The "thermal management of LiPo" thread!

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The "thermal management of LiPo" thread!

Post by jonescg » Nov 11, 2013 7:52 am

Hi guys,

I have been building LiPo packs from individual cells for a couple of years now, and I believe I have a solid system. I can discharge my 15 Ah packs at 13C and they don't get warm until the last ~25% of the discharge. Now, I have generally not found them to get particularly hot, but I certainly wouldn't be putting it on a 25 amp charger and hammering them again for another 13C hit. Most of the time my ebike would barely tickle 7C, and more like 2C constant.

My race bike will be different - I'm expecting to draw 200 A peaks from my 15 Ah pack - 13C is about 120 kW, but I can't see me needing more than 60 kW from the pack most of the time. I fully expect my pack to get warm, but I can't see it being too hot.

So what if you DID want to hit them really hard? What kind of thermal management would you build into them? I don't mean bunging a thermister on the BMS, I mean active cooling, liquid loops, forced air and the like. Applications include drag racing, circuit racing and flight.

I wanted to start this thread to explore the possibility of active cell cooling designs. Essential criteria includes:

- electrically isolated
- weatherproof
- extremely space and weight efficient
- not too complicated or expensive

I'll start with thin aluminium plates between each cell, or couple of cells to serve as a heatsink. These would have a protrusion serving as a radiator, so during and after discharge convection or forced air could soak the heat out of the pack. I don't know what the electrical isolation would be like though - aluminium tends to chafe cells and short :?

How would you ensure the pack is free from moisture ingress?

Lets throw some ideas around!

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Re: The "thermal management of LiPo" thread!

Post by dogman dan » Nov 11, 2013 8:53 am

I like the idea of thin aluminum sheets, but if the goal is to really cool it back to start of race temps fast, then some kind of liquid cooling where you could shoot salty ice water through the tubing would rock.

I assume you want a warm pack to race, but not an overheated one in the trophy heat.

Some kind of well sealed pack could be dipped in the salted ice and water, pulling the heat out of the metal sheets. For me, the best sealant has always been the caulk designed for rain gutters.

Maybe you could make your heat sink out of a welded aluminum box, with fins attached. Then you could remove the box and dip in the ice water? Or just shoot something cold on it, like compressed air or ice water.

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Re: The "thermal management of LiPo" thread!

Post by jonescg » Nov 11, 2013 9:08 am

I think they call that eutectic cooling? Brine as a freezer brick works quite well. Great ideas - keep them coming!

My theory is that most of the time, the tab is the bit that gets hot and if this isn't sunk by the copper conductor, it will go straight back into the cell. But even with big lumps of copper, a 13 C discharge (of a 40C cell) will still get it pretty warm. If you have hundreds of these little exothermic pouches flat up against each other, the whole lot will get hot pretty fast. So what is the best connection between the hot cell and the outside world? The terminals! I figure if the terminals were exposed to a cool airflow, they would successfully remove a fair bit of heat both in operation and after a race. I might yet put a few small fans on the pack, mostly for recharge.

However, additional cooling could be done with cell-aluminium-cell sandwiches. Unfortunately this doesn't render it very electrically isolated, but there are options. I also think some liquid cooling could be done with plates between whole blocks of cells could work, but at some point you take up as much room and weight as a bigger battery pack, which has lower C-rates and doesn't get as hot...

Will post some sketches of ideas up soon...

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Re: The "thermal management of LiPo" thread!

Post by nieles » Nov 11, 2013 9:14 am

i like the idea of a phase change materials to soak up the joules of generated heat. not sure if this would be viable on this scale though.

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Re: The "thermal management of LiPo" thread!

Post by Ypedal » Nov 11, 2013 10:44 am

but at some point you take up as much room and weight as a bigger battery pack, which has lower C-rates and doesn't get as hot...
.. that .. or higher C rated cells.. but then it's a matter of $$$ ...

As long as the cells are kept below 50 celcius, i really don't think it's much of a problem, i prefer to insulate the pack and keep all cells at the same temp vs trying to cool a pack and creating a variance in cell temps ( middle cells always get hotter ) .

How hot exactly is your pack getting at 100 DOD, 13C ?
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Re: The "thermal management of LiPo" thread!

Post by fechter » Nov 11, 2013 11:49 am

The cells will perform better when they are warm, so you might want some kind of thermostatic action.
The standard method used for cylindrical cells is to just leave space between the cells and blow air. This won't do for prismatic pouches.

I like the idea of placing aluminum plates between every other cell and blow air over the ends of the plates. This way you can still compress the pack and you can control the fan to maintain proper operating temperature. When the outside air is cold, you want the heat to build up to some point.

Anybody know what the thermal resistance of a typical lipo is? In other words, if you were to cool the edges of a pouch, would that take away heat from the inside? I suspect if there is any little pocket of gas there, the thermal resistance would be quite high.
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Re: The "thermal management of LiPo" thread!

Post by e-beach » Nov 11, 2013 12:36 pm

I think that it depends on the size your battery area can handle. Obviously putting anything between your cells or pouches will increase the total area of the battery, but if you can handle a wider pouch battery area here is what I think...in no particular order.

A series of Thermoelectric Peltier Cooler pads that sandwich an aluminum plate to wick some of the heat away from the cells. The electricity generated by the waste heat could run cooling fans.
(An example http://www.amazon.com/TEC1-12709-Thermo ... rid_pt_0_2)

A series of Thermoelectric Peltier Cooler pads that sandwich a series of liquid filled copper tubes attached to a radiator. The electricity generated by the waist heat could run the cooling pump. The temperature of the coolant could be regulated by an inline automobile/motorcycle radiator thermostat or similar device.

Gaps between pouches so forced air can be blown through them.

Pouch cells built into the fairing so air blows over them as you ride wicking off heat.

Slotted aluminum cooling fins top and bottom to secure the cells while leaving an air gap for air flow fan or forced air.
( An example http://www.amazon.com/Gino-20mm-Alumini ... rid_pt_2_1)

If cylindrical cells are used, the cells could be stacked directly on top of each other giving a gap between batteries for liquid filled cooling tubes to zig-zag through out the pack.

So...What is you battery area dimensions and preferred cells anyway?

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Re: The "thermal management of LiPo" thread!

Post by Hillhater » Nov 11, 2013 5:00 pm

As you say, the "hot spots" are likely going to be the tabs and other conductor areas where those Al plates wont be in direct contact.
The ultimate cooling / heat soak to move that heat quickly would have to be a liquid immersion in the same manner as power transformers.
Some ES'rs have used mineral oil filled controllers successfully, and i am sure there are better ( lighter ?) fluids available for this purpose.
It obviously adds complexity weight and cost,...but for the optimum solution it may be the best way. ?

Another thought..
to avoid the liquid complexity and weight how about a thermostatically activated CO2 ducted blast system directed at the known hot spots. ?
..sorry,..just brain storming here ! :oops:
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Re: The "thermal management of LiPo" thread!

Post by jonescg » Nov 11, 2013 7:35 pm

Gaston, Fechter, I agree that the cells will work at their best when they are warm. But we never have cold cell problems in Perth. Ever. :lol: I've been at a track day at Wanneroo Raceway where the air temp was 41'C and the track temp was 52'C. Modern race bikes were overheating! So warm is good, but hot is not ideal.

e-beach, my preferred cells are 300 Wh/kg, 500 Wh/l, 90 C and $0.25/Wh. :lol: Seriously though, I'm mostly using the 40C LiPo available from any reputable Chinese hobby supplier. These are built with cells flat up against each other, typically in muti-kWh packs. I have no problem with them getting warm, even quite warm, but if you were striving for the lightest pack possible for the application, you will be pulling high C's from the pack and getting it very hot.

I guess it would depend on where you can afford to hang a radiator off the vehicle. And also bear in mind don't want to cool cells down to a point where they will be less powerful, so perhaps one cooling plate every three cells would suffice? And lots of conductive bits of metal inside a battery is not going to make a crash particularly safe...

Edit - forgot to add, after a 13C discharge for 5 minutes (roughly 100% DOD) on a "well padded" pack we were seeing about 58'C. Not sure if this is too hot or not, but the "well padded" is relevant here. The idea was that the foam matting would prevent vibration damage to the cells, but I suspect it was serving as a giant stubby holder, keeping it nice and warm. I also think the foam was way too thick for what we were trying to do.

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Re: The "thermal management of LiPo" thread!

Post by t3sla » Nov 11, 2013 7:54 pm

Aluminium separator between each cell, but then each connecting to a base plate.
Like fins on a heat-sink, except you are wedging the cells between the fins.

Be real creative and use a non conductive liquid, submerge and pray it doesn't eat pouch :mrgreen:
Or try replicate one of those Dyson style air cools that has no blades (charges air particles)
How about an Eski type situation for the battery box?
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Re: The "thermal management of LiPo" thread!

Post by Ypedal » Nov 11, 2013 8:08 pm

Random google search result :
http://traxxas.com/forums/showthread.ph ... range-Lipo

140f is 60 celcius

What's the cycle life difference between a pack kept below this temp vs one that does the job without going up in flames.... if we consider racing, lighter, smaller, better.... for a commuter.. long term investment in reliability is worth more in personal use value.

edit, still googling, general info, open cell pic, good read
( you'd think with all the years i've been messing with these lipo's.. i don't do nearly enough reading on RC sites. )
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Re: The "thermal management of LiPo" thread!

Post by Hillhater » Nov 11, 2013 8:21 pm

t3sla wrote: Or try replicate one of those Dyson style air cools that has no blades (charges air particles) ....
Ha !... is that what you have been told ! :lol:
http://electronics.howstuffworks.com/ga ... s-fan1.htm
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Re: The "thermal management of LiPo" thread!

Post by jonescg » Nov 11, 2013 8:24 pm

Yeah I dunno what the cycle life would be under very warm cycling versus actively cooled cycling. And is the full life of the pack being shortened due to excessive heat or electrochemical discharge, or is one a symptom of the other? If racing is the main application, cycle life wont be as important as performance or safety, but it would seem to follow that a thermally managed pack would be even better under regular road use.

I think I need to do some testing on a pack under fairly 'typical' conditions. That is, not a single cell being discharged in the open air of an air conditioned room, but as a built pack with lots of cells against each other inside a weatherproof box. And then I can start to look at different cooling options. I'd really need a high current discharger and automated recharger... and someone else's power supply :)

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There are three of these to a battery. Here is the "well padded" first pack we've been flight testing with:
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Re: The "thermal management of LiPo" thread!

Post by e-beach » Nov 11, 2013 8:39 pm

jonescg wrote:e-beach, my preferred cells are 300 Wh/kg, 500 Wh/l, 90 C and $0.25/Wh. :lol:
:shock: Mine too.....How did you know :!: :?: :lol:

I think if you make the battery box solid enough it should be crash proof enough to keep the batteries from shorting due to a crash.....but then again it is racing.

If you want to avoid metal then it sounds like heat resistant plastics with gaps in the cell sets so forced air, either from your actual speed or from a fan could keep the air moving.

Are you familiar with the concept of a swamp cooler? How about a main radiator mounted behind the front wheel and a second one mounted just behind the battery box air inlet. The coolant fluid circulates through both radiators. The main radiator keeps the fluid cool and it is pumped through the smaller one inside the battery box their-by cooling any air passing through it into the battery box just before the air passes around the batteries.

Perhaps the whole battery box could be liquid cooled.....

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Re: The "thermal management of LiPo" thread!

Post by fechter » Nov 11, 2013 8:56 pm

If the cells are completely surrounded by foam, then there really isn't anywhere for the heat to escape other than the wires. 60C is pretty hot. There is a trade off between performance and cell life. I'm not sure what optimal is, it depends on your battery budget.

A simplistic approach would be to design the foam so it only covers the corners/edges and leaves a passage along the sides of the cells open. Then blow air through the passage. Direct air blowing over the edges of the cells will beat the snot out of completely sealed. If the temperature starts getting too low, simply turn off the blower. I think its important to use a centrifugal blower as opposed to a fan, as you can get a lot more air through a tight passage. I don't think it would need to be very large though.
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Re: The "thermal management of LiPo" thread!

Post by e-beach » Nov 11, 2013 9:20 pm

fechter wrote: A simplistic approach would be to design the foam so it only covers the corners/edges and leaves a passage along the sides of the cells open. Then blow air through the passage. Direct air blowing over the edges of the cells will beat the snot out of completely sealed.
+1

Doesn't just have to be the corners and edges though they are solid places to start, foam rails down the center of the cells as more support would work as well, just so you have big enough gaps and channels to blow the hot air away from your cells.

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Re: The "thermal management of LiPo" thread!

Post by jonescg » Nov 12, 2013 12:21 am

That being the case, there's probably merit in just peppering the side walls of a polycarbonate enclosure with 20 mm holes. It would let some heat out while reducing weight. It reduces the weatherproofing considerably though.

I still believe you should design your pack according to the demands of the system, such that you aren't hammering them too hard. Quite hard to do in a flight situation where weight and sustained power demands are critical, but in a road going EV it's certainly possible to overspec the battery.

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Re: The "thermal management of LiPo" thread!

Post by dogman dan » Nov 12, 2013 9:20 am

Lots to grok at once, as with any engineering problem. Screw the cycle life, the aim is winning. But don't actually cook off the pack prematurely for lack of a way to cool it. You can't have it get to 100C.

I never figured it was a question of cooling a pack below ambient, which I assumed would generally be warm on track day. But rather, avoiding a gradual accumulation of more than 100C of heat as the day progresses, assuming some fast charges are not allowing a fast cool down.

I like the idea of incorporating the heat sink into the discharge wires. Assuming you will parallel first, you could have the thin aluminum sheets between every other cell. Perhaps tape the edges of the sheets, to minimize the chafing of the sharp edge.
Those sheets could be then connected to the finned bus bars that heat sink your wiring, half of them to the + bar and half to the -. That gets your wiring heatsink also heatsinking the center of the pack, so the entire pack can cool as fast as possible.

Then you just need to air condition your bus bar heat sink fins. Assuming you have AC power, you'd just bring a small window air conditioner, and use a flex duct to connect to your air vent to the heat sinks while you charge. Unlikely you'd need to stop cooling, but a temp probe in the pack would let you know when to stop on a cooler day. You'd want 100F pack for racing. (40C?)

Or alternatively, have a liquid cooling tube as part of the heat sink. Like maybe a hollow buss bar. It's air cooled on the track, but you could circulate supercooled liquid when in the pits.

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Re: The "thermal management of LiPo" thread!

Post by powersupply » Nov 12, 2013 3:02 pm

I would take a totally different approach.

I would arrange the cells sideways, so their large area would be exposed to larger encasing/protecting/cooling aluminum.

I never understood why they would stack them as they do, because the exposed areas (except of the first and last) are the seals of the pouches and those have not as good a contact as the whole flat side.
And the heat is coming from those flat sides and is accumulated in fact because it is trapped.

So what about arranging the cells side by side and use several large aluminum sheets longside to contain the cells?
Those sheets can be arranged so that a little gap is inbetween them, enough to let air circulate, which is very tedious and complicated in the current arrangements.

It would be superior not only heatwise but also mechanically.

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Re: The "thermal management of LiPo" thread!

Post by jonescg » Nov 12, 2013 7:21 pm

Like this pack?
24spackalmostready001.JPG
24spackalmostready001.JPG (179.71 KiB) Viewed 72 times
It seems like a pretty good way to go for a narrow stack, and saves you from having to stuff lots of aluminium between cells. One of the disadvantages of a pack in this configuration is the reduced compression, but with a pair of plates on either side and a few 8 mm dividers you can certainly make it work.
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Re: The "thermal management of LiPo" thread!

Post by liveforphysics » Nov 12, 2013 8:24 pm

I hope you have some robust form of insulation between those foil pouch edges chaffing along each other my friend. Particularly with RC sourced cells, you can count on somewhere inside that cell having a pinhole imperfection in the PE film laminated onto the foil composing the pouch material. When they chafe into a foil pouch at a different voltage, slow puffing will begin as electrolysis starts going to town on the foil at the pin-holes.
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Re: The "thermal management of LiPo" thread!

Post by fechter » Nov 12, 2013 8:35 pm

The amount of heat you can remove from the pack with air will depend on the surface area exposed. Whether they are stacked one way or another probably won't change the surface area much.

If you want weather resistant, one approach would be to use a metal (aluminum) battery box and recirculate the air inside the box. If the outside of the box has air blowing on it, it will stay at ambient temperature. Recirculating the air moves heat from the cells to the box. This is also handy if a pack decides to ignite, since the metal box will help contain the fire, assuming it has limited holes for oxygen to get in.

I'm still not sure about compressing the pouches. I understand it is good to prevent delamination and may help prevent chafing from vibration if they are nice and tight. I don't know what happens when a cell tries to puff and it's clamped tightly. The gas has to go somewhere.
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Re: The "thermal management of LiPo" thread!

Post by Ypedal » Nov 12, 2013 9:20 pm

They will pop at the seams . Outer edges.
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Re: The "thermal management of LiPo" thread!

Post by powersupply » Nov 12, 2013 9:56 pm

Yes, like that but with aluiminum sheets between each 2 cells.

I figured that when at least one side of a cell is in contact with the aluminum sheet, it should help tremendously carrying the heat away from it.

So basically the two cells are stacked between two aluminum sheets.
The sheets would be held in place with spacers, and as mentiond should have some air gap to the next set of cells.
In case the cells expand, the aluminum would give way a little.


jonescg wrote:Like this pack?

Image
fechter wrote: The amount of heat you can remove from the pack with air will depend on the surface area exposed. Whether they are stacked one way or another probably won't change the surface area much.
But that is exactly the case here, instead of exposing the sealed borders to the ambient, you would have the whole side of the cell exposed.

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Re: The "thermal management of LiPo" thread!

Post by jonescg » Nov 12, 2013 10:57 pm

liveforphysics wrote:I hope you have some robust form of insulation between those foil pouch edges chaffing along each other my friend. Particularly with RC sourced cells, you can count on somewhere inside that cell having a pinhole imperfection in the PE film laminated onto the foil composing the pouch material. When they chafe into a foil pouch at a different voltage, slow puffing will begin as electrolysis starts going to town on the foil at the pin-holes.
The pictured pack was assembled as four individually enclosed battery packs. It's doing allright ;)

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