Could 18650 battery pack be cooled with airflow?

Hello, I’m yet another bypassing kiddo throwing up an idea.

It seems that the overheating battery is one of the main problems with smaller battery configurations. But if we are using 18650 cells, must they all be packed so tight? If the battery array is splitted in smaller sub-units, increased surface area will increase the cooling. If air can flow through the array, then the propeller works also as a giant cooling fan.

Here is an quick and coarse illustration of one possible way to increase cooling

There are many other ways to increase cooling. Large battery array could have ventilation holes, or many smaller battery packs could be placed sparsely in the frame.

It may be also possible to drop out closed container box, so that 18650 cell bodies are in direct contact of air flow. This would result very rapid cooling. Though, it would be probably good to prevent corrosion, so cell caps should be sealed somehow. For example, Vruzend kit may be enough to keep most of moisture out. Though I agree that container-less battery array is not the most aesthetically pleasing option.

To wrap up

  • Increased cooling would make lighter battery packs possible.
  • Cooling could be increased by dividing one large battery pack in smaller packs.
  • If battery cells were in direct contact of air flow, cooling would be even stronger.

So what do you think, could something like this be practical?

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Airflow would definitely help but this might bring on another issue, resistance. The more wires and connections you have the more resistance there is which will induce heat and reduce efficiency.

I’m rebuilding my vruzend pack from 14s20p to 13s 17p to reduce weight, and make changes by going from two to three bus bars per series connection, and stepping up to two 4 gauge wire instead of 8 gauge. As well as adding an active balancer on top of the bms. A better way to cool batteries might be to make an air duct that diverts air from in front of the paramotor to the battery, just like how aircraft engines are cooled.

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I’ve been thinking ducting air for cooling batteries would be pretty neat. Could even place a thermostatically controlled vane/valve in it to prevent over cooling when cruising or coasting at altitude. Ductwork can be very light so it’s probably worth the weight penalty. The extra drag seems minimal. Paramotors are pretty draggy to begin with. To make it worth the weight and drag you might have to design the pack to require the active cooling.

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of course you can cool batteries. basically one system should be built in such a way that all parts of it are suitable. if the battery gets too hot, it is not suitable for it! if you air-cool the cells from the outside, the internal resistance of the cells does not change. the inner heat is still the same as before. if you now cool with air from the outside, there is a greater difference in the cells. outside and inside. the cell will lose power faster, will not endure as many cycles. there is a way to keep the temperature stable: this requires liquid cooling / heating. the cells can then be brought up to the ideal temperature. The Kreisel company from Austria does this for special conversions to electric cars. to eppg. there are already cells available on the market for everyone that are ideally suited for eppg. you just have to use them. it may sound strange … currently I fly a lot at around plus +5 degrees celsius to - 15 degrees celsius. I use a special neoprene cover to protect the batteries from the cold in flight. because that would also damage the cells if the outer cells are colder than the inner ones. https://www.kreiselelectric.com/en/

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So if an aircraft engine without any cooling overheats it’s not suitable for the aircraft? My aircraft engine would overheat in minutes if it wasn’t for air and liquid cooling.

I get your point the internal heat will still be there but if you dissipate that heat with airflow it will increase battery life, and be overall healthier for the pack if it is getting too warm.

do you mean within the pack or within individual cells? Any air flow cooling would have to be designed to cool every cell as evenly as possible and have a way to prevent over cooling as well.
But if it’s not good to cool the outside of a cell while the inside is still hot…:roll_eyes:

Batteries and engines are similar. You run an engine too hot and it damages it just like batteries. Best is to have temp sensors inside your pack to watch the temp of the pack. My vruzend pack did get to 60°c only hot hot days and on full power. In cruise it was about 25°c. With the vruzend pack there is an air gap between each cell which helps keep the air flowing through and allows heat to dissipate better then having the cells closely packed together. I think if I had some active air cooling blowing on the pack I wouldn’t see this high temp. Will see if my rebuild of the pack by adding more bus bars and thicker gauge wire helps reduce heat on hot days.

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Oh yeah, I didn’t think those factors at all. Too forceful cooling may cool battery too much, and internal temperature in cylindrical cell core may be higher than on it’s surface. I did some googling and found good paper on these subjects:

Though these results are only measured with some specific Li-ion battery technology, they may be also be generalized for wider guidance.

Temperature effects on capacity

Few pick from that paper

  • Optimal temperature range for lithium ion batteries is 15°C – 35°C
  • If temperature is too low, power and energy capabilities are reduced. For example, capacity decreases by 23% if operation temperature drops from 25°C to −15°C. However, as far as I understood, discharge in low temperature does not damage batteries.
  • If temperature is too high, then electrolyte and ion layers starts decomposing. Decomposition of electrolyte becomes more rapid from 60°C up. For example, by storing batteries 6 days in 75°C, the capacity decreases roughly by 40%.

The paper contained also nice graphs:
capacity_by_heat
This shows how operating in lower temperatures doesn’t utilize the full capacity of battery. One can also see how batteries degrade slightly more quickly in higher temperatures.

So controllable ducts may be useful to keep battery in optimal temperature. However added duct weight should be less than the weight of batteries equivalent of capacity loss 10-20%.

(I have to make a cut here because new users can attach only one picture in post)

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Internal temperature difference of cylindrical shape

The paper contained some information about internal temperature difference in cylindrical 18650 cell. The measurements was done by implanting micro sensors inside the cell.


This image, (which I slightly edited for clarity,) shows how internal temperature difference gets higher if battery is cooled under forced air flow. However, as the temperature difference is only few degrees, it is not really much.

The paper also contained other measurements of internal temperature difference by using various ambient temperatures and discharge rates. For example, the highest measured temperature difference ΔT=4.7°C was recorded with 8C discharge and natural cooling. So if forced cooling doubles the ΔT, then we can roughly assume that the temperature difference between internal core and the surface is about 10°C or less. So if we keep the surface of cell, lets say, under 40°C, then internal temperature stays well under 60°C

Battery cell configuration

I also found another paper which maybe have interesting results. If we have a fan that blows air in the battery pack, what is the best cell configuration? Square (like Vruzend) or hexagonal? What should be the cell separation distance?


(Article paywalled, but it can be downloaded it from Sci-Hub)


(I edited this picture too for clarity.)

[continues]
Batteries are discharged at rate 2C, and cooling fan is blowing at 2.5m/s.
Here are the main results: Aligned configuration is better than staggered. With aligned configuration cooling efficiency is 26.1% higher and power input of fan is 54.5% lower. Optimal values for aligned battery pack are Sx=34mm and Sy=32mm. The “optimal” value is determined so that the fan power is small and cooling efficiency is good enough. For example, here is efficiency curve for aligned arrangement:


Pw is the input power of the fan and ƞ cooling efficiency. Sx is the vertical separation of aligned arrangement.

Because 34mm is almost double of the 18650 dimaeter cell, then this optimal configuration is quite sparse, and will take four times the volume compared to very tightly packed configuration. This optimal arrangement may be too sparse for us to use, and it is probably an overkill anyways, if we consider that dzubot 's pack did get up to 60°C only in the hottest days.

Also, it seems to be common advice [1] to keep temperature differences within the pack smaller than 5°C, because cells may discharge at different rates and lead to unbalance. Also, as seen from first figure, 5°C difference results roughly 5% difference in capacity, and cell aging seems to increase something like 25-40%.

[1] https://www.sciencedirect.com/science/article/abs/pii/S0306261916307279

(Whoops, this post chain was not meant to become this long.)

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I envisionied the air flowing through a pack from cell end to cell end rather than across the cells. That would make it very difficult to connect the cells or hold them together. One would have to devise very thin yet rigid connectors.