DIY Battery Cooling

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Post by weber » Wed, 26 Dec 2012, 20:53

I woke up from a dream this morning with an image of a bizarre looking arrangement on top of a set of prismatic cells like the ones in the MX-5. In the dream, this was a novel cell-cooling system that someone from interstate had brought to show to us, presumably at an AEVA meeting. It had clear flexible tubes going away from it, showing a red coloured coolant (like on our Tritium Wavesculptor). Above the cell-tops was a plastic casing maybe 20 mm high standing on "legs" that were maybe 30 mm high and 20 mm in diameter. Each leg went down to a cell terminal. The raised casing was like a rounded-rectangle inflatable pillow. Each leg was like a concertina. But these were all of one piece, in the same semi-rigid plastic, not flexible.

On waking, I tried to make sense of this. But of course dreams don't have to make sense. The best I could come up with was to have copper standoffs inside the legs, with the high-current copper links up in the continuous plastic casing above, and some kind of insulating, non-flamable coolant liquid flowing through it, with O-ring seals around the bottoms of the standoffs and the BMS boards inside the liquid. It doesn't sound very practical, but it got me to thinking about DIY battery cooling systems in general.

I came up with something that bears no resemblance whatsoever to the dream system, and is extremely simple. It only requires the cells to be in a box that is like a bin or tub made of sheet metal or composites, like the one in Zeva's RX-7. And it needs to be water-tight, or rather liquid-tight.

We all know that with EVs, heat generation comes in short bursts (max 10 seconds) and so if we have plenty of thermal mass close to the source then we can get away with fairly low heat transfer rates because we only have to deal with the average.

The most compact and light-weight form of thermal mass makes use of the latent heat of a phase change (e.g. melting - freezing). Using ordinary "sensible" heat for thermal storage is like a thermal capacitor. Using the latent heat of a phase change is like a thermal battery in that a large amount of heat can be charged and discharged at a particular temperature (or possibly a small range of temperatures in the case of a mixture).

There happens to be a cheap, readily available, completely non-toxic substance that is a supremely good electrical insulator, has 60% of the sensible heat capacity of water and 60% of the latent heat capacity of ice, and melts at around 55°C, which is an ideal temperature limit for EV batteries. It would prevent corrosion of electrical connections by excluding air and water. It does have the drawback of being combustible, but its flammability rating is only 1 (slight). In fact its name means "little affinity" in latin. I think you'll get the idea from the following photo. Ho ho ho and a belated Merry Christmas to you all.


Image

Image

If you have a non-contact thermometer I recommend you try this experiment for yourself. Let the candle burn long enough to make a puddle of molten wax, then put out the flame and monitor the temperature for some minutes. You have to hold the gun very close so only the puddle is in its "view". In the first few seconds it plummets (you can see the 71.1°C max on the gun above) but when it gets to about 57°C it's like someone slammed on the brakes. It just sits there for several minutes, slowly creeping down through the 50s until all the wax is solidified, when off it goes again.

So the idea would be to preheat the battery box and cells to about 55 degrees and simply pour molten paraffin (candle) wax into the battery box until it has completely covered the cells, terminals, straps, and BMS modules. One worry is the blocking of the vents. One could add snorkels for the vents, or one could test the theory that if they are hot enough to vent then they are hot enough to melt the wax over the vent.

Here's a Material Safety Data Sheet:
http://www.prochemical.com/MaterialSafe ... %20Wax.pdf

For some other DIY battery cooling ideas see the heading "Battery cooling:", about halfway through this wide-ranging post.
viewtopic.php?title=a-lurkers-ideas&p=3 ... 099#p37331

[Edit: Added "non-toxic" and "for several minutes"]
Last edited by weber on Wed, 26 Dec 2012, 12:22, edited 1 time in total.
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Post by acmotor » Wed, 26 Dec 2012, 22:45

Hmmm, well the idea of a coolant is that it can circulate either by convection currents or by forced flow isn't it ?
The candle wax cannot flow below 55 deg and would act more as a thermal insulator up to that point (relying only on conduction). True, once molten and having passed through the latent heat point it would act to move heat. Up to that point it would result in local hot spots ? This is the reason the wax can puddle at the top of the candle (generally !) and not melt more of it away.

Could be worth a test but my initial thought is that a battery terminal say, in open air would operate at a lower temperature via radiation and convection than it would cast in wax up to the 55 deg C point.
The ambient temperature would be a big factor in a wax cooling system ?
If say ambient was 20 deg C then battery terminals rising by 35 deg C would indicate excessive terminal or link wiring losses. If the ambient were 40 deg C then another 15 would be almost acceptable.

I do see the point of a full bath wax for the cells. Hair removal ?
Isn't the idea to keep the cells cool rather than encouraging them to rise to 55 deg C ?
Perhaps another state changing material closer to 20 deg C ?

Thermal mass of the wax may not be as useful as some forced flow cooling ?

Hey, just thinking out loud.
Interebsting dreams you have.
You might need to test the wax idea.
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Post by weber » Thu, 27 Dec 2012, 00:34

Hi acmotor, thanks for your thoughts.
acmotor wrote: Hmmm, well the idea of a coolant is that it can circulate either by convection currents or by forced flow isn't it ?
Sure. But the only place I mentioned a coolant was in describing the image from my dream, and I said that the wax idea "bears no resemblance whatsoever to the dream". The wax is intended purely as thermal mass (inertia or storage). i.e. having little temperature rise for a large amount of heat input. It's not intended to flow, even when melted.
The candle wax cannot flow below 55 deg and would act more as a thermal insulator up to that point (relying only on conduction). True, once molten and having passed through the latent heat point it would act to move heat. Up to that point it would result in local hot spots ? This is the reason the wax can puddle at the top of the candle (generally !) and not melt more of it away.
It's true that paraffin wax (solid or non-convecting liquid) is nearly as good a thermal insulator as it is an electrical insulator. In fact it's essentially the same, in that regard, as the polypropylene cases of the cells.

But it's about 10 times better than non-convecting air. And air needs at least a 10 mm gap in which to usefully convect. In so far as the wax is just filling up dead air spaces that would otherwise have existed in the battery box anyway, at the bottom and sides and between cells, it must be making a major improvement to heat conduction to the outside of the battery box, even without considering its heat storage properties.

It's when we start pouring it over the terminals and links that it gets questionable.
Could be worth a test but my initial thought is that a battery terminal say, in open air would operate at a lower temperature via radiation and convection than it would cast in wax up to the 55 deg C point.
...
Isn't the idea to keep the cells cool rather than encouraging them to rise to 55 deg C ?
Yes. But I think it possible that the improved cooling from the bottom and sides, when potted in wax, might more than offset the reduced cooling from the top. So they may not get to 55°C any quicker.
The ambient temperature would be a big factor in a wax cooling system ?
If say ambient was 20 deg C then battery terminals rising by 35 deg C would indicate excessive terminal or link wiring losses. If the ambient were 40 deg C then another 15 would be almost acceptable.
Excessive terminal or link wiring losses can be detected by voltage measurement, as they are with our BMS. So I consider them a separate issue. And we know the rate of ageing of the cells is a function of their absolute temperature, not their rise above ambient.
Perhaps another state changing material closer to 20 deg C ?
The problem with the phase change ocurring even at 40°C is that in summer (in Perth or Brisbane), after parking in the sun for a few hours, your wax would all be melted and have no remaining latent heat capacity to prevent the battery from going to even higher temperatures. At 20°C it would have no latent heat capacity left even in the shade on a spring or autumn day in much of Australia.
Thermal mass of the wax may not be as useful as some forced flow cooling ?
That is certainly possible, and needs testing.

Something else to consider is that wax expands quite a lot on melting, so we'd need to leave space for this.

Unfortunately, I won't be testing any of this myself, anytime soon. Our battery boxes are open-bottom open-sided steel-angle frames and I need to get the MX-5 finished.

[Edit: Changed "poor" to "good" insulator]
Last edited by weber on Wed, 26 Dec 2012, 13:37, edited 1 time in total.
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Post by jonescg » Thu, 27 Dec 2012, 00:41

I think that a battery company did exactly this a year or two ago, using a wax which melted and evenly distributed the heat around the cells. It was most effective for cold climates.

I can't remember the company, but it has been done. Not sure if it ever took off though.
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Post by Jeff Owen » Thu, 27 Dec 2012, 01:33

weber wrote:
and I need to get the MX-5 finished.

I agree.


That this topic exists may be an indication as to why it is taking so long.

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Post by Johny » Thu, 27 Dec 2012, 03:43

Ha. Caught!

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Post by Canberra32 » Thu, 27 Dec 2012, 04:34

What is wrong with a good old fashion computer cooling fan and air? I run 4 140mm off a 25watt solar panel for the kids cubbyhouse and they stop the ravaging heat of he sun soooooo.....

Just saying.

And yeah get back to work on the car!!!! Guys if he posts again don't reply we are only enabling his procrastination :)

Lol

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Post by Simon » Thu, 27 Dec 2012, 08:15

I think if you need to cool batteries some form of air cooling with a blower would be the easiest way to go. My car has 12 cells under the bonnet and another 24 cells in a box where the rear seat used to be. Measuring temperatures after a 25-30km drive the cells under the bonnet are barely above ambient temps but the cells in the back seat can be 15-25 degrees above ambient!
Should note my car has only 60AH cells so it is often discharging at 0.8-1C continuous.

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Post by weber » Fri, 28 Dec 2012, 02:24

Re: Procrastination

"Let he who has never sinned cast the first stone", Eh?

Jeff pointed out to me that the whole forum went very quiet after his post accusing me of procrastinating in this thread yesterday. I wouldn't know, I was too busy working on my EV. Image

But hey, nothing like a bit of public embarrassment to motivate one. Thanks Jeff. I hope you all got lots done. Here's my pitiful progress
viewtopic.php?t=980&p=40580%E9%BA%84#p40580麄
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Post by Canberra32 » Fri, 28 Dec 2012, 04:42

Yet here you are again weber :)
You are there with your car so you have no excuse :p
I'm in Adelaide so far away from mine for now and although still no excuse its still more of one than yours :p

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Post by Jeff Owen » Fri, 28 Dec 2012, 06:10

weber wrote: Re: Procrastination

Jeff pointed out to me that the whole forum went very quiet after his post accusing me of procrastinating in this thread yesterday. I wouldn't know, I was too busy working on my EV.
Weber
I made no such accusation of procrastination.
I also hit post reply just as the Doctor Who Christmas Special started. Are you saying that you missed this excellent program because you were too busy working on your EV?

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Post by weber » Fri, 28 Dec 2012, 15:35

Jeff Owen wrote:Weber
I made no such accusation of procrastination.
That's true. You only wrote: "That this topic exists may be an indication as to why it is taking so long." I guess my guilty conscience, and Johny, and Canberra32, did the rest.

Would you please explain how you think the existence of this topic might indicate why the car is taking so long?
I also hit post reply just as the Doctor Who Christmas Special started. Are you saying that you missed this excellent program because you were too busy working on your EV?

Jeff, you misquoted me by omitting the winky-face "Image" immediately after that sentence. Image
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Post by Johny » Fri, 28 Dec 2012, 18:12

weber wrote: Re: Procrastination
"Let he who has never sinned cast the first stone", Eh?
I have to say, I apologize for jumping on the bandwagon weber. I admire your work and most of your musings. The wax musing wandered around in the back of my brain most of that day even though I didn't post again. Not much EV for me this break. Mainly house work and repair and maybe some small amount of finalising EV for the engineer.

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Post by weber » Sat, 05 Jan 2013, 04:40

For those of you who are worried that Jeff and I might not be talking to each other any more. Jeff turned up to work on the MX-5 a few minutes after my previous post in this thread, and he answered my questions in person. He says it's not procrastination because that is mere delay for the sake of delay or avoidance. He agrees that some of my non-MX-5 posts may be useful, but they are still distractions from the MX-5 project. I have to agree.

He also objects to me saying he misquoted me by leaving out the winky-face, because neither "winky-face" nor "Image" appear in his dictionary, therefore it isn't a word.

Fair enough. Who am I to argue with the guy who has done so much brilliant mechanical design on the MX-5 for free. So let me spell it out: I was joking when I said I was working on the MX-5 at that time. I would never miss a Doctor Who Christmas Special. Image

Jeff and I have a far more serious disagreement now: Jeff says you should put the milk in the cup first when making tea. I say it doesn't matter. Image
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Post by weber » Mon, 07 Jan 2013, 00:08

jonescg wrote: I think that a battery company did exactly this a year or two ago, using a wax which melted and evenly distributed the heat around the cells. It was most effective for cold climates.

I can't remember the company, but it has been done. Not sure if it ever took off though.

Thanks. I went searching and found this set of tests done by the US NREL
http://www.nrel.gov/vehiclesandfuels/en ... /42544.pdf
The wax was in a sort of graphite sponge (to increase its thermal conductivity). This would presumably make it electrically conductive too, so it wasn't put anywhere near the terminals.

My summary: Hybrids may benefit from its temperature peak shaving effect, particularly in hotter climates, but they still need air cooling although less of it. But pure EV batteries are big enough to have enough thermal mass themselves, so it isn't worth the trouble.
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Post by arber333 » Sat, 25 May 2013, 11:50

Hi Can you help me with a problem?

I am using Nevilles DIY BMS modules to assist with bottom balancing. I discharged the pack to 2,8V and charged untill one cell hit 3,6V couple of times and they went together untill full discharge quite nicely. I drive 65km to work and 50 of those are highway. Speeds are 110km/h and discharge is cca 130A.

However day before yesterday i was returning home and i noticed one cell no. 22 voltage was falling below 2,6V at 130A load. I managed to get home and checked this cell. It bounced back to 3V. Nothing to cry about.

I charged them trough the day. First thing i noticed was the soc reading was 92% and cell no. 22 has shut the charger off. I went to work and by the time i arrived there i only had cca 50% SOC left!!! After work i drove back. 15km before home I noticed cell 22 and 23 voltages were falling. I went off the highway and drove at cca 50A - 90A. Nothing changed and just before home my BMS lost comm with those batteries because the modules need at least 2,2V of power to work. I checked cells in my garage and i found them quite warm. They cooled much slower then the rest. But when they cooled down they had 3,1V again. WTF!!!
My front box lid is lined with neopren rubber to press the cells in their place. Could this cause the cells to heat up? But why only two of them?

Do you have a solution?

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Post by arber333 » Sat, 25 May 2013, 11:57

Sorry double post by accident. Edited...

:)

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Post by evric » Sat, 25 May 2013, 13:36

The problem maybe the way you have mounted the cells. Normally the LIFEPO4 (eg Sky Energy blue ones) are OK on their side definitely not good to mount them flat. Maybe these black ones don't like being on their sides. Just a thought.
Definitely don't drive it if any cell drops below 2.5V for more than a few seconds. The cells will destroy themselves.
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Post by weber » Sat, 25 May 2013, 16:06

Hi arber33,

I think we need more information to help you properly. Can you give us a link to your EVAlbum page or other web page with your car's specs and photos? At least tell us what brand, quantity and capacity the cells are? How old are the cells? How long have you been doing this daily commute with them? How is SoC determined in your system? What measurement(s) is it based on? I understand charging is terminated when any cell hits 3.6 V, but what current is that at? Is that full charger current, or does it back off to a lower current before deciding the 3.6 V is grounds for termination? Why couldn't you tell that those cells were getting hot while you were driving? Surely Neville's celltop BMS modules measure temperature?

Based on the little information you have given so far, it sounds like those cells have gone high internal resistance due to past abuse, either by overcharging, or more likely by overheating. Why only those cells? First notice that high resistance causes high temperature which causes even higher resistance. So it's a vicious cycle. Cells have small differences in internal resistance from manufacture. These differences will tend to be magnified by that vicious cycle as time goes on.

Another possibility is that the high resistance (we're talking a few milliohms here) is in the terminal connections, not the cells. One way to find that out is to take the relevant connections apart, clean off the old jointing paste, sandpaper the terminals and straps/lugs, and bolt them back with new jointing paste and the correct torque. Then test drive to see if those cells are still sagging way more than the others under load.
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Post by antiscab » Sat, 25 May 2013, 19:09

arber333 wrote: I am using Nevilles DIY BMS modules to assist with bottom balancing. I discharged the pack to 2,8V and charged until one cell hit 3,6V couple of times
what charge profile are you using?

is it constant current until first cell reaches 3.6v then constant voltage until current falls to 0.05C?

or something else?

do the cells get "floated"? (held at constant voltage at low current)
arber333 wrote: However day before yesterday i was returning home and i noticed one cell no. 22 voltage was falling below 2,6V at 130A load. I managed to get home and checked this cell. It bounced back to 3V. Nothing to cry about.


3V at no load is very flat (~1% SOC)
what voltage were the rest of the cells?

have you checked the shunts (if any) on the BMS modules? particularly, have the failed on?
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Post by arber333 » Sun, 26 May 2013, 01:55

weber wrote: Hi arber33,

I think we need more information to help you properly. Can you give us a link to your EVAlbum page or other web page with your car's specs and photos? At least tell us what brand, quantity and capacity the cells are? How old are the cells? How long have you been doing this daily commute with them? How is SoC determined in your system? What measurement(s) is it based on? I understand charging is terminated when any cell hits 3.6 V, but what current is that at? Is that full charger current, or does it back off to a lower current before deciding the 3.6 V is grounds for termination? Why couldn't you tell that those cells were getting hot while you were driving? Surely Neville's celltop BMS modules measure temperature?

Based on the little information you have given so far, it sounds like those cells have gone high internal resistance due to past abuse, either by overcharging, or more likely by overheating. Why only those cells? First notice that high resistance causes high temperature which causes even higher resistance. So it's a vicious cycle. Cells have small differences in internal resistance from manufacture. These differences will tend to be magnified by that vicious cycle as time goes on.

Another possibility is that the high resistance (we're talking a few milliohms here) is in the terminal connections, not the cells. One way to find that out is to take the relevant connections apart, clean off the old jointing paste, sandpaper the terminals and straps/lugs, and bolt them back with new jointing paste and the correct torque. Then test drive to see if those cells are still sagging way more than the others under load.


Yes of course, i was a bit confused when i found out this problem.
My ride is here: http://www.evalbum.com/4534
I charge at 10A for now. I use Nevilles BMS for bottom balancing assist and it works very well now. It is voltage triggered and shunt is 3R for 1A dissipation. Charger cutoff is done by triggering AC relay.
For SOC/Fuel i use Dimitris EVDisplay, it is 1000A hall sensor and it integrates current per time passed. I find it very handy.

Thank you for your suggestion. I will now service my battery and check all links. Additionaly i will measure every cells Ri. What would you suggest i measure? Ri when full or empty? I dont have time to do both.
Also you mentioned contact paste. What kind? Dont you think this would impede conduction? Last time i used contact spray, no paste.

A


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Post by coulomb » Sun, 26 May 2013, 15:57

> Also you mentioned contact paste. What kind? Dont you think this would impede conduction?

No, the paste is supposed to squeeze out of the joint; it should only stay where there is no contact, preventing oxidisation there.

So basically, it should not impede conduction at all.

Edit: you should test Ri when the cell is almost full. When at low state of charge, the Ri will increase a lot, and te exact value will depend on how low the SOC is, and other factors.

[ Edit: Don't measure Ri when really full; things get strange up there. Thanks Weber for reminding me. ]
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Post by weber » Mon, 27 May 2013, 03:03

arber333 wrote:What would you suggest i measure? Ri when full or empty? I dont have time to do both.
Not when full and not when empty. But anywhere in between. Say 2% to 98% SoC.

[Edit: Ri will change with SoC, but we don't care about absolute Ri measurements here. We only need a comparison between cells. And at this stage I was thinking you must have an AC Ri meter since you seemed to be saying you could measure Ri directly. Thanks Coulomb for suggesting this clarification.]
Also you mentioned contact paste. What kind? Dont you think this would impede conduction? Last time i used contact spray, no paste.

Google "electrical joint compound".

Aluminium terminals particularly need to be sandpapered or filed and then coated with jointing compound within a few minutes, to prevent the hard insulating oxide layer from reforming.
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Post by arber333 » Mon, 27 May 2013, 11:31

Thank you I will search for that compound.

Meanwhile...I tried to measure voltage drop, like we used to with model airplanes. 4R7 resistor across terminals and measure voltage drop (V1-V2)*R/V2 to get Ri which is internal resistance of the cell. DOH! I remember it used to be simpler. It seems the 5W resistor load is not enough, because i cant measure any drop :(. Suggestions?

On the side note, i checked contacts and guess what? Cell 22 had one bolt not tighten. While driving the vibrations could cause overheating of the terminal trough time. And considering batteries were under 140A load for cca 35 minutes... I have to check Ri to be sure, but i decided to charge until tomorrow and then check Ri again. But how?

I guess my car is on the edge of range and i will have to buy 2 more cells anyway. It was my (weight) safety margin when i calculated range before conversion. It is scary just how accurate my calculation was including regen and all.

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Post by coulomb » Mon, 27 May 2013, 14:01

arber333 wrote: I tried to measure voltage drop, like we used to with model airplanes. 4R7 resistor across terminals and measure voltage drop (V1-V2)*R/V2 to get Ri which is internal resistance of the cell. ... It seems the 5W resistor load is not enough, because i cant measure any drop :(. Suggestions?
That's pretty much the only way to do it, but you need a much higher load (smaller resistance). With your 200 Ah cells, 4R7 would be about 0.7A, which is 0.0035C. You need at least 0.5C, or 100 A.

You could do it with a coil of wire in a bucket, with lugs and a DC rated circuit breaker; see this post for how we did this. But it's probably easier to use the motor. Put the car on stands, and run at a constant speed with the brakes on lightly (unfortunately, you have front wheel drive, so you can't use the handbrake, which can be "calibrated" in "clicks"). An assistant would be useful; I think you just want a comparative measurement against some other cells. So measure the voltage on cell 22 when under this load, and compare with the voltage on 21 or 23 and maybe a few others.

You could perhaps also use your heater; I don't remember if you have one that runs at pack voltage. Do a few measurements and write them down; turn the heater on, wait for it to settle for a minute or so, then measure the voltages of the same cells and do the calculation. The heater may even be too light of a load to get a good measurement, but it should be good enough to eliminate really bad connections.
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