Battery vault-what are the considerations?

[Bluedog225]

I need some help identifying issues with this idea and brainstorming solutions.

I’ve never felt like such a survivalist (chuckle). But I’m seriously thinking of putting my off-grid batteries underground. I can’t get ok with putting 10 grand or more into a battery bank and then baking it in the summer heat. The impact on capacity and service life is too great.

Around here (central Texas) we fairly often have many, many days over 100F. We had 71 consecutive over 100 F days in 2011. It was brutal. Everything heats up. There’s no thermal mass that is cool at night that you can rely on.

Relying on a mini split to keep the battery compartment cool would work, but a fairly expensive option and requires maintenance and replacement every 5-10 years (?). Doesn’t seem like a great off-grid solution.

But pretty reliably (depending on soil moisture, depth, ground cover, etc), the soil temp a couple of feet down is greatly moderated. 74F year round give or take. Ideal for LiFePo. In theory, with low charge/discharge rates, and proper charge limits, they could last a decade.

So…..I’ve been looking around for the best way to do this. Digging under the slab is possible. A lot of work but possible. Or excavating a hole and then covering it with foam. It would be better if the area were shaded. Maybe a hole and then putting a panel covered shipping container on top with inverter inside. Nothing extreme required. E.g.: 4’x4’x4’.

The biggest threat underground is flooding. Either surface water or ground water. I guess second would be the humidity in general degrading electronics. And a close second would be heat buildup in a closed space with high charge and discharge rates.

It would be nice to place the batteries on a plastic pallet or something that could be lifted out with an engine hoist if they needed work.

One idea that may have some merit is building the battery in one of those heavy plastic food barrels with a watertight lid. Ports for the wires could be installed. Four to a pallet. Lower it into the hole and cover with 12 inches of blue foam sheets and a thin layer of soil.

Any thoughts about making this work (or whether it’s a terrible idea) appreciated.

 

[Rednecktek]

I don't know how large your batteries are, but have you thought about a cooler and a fan? Well insulated, cheap, no flooding risk.

Another option that might work if you can feed the power for it would be a Electrical Cabinet Cooler that would keep the battery box cooled with a tiny AirCon unit mounted to the box. We use these on the ships to keep the electrical cabinets in the engine room cool.

If you're getting enough sun to cook your batteries, you should have plenty to feed a 300w cooler unit until the temperature comes down.

 

[Bluedog225]

I‘m not sure a fan will help when the ambient air is over 100F. A cooler might buffer the heat. But after many hot days, the box will come up to ambient.

I wasn’t aware of the small Kootronic units. 1000 Btu size is pretty cool. That will need more research.

I toyed with the idea of a diy box with a 12 volt cooler mechanism. Use and engel or similar, open the lid, and make a polyiso board box that separates the hot exhaust air. Still $1,000 and subject to wear, failure, and replacement. But it would work.

Thanks for the info.

 

[JoeHam]

If they fit in a cooler I would just bury that.

Or bury an old chest freezer.

 

[chrisski]

If you’re going to bury these, I’d look at tornado shelters. More of a how are they constructed thing than putting the batteries in.

The shelters I looked at in Oklahoma, where flooding was also a problem, tended to be plastic rather than concrete. Some who had a plastic shelter installed had to go out and water their shelter until the mud settled around their shelter, otherwise it could come floating to the surface on the first good rain.

I remember in San Antonio after a good long rain, digging down only a couple of inches, water would pool.

I also wonder how this 74 year round temp would hold out with the batteries starting to heat up.

 

[Bluedog225]

The batteries heating significantly would be a big consideration.

On using a freezer, I’d imagine it would rust out in a couple of years.

 

[chrisski]

I have had 50 gallon plastic barrels of water sitting in the desert sun for 5 years. The blue barrels. Still holding water. Don’t know how they would be if buried.

I could not bury my batteries like underground. How would I check on them? For me, this would end up being like a root cellar where I could get access to fairly easy.

I am putting a 24 volt lithium pack in an RV in Arizona. I plan on storing the battery for two months unused and outside temps are typically 110+ for several months straight, and highs of 120. I will just suffer the consequences. For me, I feel its less risk than any other solution I cold come up with. I don’t see any significant power use at those temps, just storage.

 

[Brot]

Hi @Bluedog225 , I've been contemplating similar ideas for burying a battery.

I'll use this post to brain dump my half-baked musings on the matter. A note for anyone with the patience to read it: Please don't take any of this as advice. Batteries are dangerous and I am in no way qualified to say what is and isn't safe .

The two main goals for me are:

• "Tying" the battery to the large and moderate temperature thermal mass of the surrounding soil (I vaguely remember a rule of thumb that ~1.5m down, you basically get a constant yearly average temperature, all year round - could be misremembering the correct depth).
• Safety, in case of battery failure. It seems hard to imagine that there would be a better way of containing the heat and toxic gases emitted in case of catastrophic battery failure.

The obvious downsides and concerns I have thought of so far are similar to what you have already identified:

• Having to seal against moisture/water.
• Thermal modelling: Is the rate of heat transfer going to be sufficient to control battery temperature under the given worst case charge/discharge loads?
• Effort/cost of digging a hole.
• Ease of access. If a deeper hole is necessary, it might also be necessary to either completely "back fill" and bury the battery, or else to have more complex insulation against higher soil strata and the atmosphere.
• Thicker wiring required for connection to battery - at least for the part which is used to connect between battery and surface.
• What depth to go for? This will determine the worst case "ambient" temperature for the battery vessel. 1.5m would be tricky - less might be sufficient.
• What material and size of container to use, taking into account:
  • heat transfer
  • corrosion resistance
  • water/moisture ingress
  • battery space requirements (probably not a big deal: I expect heat transfer to be the limiting factor on size - at least if using DIY battery)
• Since this seems like somewhat of an "out there" approach, you probably won't get any commercial battery manufacturers or installers willing to offer warranty and legislators might have something to say about whether you're allowed to do this, both from a pure "chemical" and more specific battery perspective. Could be a bit of a fly in an otherwise appealing looking ointment.
• The fact that (afaik) this dosn't seem to be done for utility scale batteries, could also be a red flag for cost/benefit and general practicality, at least.

I think the road to answering the thermal questions would look something like this:

1. Determine the battery's worst case heat losses, based on worst case charge/discharge current and internal resistance (also use worst case here, to account for increase in internal resistance at the hottest temperature acceptable for the battery to reach and increases due to aging at the target service life) plus any wiring resistance within the battery's thermal environment.
2. Figure out the right heat transfer model from the battery to the ultimate heat sink (surrounding soil). This could be tricky. It will depend on various surface areas, materials and thicknesses involved. I wuold guess there should be enough research results and standards publicly available to figure this out though.
3. Figure out the worst case temperature difference between battery cells and surrounding soil, based on worst case thermal power and Kelvin/Watt thermal resistance derived from the thermal model.
4. The constraints could potentially be relaxed a little, if distinguishing between worst _continuous_ case current flow from worst case "burst" current. In this case, the thermal mass of the battery and current temperature, relative to maximum allowed, would determine how long burst rates can be sustained. Temperature monitoring could be used to cut off power when limits are exceeded, if the implementation is suitably trustworthy.

In the setup I'm imagining (thin walled galvanised steel box with surface area larger than the cell/battery surface area approximately in the same ratio as the thermal resistance cells->air to the thermal resistance air->soil), hopefully a good approximation for the thermal model would be to ignore the thermal resistance of the vessel (thin steel). The battery to soil part through the enclosed volume of air definitely still needs a lot more thought though. Could get messy, especially if convection needs to be called upon (and thus modelled) to achieve practical cooling.

Other considerations:

• I'd definitely add thermal and moisture ingress monitoring, since that will be very cheap compared to the battery . A few bucks for an ESP plus temp/humidity and maybe water sensor together with a few lines of YAML in EspHome would hopefully do the trick . That should be able to independently monitor and isolate the battery (with an appropriate SSR) plus it integrates with Home Assistant as a bonus. Maybe multiple temperature sensors, to confirm the validity of the thermal model.
• For the vessel, I'm tending to a galvanised steel chest (e.g. a cheap "tool box"), which is hopefully sufficiently corrosion resistant (more research necessary and may depend on local soil type/pH), should be negligible for thermal resistance and hopefully easy enough to seal reliably with some neutral cure silicon.
• For managing minor moisture build up, a small reservoir of silica gel desiccant might be a worthwhile item to include in the battery box, but haven't thought this through. Could also lead to false non-detection or late detection of serious water ingress and would be a bit of a bother if it needed regular replacement. Doing a really good job of sealing the box might be better.
• Wiring access: Probably plumb a conduit from the surface into the box for battery connection and sensor cables. All sealed with neutral cure silicon. What type of conduit does one use for this? No idea yet.

I hope you find these thoughts interesting . I'd be interested to hear if you've developed the concept further yourself!

 

[Bluedog225]

Good thoughts.

As I’ve thought it over, a buried and sealed formal electrical vault would be pretty expensive done correctly. For cheaper, something like an old fashioned root cellar comes to mind.

 

[Hravn]

Hi @Bluedog225 , I've been contemplating similar ideas for burying a battery.

I'll use this post to brain dump my half-baked musings on the matter. A note for anyone with the patience to read it: Please don't take any of this as advice. Batteries are dangerous and I am in no way qualified to say what is and isn't safe .

The two main goals for me are:

• "Tying" the battery to the large and moderate temperature thermal mass of the surrounding soil (I vaguely remember a rule of thumb that ~1.5m down, you basically get a constant yearly average temperature, all year round - could be misremembering the correct depth).
• Safety, in case of battery failure. It seems hard to imagine that there would be a better way of containing the heat and toxic gases emitted in case of catastrophic battery failure.

The obvious downsides and concerns I have thought of so far are similar to what you have already identified:

• Having to seal against moisture/water.
• Thermal modelling: Is the rate of heat transfer going to be sufficient to control battery temperature under the given worst case charge/discharge loads?
• Effort/cost of digging a hole.
• Ease of access. If a deeper hole is necessary, it might also be necessary to either completely "back fill" and bury the battery, or else to have more complex insulation against higher soil strata and the atmosphere.
• Thicker wiring required for connection to battery - at least for the part which is used to connect between battery and surface.
• What depth to go for? This will determine the worst case "ambient" temperature for the battery vessel. 1.5m would be tricky - less might be sufficient.
• What material and size of container to use, taking into account:
  • heat transfer
  • corrosion resistance
  • water/moisture ingress
  • battery space requirements (probably not a big deal: I expect heat transfer to be the limiting factor on size - at least if using DIY battery)
• Since this seems like somewhat of an "out there" approach, you probably won't get any commercial battery manufacturers or installers willing to offer warranty and legislators might have something to say about whether you're allowed to do this, both from a pure "chemical" and more specific battery perspective. Could be a bit of a fly in an otherwise appealing looking ointment.
• The fact that (afaik) this dosn't seem to be done for utility scale batteries, could also be a red flag for cost/benefit and general practicality, at least.

I think the road to answering the thermal questions would look something like this:

1. Determine the battery's worst case heat losses, based on worst case charge/discharge current and internal resistance (also use worst case here, to account for increase in internal resistance at the hottest temperature acceptable for the battery to reach and increases due to aging at the target service life) plus any wiring resistance within the battery's thermal environment.
2. Figure out the right heat transfer model from the battery to the ultimate heat sink (surrounding soil). This could be tricky. It will depend on various surface areas, materials and thicknesses involved. I wuold guess there should be enough research results and standards publicly available to figure this out though.
3. Figure out the worst case temperature difference between battery cells and surrounding soil, based on worst case thermal power and Kelvin/Watt thermal resistance derived from the thermal model.
4. The constraints could potentially be relaxed a little, if distinguishing between worst _continuous_ case current flow from worst case "burst" current. In this case, the thermal mass of the battery and current temperature, relative to maximum allowed, would determine how long burst rates can be sustained. Temperature monitoring could be used to cut off power when limits are exceeded, if the implementation is suitably trustworthy.

In the setup I'm imagining (thin walled galvanised steel box with surface area larger than the cell/battery surface area approximately in the same ratio as the thermal resistance cells->air to the thermal resistance air->soil), hopefully a good approximation for the thermal model would be to ignore the thermal resistance of the vessel (thin steel). The battery to soil part through the enclosed volume of air definitely still needs a lot more thought though. Could get messy, especially if convection needs to be called upon (and thus modelled) to achieve practical cooling.

Other considerations:

• I'd definitely add thermal and moisture ingress monitoring, since that will be very cheap compared to the battery . A few bucks for an ESP plus temp/humidity and maybe water sensor together with a few lines of YAML in EspHome would hopefully do the trick . That should be able to independently monitor and isolate the battery (with an appropriate SSR) plus it integrates with Home Assistant as a bonus. Maybe multiple temperature sensors, to confirm the validity of the thermal model.
• For the vessel, I'm tending to a galvanised steel chest (e.g. a cheap "tool box"), which is hopefully sufficiently corrosion resistant (more research necessary and may depend on local soil type/pH), should be negligible for thermal resistance and hopefully easy enough to seal reliably with some neutral cure silicon.
• For managing minor moisture build up, a small reservoir of silica gel desiccant might be a worthwhile item to include in the battery box, but haven't thought this through. Could also lead to false non-detection or late detection of serious water ingress and would be a bit of a bother if it needed regular replacement. Doing a really good job of sealing the box might be better.
• Wiring access: Probably plumb a conduit from the surface into the box for battery connection and sensor cables. All sealed with neutral cure silicon. What type of conduit does one use for this? No idea yet.

I hope you find these thoughts interesting . I'd be interested to hear if you've developed the concept further yourself!

What time frame do you expect for a service life? Galvanized tin/iron/steel will corrode, especially with cyclic exposure to moisture. Stainless, some plastics/pvc/ etc. or glass ( tin bronze as well, but …) would be much longer lived. A root cellar style set up ( for access) with the batteries in sealed containers in cubbies or shelves and contact with the sidewalls would work. Much bigger than a hole in the ground, but maybe useful for others things as well.

 

[MarkSolar]

Buried gas tanks sometimes get forced up out of the ground around here if the water table gets too high. When I worked for a construction company, a few times a year we'd go to a gas station and find a tank that looked like a submarine coming up out of the ocean. We'd dig it out and replace it with the proper ballast, backfill and drainage. I have no idea what the water table or soil is like in Texas, but I'd give a think to how you keep something from floating once you bury it.

 

[Bluedog225]

Agreed. Same things with pools. Under the slab is an option.

 

[Zil]

Extreme high temperatures are a problem in vehicles as well. A small bank is easily cooled. A larger bank could be built with air con using wells as heat sink. Enough solar would be needed and could end expensive. Needs be measured against the cost of the battery bank being protected.
I have successfully cooled an insulated battery box using air cooled peltzer devices. 400ah in a rv van.

 

[Bluedog225]

I’m looking for a secop compressor and cold plate. Stand alone, not part of an chest cooler. I could make a custom box for the cells.

 

[Brot]

Thanks for the feedback .

Fortunately, ground water isn't much of a concern for me, personally: My soil type is sand and the water table is many meters down.

There is a wide variety of options to consider in the active cooling direction, of course, but I'm hoping for better long term cost and reliability with passive cooling and I'm also keen on the added safety of having the battery contained underground, in case of catastrophic failure, so I'll stick with trying to exhaust all avenues down the passive route first .

What time frame do you expect for a service life? Galvanized tin/iron/steel will corrode, especially with cyclic exposure to moisture. Stainless, some plastics/pvc/ etc. or glass ( tin bronze as well, but …) would be much longer lived. A root cellar style set up ( for access) with the batteries in sealed containers in cubbies or shelves and contact with the sidewalls would work. Much bigger than a hole in the ground, but maybe useful for others things as well.

I was hoping for something on at least the same time scale as the battery itself (which I'm dreaming will be 10 years+). I stumbled across a source which was measuring the lifespan of galvanised steel for different soil types in the time scale of decades. Based on your response and a bit more research, it looks like years - and potentially a rather small number of them - are a much safer bet (especially with my steel lacking any kind of more precise specification than "galvanised"). Darn.

I would love to have a cellar, but unfortunately in my (Australian) part of the world, they don't appear to be a thing and will thus probably have extra cost and red tape barriers around them... Much like other "modern marvels of building technology", such as double glazing and "windows and doors which actually seal".

So back to the drawing board on materials, I guess. Reviewing the options:

• Stainless steel: Would pitting corrosion not be a concern? My soil type is alcaline sand. I don't know much about corrosion chemistry. This source would probably answer my questions if I manage to get my head around it.
• Plastics:
  • Less good for heat transfer, but that might not matter and probably the cost and chemical inertness would make them the winner anyway.
  • PVC: Sounds good. I wonder if there is a type of product that effectively already is a cheap and sizeable PVC box. Maybe some sort of plumbing/drainage products could be suitable... Or maybe a plastic tank. Hmm...
  • Other plastics: Any specific ones come to mind which might be cheap, readily available and stable in moist alcaline environments? I have a passable selection of PVC, acrylic and PC sheets somewhat semi-available in my part of the world. Raw materials are often tricky to find, unfortunately...
• Glass: I had thought of using an old aquarium before. They don't seem to be super cheap though and a bit unwieldy to handle (not that the latter is a long term concern). Maybe I could find some cheap second hand glass and glue it up with silicon. Plumbing holes through for wiring sounds like it could be difficult - I've never really worked with glass. Maybe wires could just be run through a wider silicon seal at an edge between glass panels. Silicon sealed glass would presumably have the longest service life, by far.
• Maybe a coating on the box for corrosion protection is the answer. Bituminous paint seems to be cheap. Maybe that on a cheap galvanised box would do the trick? Not that it should need to be galvanised at that point.

 

[Hravn]

Thanks for the feedback .

Fortunately, ground water isn't much of a concern for me, personally: My soil type is sand and the water table is many meters down.

There is a wide variety of options to consider in the active cooling direction, of course, but I'm hoping for better long term cost and reliability with passive cooling and I'm also keen on the added safety of having the battery contained underground, in case of catastrophic failure, so I'll stick with trying to exhaust all avenues down the passive route first .


I was hoping for something on at least the same time scale as the battery itself (which I'm dreaming will be 10 years+). I stumbled across a source which was measuring the lifespan of galvanised steel for different soil types in the time scale of decades. Based on your response and a bit more research, it looks like years - and potentially a rather small number of them - are a much safer bet (especially with my steel lacking any kind of more precise specification than "galvanised"). Darn.

I would love to have a cellar, but unfortunately in my (Australian) part of the world, they don't appear to be a thing and will thus probably have extra cost and red tape barriers around them... Much like other "modern marvels of building technology", such as double glazing and "windows and doors which actually seal".

So back to the drawing board on materials, I guess. Reviewing the options:

• Stainless steel: Would pitting corrosion not be a concern? My soil type is alcaline sand. I don't know much about corrosion chemistry. This source would probably answer my questions if I manage to get my head around it.
• Plastics:
  • Less good for heat transfer, but that might not matter and probably the cost and chemical inertness would make them the winner anyway.
  • PVC: Sounds good. I wonder if there is a type of product that effectively already is a cheap and sizeable PVC box. Maybe some sort of plumbing/drainage products could be suitable... Or maybe a plastic tank. Hmm...
  • Other plastics: Any specific ones come to mind which might be cheap, readily available and stable in moist alcaline environments? I have a passable selection of PVC, acrylic and PC sheets somewhat semi-available in my part of the world. Raw materials are often tricky to find, unfortunately...
• Glass: I had thought of using an old aquarium before. They don't seem to be super cheap though and a bit unwieldy to handle (not that the latter is a long term concern). Maybe I could find some cheap second hand glass and glue it up with silicon. Plumbing holes through for wiring sounds like it could be difficult - I've never really worked with glass. Maybe wires could just be run through a wider silicon seal at an edge between glass panels. Silicon sealed glass would presumably have the longest service life, by far.
• Maybe a coating on the box for corrosion protection is the answer. Bituminous paint seems to be cheap. Maybe that on a cheap galvanised box would do the trick? Not that it should need to be galvanised at that point.

in the us at least, above and below ground water tanks made of various plastics would seem to fit the bill, widely available and a huge variety of sizes. I would avoid iron, but stainless steel, especially marine grade (316) or so isn’t going to be problem for decades. Not easy to work, but you might find existing boxes that could be fine. Not familiar with bitumen paint coatings, but they sound flammable?

 

[Brot]

in the us at least, above and below ground water tanks made of various plastics would seem to fit the bill, widely available and a huge variety of sizes. I would avoid iron, but stainless steel, especially marine grade (316) or so isn’t going to be problem for decades. Not easy to work, but you might find existing boxes that could be fine. Not familiar with bitumen paint coatings, but they sound flammable?

Thanks Hravn

Plastic tanks sound like a good idea. Something in a suitable shape and size and perhaps even with a removable lid, suitable for service access sounds very plausible. I'll have to scrounge around for what's available around these parts .

Bitumen paint may well be flammable. Probably no more flammable than plastic? Regardless, since it would just be a thin layer, sandwiched between steel and soil, I doubt it would add much to the hazard of the battery itself .

 

[Zwy]

Do this instead in a small insulated box. You could even put a plate on top of the radiator and install your batteries on top of it, you don't need a fan. Power draw is minimal.

 

[paul21625]

I don't know how large your batteries are, but have you thought about a cooler and a fan? Well insulated, cheap, no flooding risk.

Another option that might work if you can feed the power for it would be a Electrical Cabinet Cooler that would keep the battery box cooled with a tiny AirCon unit mounted to the box. We use these on the ships to keep the electrical cabinets in the engine room cool.

If you're getting enough sun to cook your batteries, you should have plenty to feed a 300w cooler unit until the temperature comes down.

 

[MarkSolar]

So, I acquired this well insulated cooler for housing my 12 floating cell 200AH batteries outside of my house.. I will be reinforcing the racks to accommodate the extra weight and providing ventilation and heating. But then I started wondering about what the electrical inspector would have to say about this arrangement. The cooler is UL listed, but not as a battery wall. Any thoughts about electrical code regarding battery storage? Thanks!View attachment 113162

Very cool. Assuming your batteries don't need to be vented, seems like a great idea to me. Which means the inspector probably won't like it. You might think about putting the batteries on something simple for the inspection, then installing your cooler after it gets signed off.