DIY Insulated Battery Box

[offgrid-curious]

In my off grid cottage upgrade show and tell post, I included a picture of the finished battery that I built. I wanted to go into a bit more detail here.

I have a few constraints at my cottage that made finding an off the shelf battery difficult. I want to keep my system operational over the winter - the cottage is not insulated and temperatures can get down to -30C outside. I also don't have a lot of space in the closet where the batteries are stored. With all of the great resources on this forum (special shoutout to Horsefly) on DIY battery packs, I decided to build my own.



I was initially going to build a wooden box, similar to what Horsefly did in his insulated battery build. However, I have a friend that owns a metal fabrication company, and he offered to help me build a custom battery box. The box is made from mostly 12 gauge steel, weighs over 150lbs and is extremely sturdy. It has 3 inches of mineral wool insulation on all sides, top and bottom. The battery box will likely outlive me. Inside the metal box, I have a smaller inner wooden box on which I've mounted all of the components (BMS, class-T fuse, etc).



The box has an aluminum plate at the bottom, to which I attached some heating wire that I bought off of Aliexpress. I can't recall now, but I believe it's about 3-4m of wire. It has a total resistance of 75ohms, which at 55v works out to about 40w of heating. The heating wire is fixed to the underside of the plate using kapton tape.



The cells rest directly on top of the plate. The heating wire is connected to a custom controller that I built that's based on a NTE-DTO60 thermistor. More details here. I really like the simplicity of just using a thermostat, as opposed to more complicated components. I tested it by putting it in the freezer for 15 minutes and it seemed to work - the real test will be over the winter!

I'm using a JK BMS, along with one of the "Peter boards" from Andy's Off Grid Garage. I'm really happy with this, as it integrates beautifully with my Victron system. I bulk charge at 55.2, let it absorb for an hour, then float at 53.6 for 6 hours.

One of the last things I added was an aerosol fire extinguisher that I also got from Aliexpress. I mounted the fire extinguisher on the underside of the inner box lid, so it's in the same area/compartment as the cells. It has a cable that supposedly will activate based on temperature and release the extinguishing gas.



Overall, I'm really happy with how the whole project turned out. It definitely wasn't cheap, although I'm sure I could probably build it for half the price now knowing what I know now (and now that I have all the tools lol). With the horror stories I've read on the forum around DIY batteries, I really tried to build something with safety in mind. Especially because this battery is in an off-grid location nearly two hours away. I feel like in the unlikely event that there was a fire, having a solid metal box, along with the insulation and fire extinguisher will go along way in preventing it from spreading (hoping to not test that theory ).

 

[yodamota]

looks awesome!!

care to share what it cost? I ask because i am about to use an old DC sundanzer freezer to put the same amount of cells in, and it cost zilch. Yea no 3" of mineral wool. But it has 4 1/2" of some kind of insulation.

custom often = $$$

 

[offgrid-curious]

custom often = $$$

That is the truth. I got a discounted rate but it was still over $1500, then add another couple hundred for freight shipping. If I were to order another one I expect it would be cheaper because I wouldn’t need to go through the design process again. I essentially gave the dimensions and requirements to my friend and iterated on the detailed spec with his designer over a few days.

 

[DIYrich]

For version 2, you might want to consider floor heating products. They are designed to bear weight, and the wire channel is non-conductive.

8006GRY11-ST - SunTouch

SunTouch HeatMatrix Uncoupling Membrane Mat for 10 Square Foot Heating Wire Installation

www.suntouch.com

 

[Stircrazy]

A huge power ser (as in less power for heating) would be to pull the mineral wool, put a 1" closed cell foam and reinstall the mineral wool on the inside of that to protect it. 3" of mineral wool on its own won't be anywhere close to the insulation value of 1" of closed cell foam.

 

[yodamota]

A huge power ser (as in less power for heating) would be to pull the mineral wool, put a 1" closed cell foam and reinstall the mineral wool on the inside of that to protect it. 3" of mineral wool on its own won't be anywhere close to the insulation value of 1" of closed cell foam.

not sure I follow this line of thinking... care to elaborate on what you mean?
is the 1" closed cell an "air barrier"?

 

[Stircrazy]

not sure I follow this line of thinking... care to elaborate on what you mean?
is the 1" closed cell an "air barrier"?

ya 1" of closed cell foam will not let any air transfer if you have leaks in your box, and is a much better insulation value than mineral wool for the same thickness. so closed cell is normally up around r7-8/inch where mineral wool is r3ish but that's only if you have absolutely no air flow, which is why we add vapour barriers to our house when we use fiberglass pink or mineral wool. so by adding that 1" of foam board you will increase the r value and stop air transfer. you built a wood case for your batteries so I would even just go 3" of closed cell foam board. that would make it so the internal heat of the batteries working alone would probably keep that heated in the winter or if not using it the power it takes to keep it warm would be very minimal.

 

[yodamota]

ya 1" of closed cell foam will not let any air transfer if you have leaks in your box, and is a much better insulation value than mineral wool for the same thickness. so closed cell is normally up around r7-8/inch where mineral wool is r3ish but that's only if you have absolutely no air flow, which is why we add vapour barriers to our house when we use fiberglass pink or mineral wool. so by adding that 1" of foam board you will increase the r value and stop air transfer. you built a wood case for your batteries so I would even just go 3" of closed cell foam board. that would make it so the internal heat of the batteries working alone would probably keep that heated in the winter or if not using it the power it takes to keep it warm would be very minimal.

Being that this battery is enclosed in a metal box(the outside). wouldn't that be the air barrier?

 

[Stircrazy]

Being that this battery is enclosed in a metal box(the outside). wouldn't that be the air barrier?

only if you have seals on any opening parts and the seams are all solid and not just screwed together. don't get me wrong, you would have to do that with the foam insulation also, but foam is a much more efficient insulation.. which will keep the electrical requirment for the heating to a minimum.

 

[ConnerLabs]

Are these disc type thermostats even rated to switch DC?

 

[offgrid-curious]

Are these disc type thermostats even rated to switch DC?

It’s not totally clear to me from the spec sheet, to be honest. I did test it a fair bit with my bench power supply and it seemed to work just fine. What makes me less concerned is that it’s not having to switch with a lot of current, only 0.7 amps or so.

 

[Chispas]

Thanks for posting this, great ideas put in to practice, inspiring me to think about LFP batteries, although we don't get anywhere near -30

 

[offgrid-curious]

It has been interesting watching the temperature fluctuation as we head into the winter season:



So far we've had a few days where the outside temperature has dipped below freezing, but the battery box hasn't yet gone below 7C. As far as I can tell, the heater has yet to have enabled (it should kick in at 5C). The forecast low temperatures for the next week are still above freezing, so I doubt the heater will kick in just yet. I'll post another update once it gets a bit colder.

 

[Tunnerus]

Great inspiration! I am already planning for what kind of battery box I will build. Foam insulation for sure, and lots of it as it does not cost any energy! Am thinking 15 cm (6”). Wooden box, but maybe with the cells lying down, stacked on top of each other and bus bars out to one side, which would be the door rather than the lid. For any heating though I like the idea of heating pads…

 

[opoliovolcx]

any update on if it worked in the winter?

 

[offgrid-curious]

any update on if it worked in the winter?

I'd say the official answer is "I don't know yet". Unfortunately I didn't take into account that my panels would be covered in snow throughout most of the winter so I ended up shutting the whole system down at the beginning of December. While the base load of the system is relatively low, I determined that I wouldn't be able to generate enough power to keep it running.

Once most of the snow on the roof had melted I turned the system back on again in March. The temperature delta wasn't huge between the battery and inside/ouside, but the heater had no trouble keeping up:







I'm thinking of a couple of options for next winter to keep the system running:

- Mounting a couple of south facing panels vertically where they can't be blocked by snow.
- Setting up an auto-start generator that is connected to my 120 gallon propane tank.

None of this is truly necessary, since I don't really plan on using the cottage in the winters - it's more that I'm interested in solving the problem

 

[Tunnerus]

Agree you would need a couple of vertical panels that would not risk being snowed in. Those vertical ones would need to be able to climb over the solar charger’s startup voltage, right?

 

[HRTKD]

The temperature delta wasn't huge between the battery and inside/ouside, but the heater had no trouble keeping up:

I run warming pads on my 48v battery. The amount of power required by the warming system is barely a blip on the graph, as long as I get some sun. My panels are nearly horizontal on my RV but even with a few inches of snow in the panels I still get enough watts to account for the warming systems load. I run the inverter in the winter to keep the batter in my ATV charged.

If I get more than a few inches of snow, I make a trip to the storage lot to clear the snow off of the panels.