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Using septic tank as battery storage vault

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mvonw

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Does anyone have an opinion (preferably based on experience or first hand knowledge) about the practicality/safety of using an underground septic tank as an accessible vault (root cellar) to store a large battery bank to provide a temperature stabilized environment rather than trying to insulate/heat them?
My plan is to burry 10 feet down, and use two risers to provide ladder access hatch and ventilation shafts.

  • Will it work?
  • Will it be dry enough for batteries
  • Will it be safe to enter to service/check on batteries?
  • Am I overlooking anything?
 
I assume this will be a brand new tank and not second hand :ROFLMAO: I know a lot of guys on here like to save a few bucks so just asking :p.

I would not use it mainly because of the chances of it getting flooded. I always think of the worst case scenarios when planning things like this. What happens if you get 2-3 days of heavy rain? Any small leaks from above etc could become a lot of water very quickly. Lastly is safety for yourself. If something goes wrong like a fire or the batteries start to vent toxic gas, do you really want to depend on being able to get up a 10ft ladder to escape in time?

Also I think your going to find that installing it and maintaining it are going to be a real pain when a ladder is involved. I don't see much of a benefit versus a separate electrical room next to your house.
 
Why would you bury it 10 feet down? If you have any experience with these type of containers you should be aware a concrete septic tank is usually buried at 10” to 36” to invert. Mine is 10” in North Central Indiana. The temperature in the tank stays above freezing because the surface area of the bottom heats the vessel. If you do that, be aware of confined space as any fumes will displace oxygen causing it to become inhabitable to humans. Even the top at ground level will prevent the tank from freezing. Good example, all of the city storm drains don’t freeze.
 
if depth is for temperature reasons, perhaps some insulation could work out at a lesser depth?
 
concrete or plastic type? where do you plan to do this ? how far down does your frost reach in given winter? what will be the overall dimension of the vessel?
 
I mean… it’s a container with several tube connections built in… could use for conduit, and ventilation…
As stated, I wouldn’t go 10’down, unless you mean the bottom of the tank, putting the top close to 24” down… not sure.
The molded plastic ones look interesting, with the manhole extensions, I could see it working…
I’d want powered ventilation.
 
Supervstech said:
The molded plastic ones look interesting, with the manhole extensions, I could see it working…
I’d want powered ventilation.
Click to expand...
?
co2 sensor, fire alarm, co sensor suggest
active powered ventilation mandatory

the portal aperture seems rather narrow.

maybe human can fit through, but what about hardware and human? use rope to lower things down?

maintenance seems like... pain...
 
Lots of considerations:
-Read OSHA stories on entering closed space. Very easy way to die. Have procedures and follow it them.
-Water-both leaks and having the thing float out of the ground. Empty concrete pools float out of mud all the time. This can be addressed but needs to be analyzed. Leaks could be handled with a sump pump, and remote moisture alarm.
-Soil temp. There are soil temperature probes for about $75 that get you about 3 feet down. Or look up soil temp maps. It becomes both cooler (edit-or warmer I guess) and more stabile the deeper you go but you are only trying to avoid extremes. So 40F to 85F (or so).
-Another way to do this is to have a gantry crane over the hole (could be diy), and lift the smaller battery vault up for maintenance. Could cover the hole with 24” of foam board then throw cypress mulch over it. Easy. Smaller vault, less digging, less expensive.
-Would the battery management system need to be remote from the batteries due to a)heat or b)moisture? I don’t know how close those things need to be to the batteries. If just burying the batteries, it makes things easier.

Fun idea. Keep us posted!
 
mvonw said:
Does anyone have an opinion (preferably based on experience or first hand knowledge) about the practicality/safety of using an underground septic tank as an accessible vault (root cellar) to store a large battery bank to provide a temperature stabilized environment rather than trying to insulate/heat them?
My plan is to burry 10 feet down, and use two risers to provide ladder access hatch and ventilation shafts.

  • Will it work?
  • Will it be dry enough for batteries
  • Will it be safe to enter to service/check on batteries?
  • Am I overlooking anything?
Click to expand...
I have experience with a 1000gal plastic/ribbed one - and yes they hold water (should keep water out as well), and sturdy / you can drive equipment on top etc. However, some things to think about....
1) Access hatches are not really big enough for a person to get in/out - it's more for cleanout - maybe 18" around, so you'll want to check on that.
2) The 1000gal was maybe 4ft high? Not enough to move around inside. You'll need something BIGer
3) Digging down 10feet + gravel bed for stability = major excavation work. HUGE pile of dirt to deal with. Most of the dirt was left behind - maybe 15yards to dispose of.
4) Be prepared for rocks or sandstone or whatever as 10ft deep is pretty deep.

To my mind, maybe think in terms of partially submerged like this 'well house'.... some clips from @Living Right Debt Free Homestead youtube
1642624434214.png
1642624487763.png
To gain moderate temps but MUCH easier build and access. I interact with my powerwall quite often to check on it, expand it,,,, more than you might be thinking.
 
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10’ down isn’t that far depending on region. Footer, drainage prep, and foundation walls with ~18-24” above grade sill in my locale is often 8’ or more.
But with the small entry I’d not be a fan.

More money, but… I know people with things done in the 1970s (one might have been ‘64?) that are… stealth and temp stable. Intentions were various reasons (or from prior owners so unknown) from unobservable storage to bomb shelters to over-zealous prepping which sorta covers everything: underground poured concrete tunnels from their basements with poured structural ceilings, hidden entrances, escape tunnels, labyrinthine “radiation proof” hallways, and one batcave-esque tunnel entrance from their basement behind a hinged shelving built-in…

It would seem that doing something similar for batteries would be far less involved and not that complicated. No confined space or access concerns but a lot more expensive than a rotomolded poly tank.
 
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^^^ steel? Underground? It will work until it rusts I guess but that seems really weird to me. Cool project though.
 
A lot of good advice, thoughts here. Thank-you. My goal with this was to do this on the cheap. My thought was to use a 2500 gallon single chamber concrete tank and cut out larger access ports. I would dig a pit down 10 feet. line with crushed stone/gravel the 2500 gallon tank has about height of about 6 feet, which is enough for me to access, but with a bit of a stoop.. That way there is 4 feet of soil on top. I'm in a very cold area in the mountains, but the soil is well drained and easy to excavate. I've already logged and stumped 5 acres for solar panels, so I have lots of space for this...
 
It's no different than any other underground battery storage, with the big advantage being that they won't freeze.

Aside from safety consideration, the biggest downside I see is lugging heavy batteries up and down ladders. If you buy high quality and know you won't be touching them for years at a time, then it's probably worth the effort. If you expect you'll be down there monthly, and moving batteries or other heavy equipment several times a year, I'd take a longer look at the cost/benefit ratio.
 
They make actual manway hatch covers for the concrete tanks. You need to get the right lid and risers. Ours is for holding rain water and we added a yard hydrant where the core drill is set up. Round risers could be for vent or ??? Ours had 4" PVC boots in the corners for the gutter pipess to enter

 

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A much easier solution would be to bury the tank and fill it with water-- circulate the water to a heat pad or radiator. Store the batteries in a well insulated space with the inverter.
 
mvonw said:
A lot of good advice, thoughts here. Thank-you. My goal with this was to do this on the cheap. My thought was to use a 2500 gallon single chamber concrete tank and cut out larger access ports. I would dig a pit down 10 feet. line with crushed stone/gravel the 2500 gallon tank has about height of about 6 feet, which is enough for me to access, but with a bit of a stoop.. That way there is 4 feet of soil on top. I'm in a very cold area in the mountains, but the soil is well drained and easy to excavate. I've already logged and stumped 5 acres for solar panels, so I have lots of space for this...
Click to expand...
Sometimes our minds get fixated on a plan and when design problems arise we tend to expand the plan to compensate. I have found from some of my wilder designs that at some point it expanded so far that it was really not worth it considering what the original plan was designed to accomplish.

You want to keep batteries warm but your plan is so expensive that it just makes sense to go with simpler options that actually work better. An above ground insulated structure that is easily accessible and well insulated can hold heat from a heating system with minimal power.
Remember that the critical point is making sure the batteries are warm enough before they start charging. They have several batteries out there that have built in heaters that will draw power from the PV array and heat their enclosure. It is only when the batteries reach the proper temperature that they will start charging and the charging will produce enough heat to keep the cabinet warm.

In a nut shell I am saying use half of that money your going to spend on the underground system towards batteries with built in heaters or an enclosure that is heated by the panels or it's own set of panels.

If it's so cold and snowy that you have no PV then your underground system is not going to make a difference anyway. If your plan is to use a generator to power the house and charge the batteries then that same power can heat the battery cabinet and also charge the batteries. Keeping a tightly packed well insulated cabinet with batteries inside of it warm is not going to require a whole bunch of extra power and it's far cheaper and much easier to access.
 
Yeah. The last two posts are right. Maybe harder for cooling in really hot temps.
 
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