battery box heater

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Acercanto

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Feb 14, 2021, 12:16:26 AM2/14/21
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Ok, so I've been reading so many threads, and trying to learn more about ohm's law that my head is spinning. So I come to you all for wisdom.
My battery will be in a well insulated box, and I am trying to decide how to heat it. I am thinking for the thermostat, I will use this:
It seems nice and simple, but with the features necessary.
The heater is where things get trickier. I am considering a few different options. I know I only need a handful of watts, but everything I've found is so oversized for this.
This seems like it'd work, but is way more heater than I need. I could just use one of the elements..

These are about the right size, but all the ones I could find seem really cheaply made. Plus I'd worry about them getting the surface of however I mount them too hot.

This has potential, but I'd worry about the thermal cycling they mention as a failure point. Possible upside would be using it to cool the box in the summer? I don't know how well it would heat if the "cold" side is already pretty cold.

Which brings us to the present rabbit hole of making a resistor bank to heat the box.
Reading these seem to make some sense, but in all the calculations, it seems to assume limited current supply. I know a resistor limits current itself, but I can't wrap my head around how much power is dissipated vs. current drawn. I think.
One guy was saying he uses 120Ohm resistors for 12v circuits, but that brings up another point. When I was building a speaker setup, I learned that resistors (speakers) in parallel subtract their resistance, and parallel adds. So do I calculate the resistance for the whole bank, and then calculate the individual resistors from that? SO MANY UNKNOWNS! :-)

Any thoughts?
Thanks!
Acercanto

Dacian Todea

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Feb 14, 2021, 2:32:23 AM2/14/21
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The thermostat is OK.
The PTC heater will use less if you slow down the fan speed.
The peltier will just not work for you type of application.

Using a power resistor as heater is also possible and calculation is not that hard.
So say your battery is 13.5V and use that to supply a 120Ohm resistor then current trough the resistor will be 13.5V / 120Ohm = 0.1125A and so power on the resistor will be 13.5V * 0.1125A = 1.52W 

Ken R

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Feb 14, 2021, 12:31:42 PM2/14/21
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Using a controller similar to your listing, I wired 2 of those in series (for 24v) with the thermal cut off and wrapped them in a piece of folded aluminum. 

Dacian Todea

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Feb 14, 2021, 12:41:02 PM2/14/21
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Ken,
Yes those seem like a good solution but not sure how uniform will they heat the battery.  It depends where are those installed and where is the temperature sensor. And thermal cut off is always a great backup solution.

Ken R

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Feb 14, 2021, 1:06:56 PM2/14/21
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The thin aluminum plates were sized to cover and pressed against the side of the battery.  They are in the middle, the temp sensor is on an outside edge.
The other option is to wire more in series (thus lowering the wattage) and fully surrounding the battery with them.  Sensor placement becomes an issue in that case.

Acercanto

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Feb 14, 2021, 10:12:55 PM2/14/21
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Dacian, thanks for the example. If I put a 4W 120ohm resistor in, it will still only dissipate 1.52 watts? Is that what is meant by "derating" a resistor so it's not operating at full capacity? What if I use 4 30ohm resistors in series, would that still be a combined 120ohms?
Ooh, that's a good idea putting the thermal cutoff switch in there, Ken. Definitely getting a couple of those. And the printer bed heater is a lot like what I was imagining I wanted. Thanks!

Acercanto

Acercanto

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Feb 15, 2021, 12:45:24 AM2/15/21
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So I'll probably still go with the digital thermostat, but for argument's sake, would it be feasible to just use one of these to do the switching, since the exact temperature isn't critical, and they're rated for 100k cycles?

Just to sanity check my math, I'm going with 4x50ohm/10W resistors in parallel (should be 12.5ohms total resistance), which should give me 14.58 watts TDP (total dissipated power). I'll have a little 150ma fan blowing over them, and with the thermostat too, we should be looking at well under 2 amps total draw when heating.
Should I put the thermal switch upstream of the thermostat, or to just cut off the resistors and not the fan?

Thanks,
Acercanto

Ross Freeman

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Feb 15, 2021, 8:21:49 AM2/15/21
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I like the temperature controller above. Also the digital thermostat. Great stuff! It's nice to have the digital readout of the first. Seems like it could let you know pretty quick if there's a problem. For heating my 2P4S pack I'm planning to use these:


I purchased two sets and will be running them off of my 12v supply so the wattage will be decreased. I may even pair them in series if they put enough heat out. I plan to attached them to some squares of sheet aluminum and place them around the sides of the pack. That with the insulation all around should be enough to keep things warm. The aluminum will spread the heat out a bit and protect the heaters themselves from damage. There isn't a rush to heat things here, you know? Low and slow... like a crock pot. We're just keeping the ambient inside the insulated box at temperatures well above freezing. We don't need heaters surrounding every inch of the pack as long as we have a good insulated box.

My two cents. Love the thread! Regards!

Ross

michael clark

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Feb 15, 2021, 12:00:44 PM2/15/21
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to Ross, electrodacus
I use a 250-watt heat lamp bulb to heat my 8-foot x 8-foot x 8-foot (512 cubic feet) insulated solar power shed. today February 15th, 2021; it is 25 degrees below zero Fahrenheit. I do not have any thermostat control at this time. it is 57 degrees inside right now so this is the ultimate test for cold temperature. I think insulating your battery space would be the 1st thing to do and then use an incandescent light bulb is likely the most inexpensive option. for 64 cells I would need 64 heating mats at 4 dollars each = 256 dollars plus thermostatic controllers. the light bulb in the well pit in the winter to keep things from freezing is still the most economical method (that is how we kept the well from freezing when I was growing up). the heat lamp bulb is running on the solar electricity generated from the PV panels. which charges a 16 cell Lishen 272Ah per cell battery 2P8S configuration at 24 volts. presently 4 260 watt panels facing south connected to 2 Electrodacus DSSR20's via the Electrodacus SBMS0. I have the 24-volt inverter running 24/7 for the light bulb etc as I build assemble inside the solar power shed. I also use some dc directly connected to the battery lights some but only the heat lamp bulb and 3 dc direct connected light bulbs used overnight to keep it at a balmy 57 degrees Fahrenheit even though it is 25 degrees below zero outside. just a thought on economics. cheers from South Dakota USA

Dacian Todea

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Feb 15, 2021, 12:13:10 PM2/15/21
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Acercanto,

Yes a 4W resistor will be capable of max 4W of heat dissipation but if is 120 Ohm and connected to a 13.5V battery it will only dissipate 1.52W
Yes the 4x50 Ohm resistors in parallel should be a good choice just make sure you have them mounted maybe on a good side heatsink (or aluminium plate) and have a fan working.
Maybe have a 35C thermostat on the heatsink and a 15C one on the battery then an electronic one set at +10C also connected to battery thus the electronic one will do most of the job and if that fails for some reason the bimetallic ones can take over.

Ross,

Yes a properly insulated box is the key and then you just need to cover the box loss at the specific max delta in temperature.  30W will be way to much for a single pad. I will say 5 to max 7W for a pad glued on an aluminium plate.

Michael,

an incandescent light bulb is also a good option especially if used at lower power than rated (else they do not last to many hours).
So say for a 12V battery use 4x 20W automotive light bulbs as groups of two in series (6V per bulb) that will be around 5W of each bulb 20W in total and maybe a small fan also used at lower voltage so it lasts longer to circulate the air in the box and make temperature much more uniform bulbs may need to be in a steel or aluminium box so there is no direct heat from the light heating some part of the cells.
If heat was all you needed in that shed then thermal storage will have been a much better option instead of the expensive LiFePO4

Ross Freeman

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Feb 15, 2021, 1:01:51 PM2/15/21
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Dacian,

Yes... 30 watt pads at 24v... 15 watts at 12v... 7-1/2 watts each when series paired 2s at 12volts. Aluminum flashing should be fine. Anything to carry some heat away from the heaters.

Ross

Dacian Todea

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Feb 15, 2021, 1:24:11 PM2/15/21
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Ross,

If the pad is 30W at 24V that means 19.2Ohm and so at 12V it will be just 7.5W so a quarter of the power at half the voltage.

Ross Freeman

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Feb 15, 2021, 1:28:58 PM2/15/21
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Ha... you know Dacian, I was doing the math and kept getting 7.5 watts. Stupid higher math! Can't seem to get my planets lined up today!  :-)

michael clark

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Feb 15, 2021, 2:22:49 PM2/15/21
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the solar power shed is to put inverters, SBMS0, DSSR20's, mppt, ac load center, LiFePO4 batteries, etc in. and keep it safe from children and other snoopy people out. it has a locked door and workbenches on 3 sides. the light bulb idea is only another inexpensive idea. the 250-watt heat lamp bulb has been running almost constantly for 3 months without fail. I think a smaller bulb would have worked most of the time though and still working toward a thermostatic control to turn on and off or maybe a simple timer. insulation is the key to keep the batteries from freezing then very little supplemental heat will be needed. my primary concern was keeping the batteries from freezing and keeping people from contact with 544 amps at 24-volt batteries. i also have 48-volt batteries in it. again the reason to heat the solar power shed is to keep batteries from freezing.
 I was just looking at the heat mat prices and thinking how expensive that is but then again I bought 64 272Ah Lishen LiFePO4cells specifically to build solar pv storage and hope for a long lifespan with them. just an idea for some it may work for others a more expensive complicated way will work better. cheers, all..
i have 24-volt water heater elements to try the diversion heating but have not got to that part yet. slow but sure.....

Acercanto

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Feb 15, 2021, 9:20:50 PM2/15/21
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Holy crap, Michael, a single (albeit large) light bulb keeping your shed warm in -25F? How did you insulate that thing? I'm working on plans to build a workshop/outbuilding/hobby place out back, and want to build it really well. We don't get much below the teens F here in SW Virginia, but a tight building makes an efficient building. I had considered using an off-the-shelf 120vAC heating mat with thermostat, but didn't want to have to run the inverter all the time just for that.

I'm glad I started this thread. I was worried it was just me with this question, but there's been some really good discussion! Thanks!
Acercanto
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