Custom Wood Battery Rack for 16 x Pylontech US5000 Off Grid

[AlaskanNoob]

So the 8 x Pylontech US5000 we currently have stored in our uninsulated solar shed are not braving Alaska winters well. I wasn't sure how much they would heat up with charging and discharging, turns out not much. So when they get below freezing, it throws a wrench into the system. Fortunately, I built the shed on top of a root cellar (made from a buried 12' diameter culvert) so this summer I'm going to move the batteries down there which should keep them above freezing even when it gets down to -5F or so.

But, just in case, I'm going to build an insulated cabinet for them. I have no meaningful woodworking experience, but I want to get this right and I'm going to build it for 16 batteries since we're planning on doubling them.

Not shown is a plywood front and back. Front will open up in some way for access. Both front and back plywood will share four 12V heating pads. Also not shown is a base I need to construct for this thing to sit on, that will keep it about 2 feet off the concrete floor of the cellar and will keep the batteries as close to the root cellar ceiling (floor of the solar shed) to minimize wire lengths up to the bus bar in the solar shed.

Shown on the top of each battery (screwed into the shelf above it) is a t-fuse holder. And the center 8 batteries will also have a "bridge" where I'll take the small factory power cables and connect them to thicker cables (probably 4/0) so the extra battery length doesn't provide too much resistance.

My three questions for the generous in the group:

A) should I put a vapor barrier inside the cabinet since the air will be humid, and the battery compartment air will be much warmer than the outside air? Or should I put the vapor barrier on the outside of the cabinet? For homes, the vapor barrier goes inside the home, but I want to ensure any condensation is outside the battery cabinet.

B) each shelf will hold 352 pounds of batteries -- should I construct this shelf differently for that load? I did a sag calculator and with my planned 2" thick shelves, there is no sag to speak of. Planning on 3" side walls, likely with a metal plate on the outside to connect them together, along with screws through the wood on the sides.

C) should I insulate with XPS foam on the outside, or spray closed cell polyurethane instead (which would also serve as a vapor barrier)?

Many thanks for any words of wisdom!




Also not shown are the holes for the cables from the center two columns going up to the bus bar. The outer columns will just be connected to the interior columns with the factory cables.

 

[Marobel]

I think this is a great solution but you may not need more than the rack you’re building to accomplish your need to keep the temperature above freezing. Are you going to bring the inverter(s) down here too? That will generate pretty good heat for this space. I’m assuming you’ve had your stored vegetables down here over winter. Did they freeze? And for moisture control: how about a heat exchanger device that will take the warm, moist air from below and exchange the heat and drop the moisture out as it passes by the incoming cold, dry air. There are many diy units that seem very much in your wheelhouse. Would love to hear what you come up with and the results.

 

[Sennen]

Personally I'd go metal for fire reasons as well as insect/critter resistance.

Vapor barrier goes on the hot side.

 

[LakeHouse]

should I put a vapor barrier inside the cabinet since the air will be humid, and the battery compartment air will be much warmer than the outside air? Or should I put the vapor barrier on the outside of the cabinet? For homes, the vapor barrier goes inside the home, but I want to ensure any condensation is outside the battery cabinet.

I'm inclined to say that I wouldn't worry about vapor barrier here. This is different from a house in an important way... It's not full of humans who breathe out humid air constantly, and you aren't going to be intentionally adding humidity with something like a humidifier. If the air in the box is warmer than the air outside, the relative humidity in the box will be lower than the relative humidity outside the box, and it will stay that way. But if the location is really that humid, I'd probably look at a small dehumidifier and not worry so much about vapor barrier.
But as Sennen says, vapor barrier goes on the hot side of the insulation. Where you live, and where I live that's the inside. In warmer climates vapor barrier goes outside the insulation.

C) should I insulate with XPS foam

Yes, this would be a good choice for insulation.

 

[CaliSunHarvester]

I too was very concerned about humidity. My AIO from Sungoldpower emits so much heat, humidity is as low as 20%, while outside it pours 3+ inches in a day.

 

[AlaskanNoob]

I think this is a great solution but you may not need more than the rack you’re building to accomplish your need to keep the temperature above freezing. Are you going to bring the inverter(s) down here too? That will generate pretty good heat for this space. I’m assuming you’ve had your stored vegetables down here over winter. Did they freeze? And for moisture control: how about a heat exchanger device that will take the warm, moist air from below and exchange the heat and drop the moisture out as it passes by the incoming cold, dry air. There are many diy units that seem very much in your wheelhouse. Would love to hear what you come up with and the results.

I agree, even without insulation or the 12V heaters the batteries should stay above freezing since they're in the root cellar. I'm anticipating needing to get these batteries up to roughly 60-70F when it's -5F outside though, because the batteries limit what they can take based on temps and our system is designed in the hope of charging with 26KW on a clear winter day. We only get about 3 hours of sun in December, so we want to make sure we can take everything the panels can produce.

We're just putting the batteries in the cellar. Was thinking about moving it all down but it would get a bit crowded and, most importantly, I do not want to ever move the Quattro 15KW inverter ever again. That thing is a beast and I'm amazed I was able to get it on the wall in the first place.

We did have freezing in the root cellar this year when it got really cold, but I'll fix that this summer with some insulation around the top of the culvert.

I might have to look into an air exchanger or dehumidifier, but we'll see. We poured the concrete while it was raining and there were pools of water during the pour. So it's going to be awhile before this concrete cures, hopefully the humidity drops even though it's a root cellar so it might stay pretty humid.

 

[AlaskanNoob]

Personally I'd go metal for fire reasons as well as insect/critter resistance.

Vapor barrier goes on the hot side.

Metal might make more sense, but based on what I've seen, if these batteries went up, there would be no metal that would contain it. 76KWh of lithium would likely not be controllable. But they are LifePo4 and from what I've seen, so long as they are not overcharged they shouldn't be an issue and from what I understand they don't suffer from thermal runaway anyway.

We don't have any termites up here, the wood will be pretty thick, and I doubt voles would chew through it but I'll have a thermal camera monitoring the battery cabinet at all times and will audible if that changes. We have a bunch of food in the cellar and so far the voles that have entered did not go into the culvert (they'd have a tough time getting out of it). They did climb up through a hole into the solar shed though, but a mouse trap fixed that followed by some foaming of the hole.

 

[AlaskanNoob]

I'm inclined to say that I wouldn't worry about vapor barrier here. This is different from a house in an important way... It's not full of humans who breathe out humid air constantly, and you aren't going to be intentionally adding humidity with something like a humidifier. If the air in the box is warmer than the air outside, the relative humidity in the box will be lower than the relative humidity outside the box, and it will stay that way. But if the location is really that humid, I'd probably look at a small dehumidifier and not worry so much about vapor barrier.
But as Sennen says, vapor barrier goes on the hot side of the insulation. Where you live, and where I live that's the inside. In warmer climates vapor barrier goes outside the insulation.

Yes, this would be a good choice for insulation.

Good point on the moisture. The root cellar is high humidity right now because the concrete floor is still curing and it was a particularly wet pour. We've put some water absorber buckets in there but the humidity still stays 80-90% even in the winter. Might have to get a dehumidifier. I'll also have a wireless sensor monitoring temp and humidity inside the cabinet so I'll adjust as needed.

 

[AlaskanNoob]

When we got below 0F we had some freezing in the culvert, but I will fix this in the summer...I have an exposed lip area above the culvert and below the floor of the solar shed that I need to insulate. I'm going to put a ton of XPS insulation on that lip, then spray closed cell foam over that and on the sides of the culvert, and then put XPS insulation on the cellar roof (shed floor).

Hopefully this concrete cures and this humidity drops though...

 

[robbob2112]

This might be good to read before you settle on wood

Shunt snapped and caused a fire🔥

Got a call Sunday from the wife about smoke coming from the garage and fire trucks were already on the way. After they had there fun spraying many cans all over the garage I got to have a look inside. I have 5 rows of shelves with 2 shelves for in each with a battery and then busbar along the...

diysolarforum.com

 

[Brucey]

This might be good to read before you settle on wood

Shunt snapped and caused a fire🔥

Got a call Sunday from the wife about smoke coming from the garage and fire trucks were already on the way. After they had there fun spraying many cans all over the garage I got to have a look inside. I have 5 rows of shelves with 2 shelves for in each with a battery and then busbar along the...

diysolarforum.com

I don't think he's going for particle board shelving with support only around the edges like that construction.

 

[robbob2112]

I don't think he's going for particle board shelving with support only around the edges like that construction.

I agree, I was just poihting out that wood shelves have a downside. Being in an enclosed space might be a good way to prevent a fire either way since it would smother anything that starts in short order if it is even close to airtight.

 

[AlaskanNoob]

I agree, I was just poihting out that wood shelves have a downside. Being in an enclosed space might be a good way to prevent a fire either way since it would smother anything that starts in short order if it is even close to airtight.

That's an interesting thought. If an airtight environment actually prevented fire (which obviously it does), that would make me consider putting an air barrier around the cabinet and maybe some metal on the insides to slow any fire that starts while the oxygen is depleted.

But I think lithium will put off massive heat (even if it's not fire) that may be just as problematic. So I'm not sure it would be worth it in the end. Interesting question though.

 

[CaliSunHarvester]

That's an interesting thought. If an airtight environment actually prevented fire (which obviously it does), that would make me consider putting an air barrier around the cabinet and maybe some metal on the insides to slow any fire that starts while the oxygen is depleted.

But I think lithium will put off massive heat (even if it's not fire) that may be just as problematic. So I'm not sure it would be worth it in the end. Interesting question though.

My batteries are in a wood shelf "rack" and the batteries are warm to the touch, but not hot. Maybe it will be different in July. The heat comes from the inverter. Metal has its downside as well.. it conducts.

 

[marionw]

Typically, a battery fire starts in a single cell inside a larger battery pack. There are three main reasons for a battery to ignite: mechanical harm, such as crushing or penetration when vehicles collide; electrical harm from an external or internal short circuit; or overheating.

Battery short circuits may be caused by faulty external handling or unwanted chemical reactions within the battery cell. When lithium-ion batteries are charged too quickly, chemical reactions can produce very sharp lithium needles called dendrites on the battery’s anode – the electrode with a negative charge. Eventually, they penetrate the separator and reach the other electrode, short-circuiting the battery internally.

Such short circuits heat the battery cell to over 212 F (100 C). The battery’s temperature rises slowly at first and then all at once, spiking to its peak temperature in about one second.

Another factor that makes lithium-ion battery fires challenging to handle is oxygen generation. When the metal oxides in a battery’s cathode, or positively charged electrode, are heated, they decompose and release oxygen gas. Fires need oxygen to burn, so a battery that can create oxygen can sustain a fire.

Because of the electrolyte’s nature, a 20% increase in a lithium-ion battery’s temperature causes some unwanted chemical reactions to occur much faster, which releases excessive heat. This excess heat increases the battery temperature, which in turn speeds up the reactions. The increased battery temperature increases the reaction rate, creating a process called thermal runaway. When this happens, the temperature in a battery can rise from 212 F (100 C) to 1,800 F (1000 C) in a second.

 

[AlaskanNoob]

Typically, a battery fire starts in a single cell inside a larger battery pack. There are three main reasons for a battery to ignite: mechanical harm, such as crushing or penetration when vehicles collide; electrical harm from an external or internal short circuit; or overheating.

Battery short circuits may be caused by faulty external handling or unwanted chemical reactions within the battery cell. When lithium-ion batteries are charged too quickly, chemical reactions can produce very sharp lithium needles called dendrites on the battery’s anode – the electrode with a negative charge. Eventually, they penetrate the separator and reach the other electrode, short-circuiting the battery internally.

Such short circuits heat the battery cell to over 212 F (100 C). The battery’s temperature rises slowly at first and then all at once, spiking to its peak temperature in about one second.

Another factor that makes lithium-ion battery fires challenging to handle is oxygen generation. When the metal oxides in a battery’s cathode, or positively charged electrode, are heated, they decompose and release oxygen gas. Fires need oxygen to burn, so a battery that can create oxygen can sustain a fire.

Because of the electrolyte’s nature, a 20% increase in a lithium-ion battery’s temperature causes some unwanted chemical reactions to occur much faster, which releases excessive heat. This excess heat increases the battery temperature, which in turn speeds up the reactions. The increased battery temperature increases the reaction rate, creating a process called thermal runaway. When this happens, the temperature in a battery can rise from 212 F (100 C) to 1,800 F (1000 C) in a second.

Good to know. I've thought a bit about fire suppression methods, but at the end of the day, I just don't think there is anything affordable. My batteries will be well protected in a root cellar, have the Victron system only charging them according to what the batteries demand since they tell the MPPTs what current and voltage to provide, and at normal temperatures the batteries can take the full 27KW of power that my panels can produce. So I would hope the risk of fire would be low.

I'm also banking on smarter folks saying thermal runaway with LifePo4 is unlikely or impossible.

Can LiFePO4 Batteries Combust in Thermal Runaway Event?

Just to clarify the use of the term "exothermic". Exothermic means more heat (energy) is released than is used to start and sustain the process. It does not mean thermal runaway. A thermal runaway of an NMC cell is clearly exothermic. A camp fire is also exothermic, as is a burning candle...

diysolarforum.com

 

[robbob2112]

If being airtight prevented a fire it would be a time bomb waiting for the first bit of oxygen to flash over. Unless you had a pipe for the firefighter to hook on to and just fill it with water there isn't a good way to prevent a total loss I don't think

I listed what my fire fighter brother-in-law would use in the thread. Seems like it was a class D dry extenguisher.

 

[SparkyJJO]

LFP is not self oxygenating. Doesn't mean it won't burn (because it will) but LFP won't destroy you just for looking at it funny like NMC or such.

 

[Checkthisout]

Fire isn't a worry as much because this particular install is keeping the batteries basically underground in a structure that is separate from the house.

Old concrete form boards work good but I'm a fan of old pallet racking or a new smaller pallet rack like Costco sells.

 

[AlaskanNoob]

If being airtight prevented a fire it would be a time bomb waiting for the first bit of oxygen to flash over. Unless you had a pipe for the firefighter to hook on to and just fill it with water there isn't a good way to prevent a total loss I don't think

I listed what my fire fighter brother-in-law would use in the thread. Seems like it was a class D dry extenguisher.

No firefighters out where I'm at... Thinking we might put some of those balls of fire extinguishing agent in a net above the cabinet so they might do something. And above the batteries the celling/floor will have a couple layers of firebird. I guess something is better than nothing.