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Thoughts on a mineral oil bath?

ThaiTaffy

Jack of all Trades
Joined
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As I go deeper down the rabbit hole and now looking at the future and my next project I starting to be torn between FLA and Lifepo4, I was thinking FLA and Purley battery backup. Silly thing is here in Asia it's actually far cheaper to go down the Lifepo4 route but the hurdle being temps especially here in SEAsia.
CALB SE cells work out less than half the price of FLA, though my cooling ideas might even the price out some.
My ideas are leaning towards some form of compressor coil cooling a mineral oil bath as I've built similar stuff before cooling computers, it seems easier and cheaper than individual cooling plates and likely safer. I see there's some research papers available (yet to read anything) anyone have any suggestions or ideas?
 
Will a bath reduce the temp below ambient?

Around here ambient can be 110 F. I hear it’s pretty high in Thailand as well.
 
Oil is not a good cooling medium, as you cool it the viscosity drops so by the colling coils the oil thickens and stops moving. Safer than water but not as effective.
 
Water is problematic, its electrically conductive, and a great promoter of corrosion.

Oil cooling is fine, ALL large commercial power transformers used by the power utilities are cooled by oil.
That is not a problem provided there is enough oil and enough surface area to transfer heat.

Powering a piston compressor is going to be pretty energy intensive.

What is the local ground temperature, down perhaps a metre or so?
Easy enough to measure.
Temperature should be pretty stable down there, day/night and summer/winter.
 
Will a bath reduce the temp below ambient?

Around here ambient can be 110 F. I hear it’s pretty high in Thailand as well.
Not that high 115f in the open air shade last year some rooms were 130f+ with little ventilation in the summer plus the heat from charging/discharging it's liable to go boom in ambient or at least cause some issues.
Oil is not a good cooling medium, as you cool it the viscosity drops so by the colling coils the oil thickens and stops moving. Safer than water but not as effective.
?? I'm talking cooling down to 95f nothing that would cause viscosity issues.
Water is problematic, its electrically conductive, and a great promoter of corrosion.

Oil cooling is fine, ALL large commercial power transformers used by the power utilities are cooled by oil.
Yes I have seen them being worked on up close the oil allows the heat to dissipate evenly and then the contact with the external shell which normally has cooling fins can keep temps well within operational limits.
That is not a problem provided there is enough oil and enough surface area to transfer heat.
Haven't worked out the actual battery size yet so no idea of the bath size but I'd be inclined to over engineer it rather than skimp
Powering a piston compressor is going to be pretty energy intensive.
I was more thinking of repurposing an inverted mini split system and making a heat exchanger but only used when the temps got really high.
What is the local ground temperature, down perhaps a metre or so?
Easy enough to measure.
Temperature should be pretty stable down there, day/night and summer/winter.
This is my thinking as a primary way to cool if I get the deep well guys to dig a few boreholes a couple meters deep then sink coils in them It should work as cooling alot of the time I'd need to check how much cooling I can actually get from this then if it's particularly hot, swap over to the heat exchanger.
 
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Just worked out the pricing and I'm saving $500 by using 200ah Lifepo4 cells compared to 160ah FLA and that's just a single 48v bank so I might still be better off even with the cost of the cooling.
I doubt I'll start the project till maybe the end of the year or the start of 2025 but at least I'll have the time to do my research.
 
Just worked out the pricing and I'm saving $500 by using 200ah Lifepo4 cells compared to 160ah FLA and that's just a single 48v bank so I might still be better off even with the cost of the cooling.
I doubt I'll start the project till maybe the end of the year or the start of 2025 but at least I'll have the time to do my research.
What about an insulated room with a minisplit? Seems like an oil bath is going to be a bespoke solution that only you know how to operate or maintain.
 
What about an insulated room with a minisplit? Seems like an oil bath is going to be a bespoke solution that only you know how to operate or maintain.
Because that's boring....no in honesty if the theory works it shouldn't really need any maintenance and if the ground source cooling works extremely efficient, I could also put in redundancy pumps for any emergencies and as far as operating it's likely going to be connected to the house via esp so I should be able to operate it anywhere with an internet connection though I'd plan to automate it rather than have to go playing with switches or valves manually.
 
Because that's boring....no in honesty if the theory works it shouldn't really need any maintenance and if the ground source cooling works extremely efficient, I could also put in redundancy pumps for any emergencies and as far as operating it's likely going to be connected to the house via esp so I should be able to operate it anywhere with an internet connection though I'd plan to automate it rather than have to go playing with switches or valves manually.
So it’s not going to be a bespoke solution that only you know how to operate or maintain?

Maybe I’m just getting old, and this is a DIY forum, but my Favorite Wife (and by extension myself) has realized that if I’m the only one who understands it and can maintiain, repair, or upgrade it, then it’s only functional till I get hit by a Beer Boat, then it’s have a professional rip it all out and install a new, supported system at 4X the cost. So we’re mutually working on documentation that she groks.
 
So it’s not going to be a bespoke solution that only you know how to operate or maintain?

Maybe I’m just getting old, and this is a DIY forum, but my Favorite Wife (and by extension myself) has realized that if I’m the only one who understands it and can maintiain, repair, or upgrade it, then it’s only functional till I get hit by a Beer Boat, then it’s have a professional rip it all out and install a new, supported system at 4X the cost. So we’re mutually working on documentation that she groks.
That's the thing my wife will understand it and also understands that once I kick the bucket she will have either maintain it herself or rip it out and not replace.
We live in rural Thailand and have more respect for ourselves and the planet to let anyone service anything we own. Service and maintenance isn't a highly paid job here so anyone with a brain doesn't make that career choice.

Air conditioner service agents who work for big brands won't check a system for leaks,so as soon as that vacuum pump hits the pressure they will dump the refrigerant in, which then leaks into the environment but they don't own a recovery tank so they just let it all out and start again.

Electricians regularly bond the neutral to a steel frame house so they can forget about having to run a second wire, just take it from the framework because that's what they do on cars and motorcycles.

Builders add so much water to concrete you could swim in it, because that's easier to mix.

The list goes on and on, if you think that a detailed instruction set would do any good you'd be off your rocker. I'll leave a will that states that once I die the house must be dismantled by a qualified western trained engineer otherwise my inheritance goes to the wildlife trust.
 
That's the thing my wife will understand it and also understands that once I kick the bucket she will have either maintain it herself or rip it out and not replace.
OK, you’re good. I definitely understand the self-sufficient thing, Around here it’s mostly do it yourself or pay someone else to do it over (again and again and again) till they get it right or you decide to live with it.
 

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Water is problematic, its electrically conductive, and a great promoter of corrosion.

Oil cooling is fine, ALL large commercial power transformers used by the power utilities are cooled by oil.
That is not a problem provided there is enough oil and enough surface area to transfer heat.

Powering a piston compressor is going to be pretty energy intensive.

What is the local ground temperature, down perhaps a metre or so?
Easy enough to measure.
Temperature should be pretty stable down there, day/night and summer/winter.
Mineral oil for power transformers is used as a fair amount of heat is generated. Oil moves that heat away and allow for higher thermal ratings of the same design being dry. Talking 100c with oil, or 1/4 the thermal rating at the same temp due to hot spots and eddy currents.

How hot do cells go above ambient under normal use?

I never considered them really heating up, but then again I wouldn’t design a system where constant 1-.5c load is applied, I’d add more storage to bring current down.
 
Seen heaps of oil bathed PC's in my days ... thumbs up from me (y)

I would keep the tops of the cells clear of the oil for maintenance to the terminals, etc.

What I'd love to see is an oil cooled inverter
 
Seen heaps of oil bathed PC's in my days ... thumbs up from me (y)
That's what jumped in my head when the idea first came to me.
I would keep the tops of the cells clear of the oil for maintenance to the terminals, etc.
That's a point I'm wondering about wouldn't the oil prevent oxidation making it maintenance free
What I'd love to see is an oil cooled inverter
Sadly I'm not DIY'ing the inverter otherwise I could have thrown that in the bath also.
 
Mineral oil for power transformers is used as a fair amount of heat is generated. Oil moves that heat away and allow for higher thermal ratings of the same design being dry. Talking 100c with oil, or 1/4 the thermal rating at the same temp due to hot spots and eddy currents.

How hot do cells go above ambient under normal use?

I never considered them really heating up, but then again I wouldn’t design a system where constant 1-.5c load is applied, I’d add more storage to bring current down.
I don't think I'll be pushing the batteries but ambient temps are already pushing the limits and we haven't even hit summer yet here.

Below is my garage temps so far this year this is a shaded thermometer 2ft below a tin roof.
Screenshot_20240609-204945.png
 
Though boring, a small window unit or dual hose portable air con can easily keep up with the tiny volume the batteries occupy.

I’m about a month into my trials and keeping the insulated

battery and inverter space at 75 eff appears to be no problem. Batteries are near full each morning.

IMG_0894.jpeg
 
Though boring, a small window unit or dual hose portable air con can easily keep up with the tiny volume the batteries occupy.
Sadly they don't work here, the humidity is that great they just can't keep up even our oversized mini-split's will struggle to keep up in the next few months as the rains continue but the heat rises.
 
Interesting. They seem to work fine on the Texas coast. Which part of Thailand?
 
Interesting. They seem to work fine on the Texas coast. Which part of Thailand?
East on the river Mekong it's currently 80% humidity and monsoon season isn't due to finish till august so temps will likely reach 120-130°f if it's anything like last year as well as stupid humidity. Texas is lucky you just get high humidity during the night and then it tapers off as the day goes by here it's just like breathing soup 24/7.
 
You need to think like an earthworm.
If you draw a straight line through all the temperature variations, that is what you will get some short distance below ground level.
Find a nice shady spot, and dig a bloody great hole.
Let your battery rest in peace down there.
It will be completely happy.

In really cold climates which are below freezing for most of the year, the ground freezes as solid as concrete with permafrost, but only for maybe a couple of feet. Below that the ground stays relatively warm, or at least above freezing even right through winter.
I am pretty sure you will find a similar effect in the tropics.
Ground temperature will average out over months or even years if you go down just a few feet.
You will need a really good air tight insulating lid or cover though, for it to work as intended.

If your cells have good thermal coupling to the surrounding ground via an oil bath, I expect it should all work quite well.
Temperature.png
 
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Water is problematic, its electrically conductive, and a great promoter of corrosion.

Actually pure distilled water is not conductive. It is the minerals in the water that are conductive. With non-conductive anti-corrosion additives it is used for cooling in a number of ways. Antifreeze in your car is an example depending on the type used.

You could use the monsoon rain to your advantage... car radiator horizontal where down spout from the roof channels water through it. In the off season pump water over it or a fan attached. Circulate water or oil through it.
 
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Actually pure distilled water is not conductive. It is the minerals in the water that are conductive.
You are quite right.
But perfectly de-ionised water is very difficult to make and keep that way.
Its like "a perfect vacuum" simple in theory, but rather mode difficult to actually achieve.
 
You are quite right.
But perfectly de-ionised water is very difficult to make and keep that way.
Its like "a perfect vacuum" simple in theory, but rather mode difficult to actually achieve.

Very true, it would have to be constantly filtered and run through a di-bead bed
 
You need to think like an earthworm.
If you draw a straight line through all the temperature variations, that is what you will get some short distance below ground level.
Find a nice shady spot, and dig a bloody great hole.
Let your battery rest in peace down there.
It will be completely happy.
Earthworms likely drown here, high water table and high likelihood of flooding that's why I was thinking about the coils in my earlier post. It's mainly tobacco and paddy fields here.
If you dig a hole here it will likely be a pond, though ATM with the rainfall you don't even need to dig a hole, my driveway is starting to resemble a mudding track.
 
Earthworms likely drown here, high water table and high likelihood of flooding that's why I was thinking about the coils in my earlier post. It's mainly tobacco and paddy fields here.
If you dig a hole here it will likely be a pond, though ATM with the rainfall you don't even need to dig a hole, my driveway is starting to resemble a mudding track.
Might be even better !
What is the water temperature ?
It will always cool a bit at night, and evaporative cooling during the day should keep it well below peak mid day air temperatures.
 

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