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Discussion Starter #1
I am building a 18650 li-ion battery pack for my electric motorcycle. It has over 1000 cells and will hold over 9kWh of power. I am running the cells at a 1C discharge with peak discharge of 3C for no more than 30 sec.
I am wondering if air cooling will be sufficient or if anyone has ever tried mineral oil cooling on 18650 cells?
Bad idea, good idea? What do you think?
 

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I am wondering if air cooling will be sufficient or if anyone has ever tried mineral oil cooling on 18650 cells?
Bad idea, good idea? What do you think?
I used to run mineral oil in my oil lamp... it definitely burns... so it may not be the best choice if something should short out for whatever reason. Keep in mind that most 18650 cells go into thermal runaway at only 300 F.
 

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I am building a 18650 li-ion battery pack for my electric motorcycle. It has over 1000 cells and will hold over 9kWh of power. I am running the cells at a 1C discharge with peak discharge of 3C for no more than 30 sec.
I am wondering if air cooling will be sufficient or if anyone has ever tried mineral oil cooling on 18650 cells?
Bad idea, good idea? What do you think?
What cells are we talking about? There are 2A cells, and there are 30A cells, huge difference :)

Also, it's 9 kWh of energy, not power ;)
 

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Mineral oil is combustible but not flammable. It will burn off a wick or if you atomize it into a flame, but not otherwise. It'd be a safer coolant than anything water based. There are various grades with different viscosities like any oil.
 

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Yes, But for extreme applications, for racing vehicles or sports hybrids. A fluid buffer might just be good to try out, but expensive battery cases.
 

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Discussion Starter #9
Thanks for all the information guys!
For everyone saying fire risk, watch these idiots put lipo packs in mineral oil
https://youtu.be/1PdLzYufSqw

Now as for glycol, Its slightly conductive (about 1.5 to 5 Meg ohms) Which will be a parasitic drain on the pack for its whole life. Wouldn't that be bad? Where as mineral oil has a resistivity that is greater than 1000 Meg ohm (whats that a gigaohm?)

More info about my cells 2600mAh 8A max discharge rated. The pack will be 20S51P 72v nominal. I have a BMS and will be pulling 300A for no more than 30 sec at a time. Normal draw is 75A to 130A. I am doing tesla style cell level fuses and buss fuses. I am building a custom aluminium enclosure to protect the pack while it is installed on the motorcycle.

The cooling is really only to make sure that I get the maximum cycles out of my investment please let me know if you think this is all not worth it.
 

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What model cell are you using? 8A rating doesn't really tell us much, the internal resistance is what will determine how hot they'll get.
 

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Discussion Starter #11 (Edited)
2600mAh 18650C is the label.
I did load testing on 5 cells and at a 1C load the temp rise was about 2 deg C. Even when I abused them with a 10A discharge on 1 cell it got warm but you could still hold it in your hand.

Here is the datasheet from the manufacturer.
 

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As is often the case, the datasheet doesn't list DC resistance, only AC impedance. But you can get somewhat of a relative idea, and this sets a lower bound for DC resistance. Fifty milliOhm is pretty high for impedance, and resistance is probably at least 55. That would correspond to about 3.5 Watts when running at 8A. That comes out to just about 1kW for your whole pack. So it definitely will get warm. You might want to do some better tests, running the cell at the expected load profile, and actually measuring the temperature rise--if you are relying on "too hot to touch" it is already way too warm for the health of the cell.

I suspect that you could benefit from liquid cooling. That's not to say that I endorse the idea of filling the box with mineral oil. But these types of cells benefit a lot from staying cool. If you can keep them below 40C at all times, that's really helpful for their life cycle.
 

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Discussion Starter #13
Also, could you break your numbers down and explain it. I can't make the math jive out for the numbers you cited. Trying to learn, batteries are a different sort of electrical work than I do for my day job.
 

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Jintion batteries (Great Power Battery Company)--curiously there is no specification listed for the weight of the cell? Seems like that would be somewhat important...

they seem to have a fairly low charge rate compared with other 18650's.

What are you building--a race bike or daily commuter?
 

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Discussion Starter #15 (Edited)
This is a commuter bike. Its been running on lead acids for about 2 years now. But I am tired of the voltage sag and running out of power all the time. Also my charger crapped out for the second time so I figured now was a good time to switch.
Glad to hear you think highly of Jintion. They have been really awesome working with me and getting these batteries to me on time, and they are exactly what they promised so far.
The cells weigh in at 55g if I am remembering correctly.

The charge rate is pretty low but with the number of cells I will have in parallel I will only be charging at about a .1C rate per cell.

Are most people using passive cooling or fans to cool their batteries?
 

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So, you bought chinese cells? Sorry to tell you, but that's epic fail, there are cheap brand 18650 cells like sampsung or panasonic that are miles better than any chinese cell. Your cells wont last long whether you cool them or not, so don't bother. I wouldn't even bother with assembling the pack with them, you will invest your time and hard work for nothing.

Why didn't you ask for advice on cell model before buying? You can't simply trust some chinese datasheet on batteries. Most of it looks copied from samsung or LG...

And most people use absolutely nothing to cool their cells because there's no need to :)
 

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Discussion Starter #17
I know I bought cheap cells. That is what my budget was for the project. However, I do have access to some really good battery testing equipment and these cells test out about 10-20% better than what that datasheet says. Is it going to last as long as it could with name brand batteries? Probably not. But your attitude of "why even bother putting it together" is not in the DIY spirit. By your logic I should probably just buy a production electric motorcycle. I mean why bother building it myself?
Their was a guy I talked to just like you that said my kelly controller would explode after 6 months of use because it is cheap chinese crap. That was really wrong! Kelly is an awesome company to work with and that controller has been working flawlessly for over two years now!
Anyways sorry for the rant.
 

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If you can build your pack such that you closely monitor every string and can readily access the cells to swap out any laggards, then it shouldn't matter.

Maybe you can run some bench testing with the entire pack. There will likely be some defective cells that don't hold up, but if you can detect them early and change out before installation that would be easier. Or test each individual cell before installation.
 

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Discussion Starter #19
I am, at this point, not worried about the cell quality. I have tested over 100 of the 1020 cells and have had 0 defects so far. I will of course be doing some pack testing when it is all assembled.
 

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Hi - my 2 cents is to measure the internal resistance of the cells, then calculate how much heat you will generate while riding normally and during short bursts. Then use the heat capacity of a lithium ion cell (1350 J//kg/degree Celcius) to determine the rise in temperature of your batteries.

You will have to find someone-else who can estimate cooling, or just try a fan and see. You have a BMS, so can monitor temps.

Jim
 
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