RC LiPo - cool or compress?

[Sunder]

Hi all,

As the title suggests, I'd like to know what people think about how to treat large lipo packs.

The old wisdom was that a light compression helps protect cells especially under heavy load.

I heard a bit of hearsay from RC guys that hard cases were more compression that you needed: Shrink wrap was enough and allowed for better cooling.

So I am tossing up between a custom perspex hard case for protection and compression and using fibreglass tape to allow for cooling.

Thoughts?

 

[Sunder]

Sorry, meant to put this in the Battery Tech forum. Would an admin mind moving it? Thanks.

 

[Punx0r]

RC guys run small batteries hard, so they're problems are weight and over-heating batteries. For ebike use C rates tend to be much lower and heating of most RC packs isn't an issue (expect maybe the high-energy, low-power packs like Multistars).

They are wrong about compression: it is good & beneficial. Shrinkwrap doesn't do much useful, just puts point loads on the corners. The plastic hard-cases are probably better than nothing, but still not exactly stiff in the middle and cells need even pressure over the whole surface (IIRC A123 recommends ~2psi for the their pouch cells).

It's best to use stiff plates of a material of your choice on each end of the pack, squeeze it up with a clamp and applying banding or filament tape.

 

[dogman dan]

I never tried compressing, but I VERY quickly learned to keep the c rate low. Our bike batteries are much larger, hence more expensive, than a drone pack. So it's worth it to carry at least 10 ah, or more if needed, to keep the c rate low. This eliminates the really hot pack problem. You want two years, not six months from your pack.

I'm not so sure squeezing them will do much good, if you cook em off. Warm sure, but not so hot you really notice it when you hold them bare handed.

 

[wesnewell]

If there's no room for gas expansion, what do you think happens to the pouch? Gases heat up during compression. Great for freon in an ac unit. Not so good for batteries. Even the hardcase packs leave room for expansion. And once they puff enough, the case will pop.

 

[Punx0r]

You think a contained cell will somehow experience extra heating when cycled due in some way to adiabatic compression? Just...no.

I'd ask what you think happens in a cylindrical cell like the 18650, but I know you think they're the Devil's work.

Compression reduces cell IR. How someone can argue that isn't a good thing is beyond me

 

[Sunder]

I think you might be confusing me for someone else. I have nothing against 18650s, and use them extensively. I simply haven't here for price and ease of assembly.

Edit: Sorry, thought you were referring to me, not Wesnewell. Apologies.

I'm not concerned about "adiabatic compression" (Not heard adiabatic used in that context before...) but rather, 6mm of perspex is a pretty good insulator, making the whole system closer to adiabatic.

I am using multistars, with a constant load of 0.5-1C with bursts of 6C up to 30 seconds.

The replies here also have been a bit mixed, but I am leading towards compression. Possibly only on one axis and ventilating on the other. It won't be all that effective, but better than nothing.

 

[Hillhater]

Hi Sunder,
To me its a case of theory vs reality. !
The theory says light compression/containment is benificial, but in practice there is little real feedback to verify the theory or justify the complication.
Stiff outer sides and strong fiberglass reinforced tape, is as far as i have bothered for my Ebike packs,....
....but i know you are looking at something larger than 1kWh and probably not wanting to check it too often.

 

[Punx0r]

A123 did the research for their cells in order to come up with the guideline. That's just one that has been made publically available on this forum, other manufacturers may well have done similar.

Sunder, apologies, my comments were directed entirely at Wes' post and not you. I should have quoted his post. It's just his opinions always seem contrary to accepted wisdom or convention.

 

[dogman dan]

Again with the c rate. One good way to puff em is to run em hard.

Since you were talking large, I'm assuming you won't be heating enough to worry about the insulation. A warm battery perks up, so you do want them to reach body temp, or at least room temp, when it's cold. When it's hot, they are already a bit above body temp, and run correctly, won't heat up much more.

I see no problem with compression, but again, have no personal experience with the results. On bikes, I provide a very mild compression with the coroplast boxes I make for each 10 ah brick. The box is not for compression, but for protection from chafing and corner dings. Once protected, I can just toss the pack in a motorcycle saddle bag, and they don't get dinged or rubbed. The coroplast is a decent insulator, so the packs stay warm enough to use in cold weather (starting at room temp). I don't hammer them hard enough to overheat in summer, despite the insulating box.

But in a stationary application, no weight or size penalty to worry about with a compression system.

 

[Sunder]

Thanks Punx0r and Dan.

Looks like the jigsaw and router table will be coming out this weekend. I'm going to try to make a custom case with only very minimal compression. Maybe just 1 - 2 mm shorter than the battery, then use bolts to apply a small amount of pressure. See how I go. I'm not that proficient in that kind of handiwork.

Speaking of battery temps, by pure coincidence, my internal resistance tester came today. A 12S pack of these multistars rate at 14mOhm at 4°C (fridge) and 11mOhm at 25°C (room). So, I'm expecting a 36S pack assembled to be around 35-40mOhms including connectors. Really not bad. I was a little skeptical when I read the per cell resistance was meant to be 1-1.5mOhms.

The battery I'm boosting with this was measuring 160mOhms - less than I estimated through sag, so it should be taking a bit more of the load, meaning the multistars will probably never hit the 6C I estimated.

 

[wesnewell]

Sunder, apologies, my comments were directed entirely at Wes' post and not you. I should have quoted his post. It's just his opinions always seem contrary to accepted wisdom or convention.

At one point, the excepted wisdom was that the earth was flat and the sun revolved around the earth. I can't count how many times I've proven the excepted wisdom wrong over the years. Restrict expansion too much and something has to give.

 

[Punx0r]

Those are pretty good IR results for the Multistars.

Wes, please see the flat earth myth: https://en.wikipedia.org/wiki/Myth_of_the_flat_Earth

A flat earth and earth-centric universes were intuitive beliefs that were quickly dispelled using scientific methods such as observation, measurement, experiment and logical argument. Your opinions on battery compression, 12ga spokes and RC lipo are based on nothing but your own intuition, assumptions and biases. In your own example you'd be on the side arguing for a Flat Earth (you'd at least have NutSpecial for company...).

The teams of scientists and engineers who designed, develop and test batteries are pretty smart and experienced. As average-joes (and armchair experts) we should be very cautious about dismissing their findings and recommendations.

 

[wesnewell]

There have been humans on earth for millions of years. It was only in the last 1000 or so that it was thought to be round. All my opinions are based on facts. Pouch cells are just like a ballon. Put too much pressure in them and they pop. That's fricking common sense. It's also a proven fact that when you compress gases the gas heats up. The more compression, the faster they heat up. Just another fact. A 2.5mm rod is stronger than a 2mm rod when made from the same material. Just another fact. These or facts, not just my opinion.

 

[MrDude_1]

Pouch cells are just like a ballon. Put too much pressure in them and they pop. That's fricking common sense. It's also a proven fact that when you compress gases the gas heats up. The more compression, the faster they heat up. Just another fact. A 2.5mm rod is stronger than a 2mm rod when made from the same material. Just another fact. These or facts, not just my opinion.

They're also just like a balloon.. they will expand to fill the cavity they are placed in. once they expand to match that cavity, the balloon no longer has additional pressure nor does it expand. :lol:

 

[Hillhater]

......my internal resistance tester came today. A 12S pack of these multistars rate at 14mOhm at 4°C (fridge) and 11mOhm at 25°C (room). So, I'm expecting a 36S pack assembled to be around 35-40mOhms including connectors. Really not bad. I was a little skeptical when I read the per cell resistance was meant to be 1-1.5mOhms.

The battery I'm boosting with this was measuring 160mOhms - less than I estimated through sag, so it should be taking a bit more of the load, meaning the multistars will probably never hit the 6C I estimated.

What is you IR tester ?
..im guessing its measuring AC IR rather than DCIR which would be higher.
Sag is a function of DCIR hence why your estimated IR varies from that measured ?
If you want to double check, DCIR is very easy to measure,.....search out the threads on it.

 

[Sunder]

......my internal resistance tester came today. A 12S pack of these multistars rate at 14mOhm at 4°C (fridge) and 11mOhm at 25°C (room). So, I'm expecting a 36S pack assembled to be around 35-40mOhms including connectors. Really not bad. I was a little skeptical when I read the per cell resistance was meant to be 1-1.5mOhms.

The battery I'm boosting with this was measuring 160mOhms - less than I estimated through sag, so it should be taking a bit more of the load, meaning the multistars will probably never hit the 6C I estimated.

What is you IR tester ?
..im guessing its measuring AC IR rather than DCIR which would be higher.
Sag is a function of DCIR hence why your estimated IR varies from that measured ?
If you want to double check, DCIR is very easy to measure,.....search out the threads on it.

It's ACIR.

DCIR is easy to test when you can put a decent load on a 10C x 16Ah x 150V battery!

In some ways, the actual figures don't matter, as long as the difference is linear - I.e. if the new pack ACIR says 40mOhms, and the old pack says 160mOhms, then if the real DCIR for the new battery is 80mOhms, then the old battery is 320mOhms.

If it's not linear, then yeah... I'm stuffed.

 

[Sunder]

I'm calling Punx0r the winner of this debate.

Cells manufactured with higher levels of stack pressure are found to exhibit shorter cycle lives, although small amounts of stack pressure lead to increased capacity retention over unconstrained cells. Postmortem analysis of these cells suggests a coupling between mechanics and electrochemistry in which higher levels of mechanical stress lead to higher rates of chemical degradation, while layer delamination is responsible for the capacity fade in unconstrained cells.

http://www.sciencedirect.com/science/article/pii/S037877531301197X

 

[Hillhater]

In some ways, the actual figures don't matter, as long as the difference is linear - I.e. if the new pack ACIR says 40mOhms, and the old pack says 160mOhms, then if the real DCIR for the new battery is 80mOhms, then the old battery is 320mOhms.

If it's not linear, then yeah... I'm stuffed.

it seems ACIR does stay fairly linear, but not necessarily DCIR which can be significantly influenced by SOC, Temp, etc
This thread had some useful info on IR for a few cells.. ( sadly not those Multistars though)..
https://endless-sphere.com/forums/viewtopic.php?f=14&t=73701&hilit=internal+resistance+testing
this was a typical GA cell i believe..

 

[Punx0r]

This thread has some good info on pouch cell compression, with links to a few papers and tech sheets: https://endless-sphere.com/forums/viewtopic.php?f=14&t=52244 "A123 20Ah Pressure Thoughts--and bits for experiments."

Skip to page 10 in particular for the best bits

Wes, none of those points are facts. They're an over-simplification of complex technical issues that ignores the interplay of the various components of a system.

 

[dogman dan]

FWIW,, what I do is make my box to fit tight as possible. Essentially, tight as I can wrap the tape that holds the corners together. The packs can then slip easily in and out of the box.

The main purpose of this box is not compression, its preventing chafing. But the packs can still slip around in there. how does this work.?

The packs swell as soon as you start using them. Not a lot, but they do grow. Now they fit tight, and no amount of bouncing around in a saddlebag can chafe the actual cells or ding their corners. It's not a lot of compression, but the swelling of the packs makes it tight. By the second year of use,, very tight.

So my advice is to build your box snug. loose will just let them slide around in there chafing till they swell enough.

 

[Sunder]

Not sure how many people will be interested in this. But here's the end result - electrically at least. I forgot to take photos before I fitted it to the motorcycle, and I'm not taking it out again.



The original battery is a 40Ah, 43S LiFePo4 pack with about 160mOhm of internal resistance measured with an AC IR meter.

The new battery is a 16Ah, Multistar 9 x 4S pack to get roughly the same voltage. I am under-charging these to more closely match the resting voltage of the LiFePo4s, and also to extend the life of the Multistars.

So we've gone from a peak power of 14.5kw at a sag of 48v, to a max power of 23kw, which is roughly the maximum the bike can draw at a sag of 15v. At the time I was doing 90km/h up a hill. I had to release the throttle, as I was about to run up the back of another car.

Wow. I am impressed. It now feels like a sports bike. Thanks all for your help in this thread and the other one.

 

[dogman dan]

And there you have why I still own some lipo, even though I hardly need it. That kind of performance sure is addicting.

 

[Sunder]

I'm of the same opinion. Derate the capacity 20% and the power rating 40%, and you've got the best overall batteries in rhe world.

Sure, I'd use LiFePo4 for stationary applications and LTO as super capacitors, but they're very narrow application uses.

 

[liveforphysics]

......my internal resistance tester came today. A 12S pack of these multistars rate at 14mOhm at 4°C (fridge) and 11mOhm at 25°C (room). So, I'm expecting a 36S pack assembled to be around 35-40mOhms including connectors. Really not bad. I was a little skeptical when I read the per cell resistance was meant to be 1-1.5mOhms.

The battery I'm boosting with this was measuring 160mOhms - less than I estimated through sag, so it should be taking a bit more of the load, meaning the multistars will probably never hit the 6C I estimated.

What is you IR tester ?
..im guessing its measuring AC IR rather than DCIR which would be higher.
Sag is a function of DCIR hence why your estimated IR varies from that measured ?
If you want to double check, DCIR is very easy to measure,.....search out the threads on it.

It's ACIR.

DCIR is easy to test when you can put a decent load on a 10C x 16Ah x 150V battery!

In some ways, the actual figures don't matter, as long as the difference is linear - I.e. if the new pack ACIR says 40mOhms, and the old pack says 160mOhms, then if the real DCIR for the new battery is 80mOhms, then the old battery is 320mOhms.

If it's not linear, then yeah... I'm stuffed.

AC Ri measured at high freq is unrelated to how the cell will do in a DC application (like running an EV), and almost exclusively related to cell capacitance/capacitor effects. It's used in industry for QC screening to know if you got the right foil area and that it's all wetted because the entirely test can happen in 0.2seconds, where true DC impedance takes a bit of time to measure.

The useful Ri value for the sake of vehicle performance (DC Ri), should increase resistance by at least 50-100% for every 10degC cooler the cell gets, or drop by 50%-100% in DC impedance for every 10degC warmer the cell gets (up to say ~50-70degC depending heavily on the blend of electrolytes and separator design, anode/cathode structure etc).