooms LFP battery
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Cut a hole in the side to be able to put a temperature sensor in the cells.
I used Will Prowse's tear down video to find a safe place to gain access. Interesting enough, the internals are a bit different than the Prowse video.

I think maybe I've voided my waranty :)

I'd hoped to be able to get between the cells, but they are glued to each other by the about 1/16 th inch thick black layer you can see in the photo. The black stuff is pretty hard and very well stuck to the cells. Can't figure out how to remove enough of it to get a temp sensor in. I do have a very thin temp sensor that would fit if I could remove a bit of the black stuff - any ideas on how to get it out without damaging the cells?

I think for the first test I'm going to glue the temp sensor to the cell wall. The black material you can see at the bottom of the hole I cut in the side is about half inch thick foam insulation/padding. I'll put the sensor between the foam and the cell wall. Not as good as between two cells, but it should be pretty good. I'll tape the rectangular piece of plastic back into place.

First test:
With temp sensors 1) inside battery, 2) on the case, and 3) for the ambient air being logged....
I'll place the battery outside overnight - this will get it down to about 20F.
Then bring it inside tomorrow to 70F air and see how long the temp sensor on the cell takes to warm up.

I'll try to work up to adding a heating pad under the battery with and without insulation around the battery.
And check how fast it warms when supplying current to a load.

Any other ideas?

One quick observation is that with the cells being spaced away from the battery case on all sides by the black foam, it does not seem like heat applied to the outside of the plastic case is going to warm up the cells very quickly?

Gary
 

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@GaryBIS
wasn't expecting that

this guy froze a battery and charged it up, pulled full capacity 2 times, he's not sure what's going on though
charging frozen LiFePO4 battery
That is fascinating that he saw no degradation when charging the battery well below freezing. Maybe if only done once in a while its not such a big deal?

I suppose it might also depend on the cell construction and may hurt some batteries more than others.

Gary
 

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2014, 138WB, High Roof, Gas, SW MT
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Just put the Zooms outside for the overnight test.

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Temp sensor is at end of grey wire under the black foam.

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Patched up the hole with duct tape.
That is snow just to left of battery.


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On the front porch with logger.
The battery case temp sensor is under the duct tape on top of bat.
Ambient air temp sensor taped to top step.

Gary
 

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2016 3500 ext-ht
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If you are using a BMV712 and the temperature sensor for same, you can use the relay of the bmv to control the heating pad. The temp sensor goes on the battery post so the sensor is "connected" thermally to the cells. I found that method to be more accurate than a sensor placed between the battery cases. Eliminates the wide range of the Facon integrated thermostat and also gives you a failsafe should the Facon thermostat fail to turn off at high limit.

You can also use the relay on a Cerbo for controlling a heater.
 

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If you are using a BMV712 and the temperature sensor for same, you can use the relay of the bmv to control the heating pad. The temp sensor goes on the battery post so the sensor is "connected" thermally to the cells. I found that method to be more accurate than a sensor placed between the battery cases. Eliminates the wide range of the Facon integrated thermostat and also gives you a failsafe should the Facon thermostat fail to turn off at high limit.

You can also use the relay on a Cerbo for controlling a heater.
On Lead acid batteries, the terminals are well connected to the battery cells, but it seems like on the Li battery tear downs I've looked at the battery terminals are connected via a wire and (I think) the wire goes to the BMS and then to the cells. So, maybe not too well thermally connected to the cells, but maybe better than the plastic case.

Gary
 

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On Lead acid batteries, the terminals are well connected to the battery cells, but it seems like on the Li battery tear downs I've looked at the battery terminals are connected via a wire and (I think) the wire goes to the BMS and then to the cells. So, maybe not too well thermally connected to the cells, but maybe better than the plastic case.

Gary
Can I get back to you on that Gary?
 

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Hi,
Here are some results for the how fast does my Zooms LiFePo4 battery warm up when moved from 20F outside temp to 70F inside temp.

Bottom line is that the cells warm up at about 8F per hour - at least for my Zooms LFP battery.

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Click on the attachment at end of this post for a more readable plot.

The plot begins at 4pm yesterday when the battery was placed outside.
At 8 am this morning, the battery was moved inside the house which was at 70F.

The red line is the temp sensor that is mounted inside the battery case between one of the cell walls and some black foam insulation that isolate the cell from the battery case (see pic).
The blue line is the temp sensor taped to the outside of the case (see pic).
The green line is the temp sensor placed in the ambient air near the battery.

Starting at 4pm, all three temps move downward as the night gets colder. By 8 am this morning all three temps are down to about 23F.

At 8am the battery is moved inside, and the air temp immediately bumps up to 70F.
The case temp also rises pretty rapidly to up toward 63F.
The cell temp rises much more gradually and reaches about 52F after 5 hrs at 1 pm when the test ends.

For this particular set of circumstances, it takes from 8 am to 10 am (2 hours) to warm the battery from 23 F up to 40F.
During this time the battery was exposed to 70F air and with a heat vent about 4 ft away from it, so slow circulation of warm air at the battery location.
This is NOT representative of an RV battery compartment cut off from the interior air and likely at a temp that is well below 70F.

As a rule of thumb you might say the battery warms at about 8F per hour when the air around the battery is considerably warmer than the battery.
So, if you managed to cool the battery down to (say) 0F, it might take something like 5 hours to warm it up to 40F.

One thing to note is that the case temp is not a very good indicator of cell temp. The case is apparently coupled more to the ambient air temp than the cell temp.
So, a temp sensor on the case may not be much use in determining when the cells are up to a safe temp.

Another thing to consider is that my cell temp sensor is placed on the outer cell wall, not in the middle of the cell. Hard to tell how far behind the center of cell temp lags the outer wall of cell temp. This is why I used 40F above instead of 32F - provides some margin.

Another thing to consider is that how long it takes to warm the cells up will depend on how thermally coupled the cells are to the case. For my Zooms, there is dead air or insulation between the out plastic case and the cells on all side - so, not to well coupled.

I'll see if I can have a go at heating the battery by placing a load on it and I've ordered a silicone 12 volt heating pad to see how that works as a heater.
Let me know if you have any other thoughts on what to check.

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The cell temperature sensor is at the end of the grey wire between the blue cell wall and the black foam, which is about half an inch thick.


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All set up for logging.

This is a test of an LFP battery with an internal heater:

Gary
 

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