the importance of heating/cooling your batteries

[auraslip]

Update:

It seems above a certain point heat does decrease cycle life. Above 45c their is a decrease, but it's not too bad. Around 60c things get really bad.

On the other hand, a warm, hot cell has a lower internal resistance. This means less voltage sag, less I^2R losses, and overall more capacity.

So while heat is bad for batteries, in practice most of us will have problems with cold batteries.




These are from a 2002 report on sony li-ion 18650 cells. The chemistry is different than lipo and lifepo4, and technology has progress, but this should give ya'll an idea of what I am talking about.

You can see even at 45c(113f) a substantially decrease in cycle life. 55c(122f) and 55c(133f) are even worse. It's kinda insidious because the immediate effect is small. However, at 300 cycles the difference between room temp and 55c is 1.6 ah compared to 1.3 ah. The 55c cells don't even make it to 500, while the RT cells make it to 800 with a 1.2 ah life or %60 capacity.

This might be more interesting for those interested in high power draws. Once again the difference between 45 and 50 Celsius is quite large.

The date is pretty clear. Above 113f, or 45c, bad things happen to the life span of your batteries.





So them more I read about batteries, the more I come to realize the importance of cooling. For example Pings pouch cells are tightly packed together making it hard for the cells in the center to dissipate heat. This contributes to it's low C rate. Another thing I've been reading is that all the new mass market EVs use air cooling, and some even water cool the batteries! You can see it's an important subject. Especially for those of us in warmer climates.

While it's hard to find specific information about chemistries such as Ping, Headway, or hobbyking lipo a google scholar search will come up with some decent info. It seems most of the journal articles are seeking ways to improve lithium batteries, or experimenting to find the source of their weakness's. If someone can find a good article discusing cycle fade at different temps and C rates, I would love them for it.

In any case, it seems charging and discharging over 35celcius is where live span starts to really erode. That is a shame because it regularly gets to 95f(35c) where I live! If that's not bad enough, many of the batteries many of us use (ping - headways - cheap stuff) have fairly high IRs. Combine the high ambient temp, with a high IR, and a battery often "protected" from physical damage in a closed box or bag and you have a recipe for a much shorter life.

Summer time is fast approaching, and I'm wondering how to mitigate the effects of heat! At the very least I'm going to get some fans and a temp sensor. I've been checking out the stuff cpu overclockers use. A lot of it is designed around 12v or 5v systems, so that could be easy for those of us with dc-dc conveters / switching power supplies.

Their water cooling is stuff is probably over complication and overkill, but DAMN it's cool! I had a dream of running tiny copper pipes through a battery pack to a radiator suspended from the down tube with a tiny aquarium pump to move the coolant. It would probably be better to just let the copper cool passively though like a heat sink. Heatsinks, radiators, and water cooling probably won't do to much good on an ebike, but I'm hoping some good fans will keep my batteries cool.

 

[liveforphysics]

Heating your batteries is more important than cooling.

The reason some use liquid is to maintain even temps through the packs while heating them to get them to the minimum safe charging temp more than to cool them(like the Tesla Roadster for example).


Any modern high performance pack design should be wrapped an insulated with heating pads inside it.

If you have to cool a battery in an EV application, it means you chose the wrong cells, or sized the pack incorrectly.

 

[auraslip]

Given you work in a data center and do a lot of cooling I was hoping you'd respond

What do you think about Dana and the work they've done on the Tesla and Volt?

Dana battery cooling pdf

 

[flip_normal]

There was a video posted recently of a guy giving a lecture on lithium ion cell chemistry, can't find it atm. IIRC he said that 50degC was the temp at which the organic solvent in the cells starts to degrade. My headways get up to 35degC after a half hour run at 25degC ambient and I haven't noticed any increase in voltage sag over 2 years.

 

[neptronix]

Gee, no matter how much i beat on my hobbyking lipo i can never get it hotter than the outside temperature.
Something about cells with low internal resistance not generating heat.... hmmm..

Also, who lets their house get to 95deg? or goes riding in the sweltering sun? i don't.
I guess lifepo4 could be helped out some by cooling as it is often used close to it's discharge rate limitations by us.

 

[AussieJester]

Given you work in a data center and do a lot of cooling I was hoping you'd respond

Used to work in a data center...Luke has relocated to a different state and is not working in a 'data center' anymore...He has a far more interesting job now IMO... :wink:

Gee, no matter how much i beat on my hobbyking lipo i can never get it hotter than the outside temperature.
/quote]


Same...but if i put a single 22v 25-30c pack in a Trex600 RC chopper and beat the piss out of it with some 3d flying, it's a whole different story... No wonder the rc fliers only get ~500 charges from lipo....

KiM

 

[auraslip]

There was a video posted recently of a guy giving a lecture on lithium ion cell chemistry, can't find it atm. IIRC he said that 50degC was the temp at which the organic solvent in the cells starts to degrade. My headways get up to 35degC after a half hour run at 25degC ambient and I haven't noticed any increase in voltage sag over 2 years.

That was a damn good video! I do disagree with him about not using a bms, but he worked for nasa so maybe he knows something we don't.

> Also, who lets their house get to 95deg? or goes riding in the sweltering sun? i don't.

People that don't have cars and live in texas where it get's 105 degrees some days

Heating your batteries is more important than cooling.

I think you are talking for performance during races? Or perhaps during cold weather? I'm talking about preserving cycle life during hot weather.

 

[dogman dan]

Yes cooling the batteries is important. Gotta keep em below what? 150 F? Read those specs again, you'll die long before the batteries if it's kinda hot. :lol:
My pings run exactly ambient temp in summer, so they get to about 115 max. Really perks em up above 100.

Chargers now, that's a horse of a different color. In my climate, some of my chargers have a lot of additional 1/4 inch holes in the case. Only the ones with no fan though. Been charging my pings in 100F-120F for three years now. 8)

 

[auraslip]

Yes cooling the batteries is important. Gotta keep em below what? 150 F?

That's what I'm trying to figure out.... most of the studies are done on regular li-ion though. Our ping packs are especially bad because (besides the IR) the way the pouches in the middle can't properly radiate heat. Also keeping it in a closed bag or box like I do, probably doesn't help. I'm going to get one of the hobby king temp/voltage sensors and stick it down the middle of the pack. for science!

Cylindrical batteries at least have room around them or air flow, and IIRC the headway cell holder blocks leave a little room between each cell.

My pings run exactly ambient temp in summer, so they get to about 115 max. Really perks em up above 100.

That's a cool side effect. I'm sure that's what luke was talking about with the insulated heat blankets. I've read a lot of ev racers get pretty complicated with not just heating, but making sure the battery pack is heated evenly. If you heat just the outside, the inside stays cool and this dis-balances the pack when it's used. Some of them use water cooling/heating to equalize temp. Pretty nifty.

Been charging my pings in 100F-120F for three years now. 8)

Even cheap low-tech undoped lifepo4 is pretty good investment for us mere mortals

 

[liveforphysics]

Cylindrical batteries at least have room around them or air flow, and IIRC the headway cell holder blocks leave a little room between each cell.

You choose the right cell, and you should be trying to insulate your pack ONLY. Trying to keep all the natural heat in that you can, and if you're in a cold area, then you're going to need heaters if you're looking for best cell range, performance, cycle life, etc.

If you choose the WRONG cell (or spec'ing the pack too small) for an EV application (as many folks do, due to cost or perceived safety or whatever), then you might have need to cool them. But again, this is a band-aid to cover choosing the wrong cell/battery in the first place.

If you choose the right stuff, you want to be insulating it not cooling it.

 

[neptronix]

If you choose the WRONG cell (or spec'ing the pack too small) for an EV application (as many folks do, due to cost or perceived safety or whatever), then you might have need to cool them. But again, this is a band-aid to cover choosing the wrong cell/battery in the first place.

yep, yep, yep & yep

 

[northernmike]

Suppose I intend to run LiPo in conditions varying from -20C to +30C. (-4f to 90f) (Toronto, Canada)

How do I manage the pack, thermally, based on the analysis of the OP?

The more >I< read about Lithium batteries, the more I realize that chemistry, above all else, determines available output.

The better cells, AFAIK, don't run into problems until well above 90f.

And, most importantly, they don't get to 90f as a result of high-rate discharge. Internal resistance heating on high quality LiPo in "normal" ambient temps ends up BOOSTING output, is my understanding.

And yet, until a year ago, my girlfriend believed that storing AA alkalines in the fridge was worthwhile... :?

Can someone show us a plot of internal resistance vs temp for the cells in question? This seems a bit redundant, otherwise, no?

 

[auraslip]

I'm looking at these kits:

http://www.quadrikits.com.pk/temp.html

I figure I can rig up some switches to switch temp probes, and monitor several different temps. I know they only go up to 99c, which isn't high enough to monitor hubs. It'd be more just for fun. I know their are better cheaper options, but I'm wondering about the kits. Is it just a board, and I have to order the parts? It it pre built? What's the deal? Has anyone used this site? They have some delay on timer kit for $5, which would be really useful for a precharge relay I'm making!

 

[auraslip]

> Can someone show us a plot of internal resistance vs temp for the cells in question? This seems a bit redundant, otherwise, no?

Search google scholar for lithium temperature and batteries. Report what you find!

You choose the right cell, and you should be trying to insulate your pack ONLY.

So basically what you are saying is, "Use HK lipo or a123 or GO HOME?"

And you, as always, are right. A proper battery pack SHOULD need only insulation, but on a bike or car that is practical for most of us, including G.M., Nissan, and Tesla, we are forced to use inferior cells. I'm sure that will change rather quickly, but as of right now I can't afford a123 or lipo.

Hopefully in ten years every tow truck/aaa guy will carry a HK pack of nano-tech lipo in his pocket to jump start cars

 

[oldpiper]

IMHO, storing your alkaline batteries in the fridge is OK. The increase in internal resistance slows the self-discharge rate so they keep more of their original power. But, obviously, you have to warm them up to room temperature before you use them. :lol:

Cameron

 

[neptronix]

Suppose I intend to run LiPo in conditions varying from -20C to +30C. (-4f to 90f) (Toronto, Canada)

How do I manage the pack, thermally, based on the analysis of the OP?

Dr Bass has a couple threads in the battery section about running the batteries in the cold, check those out.

I dunno if anyone has done this, but dealextreme sells some little glove warmers, and their power usage is very low. I am sure they can be used to keep the batteries warm in freezing cold, providing a massive performance increase in the winter, at a very low power cost ( sucking down maybe 200 watts, yet giving you 2x more capacity )

heat.. good question!

 

[liveforphysics]

Nitrous bottle warmers.

They are flexible resistive heating pads sewn into wattages from 100w to 1000w, then impregnated with silicone to form a little 2mm thick flexible silicone rubber pad with wires coming off it. You could easily stick them between stacks of packs or whatever. They cost between $20-50 each, and in 10 years, I've never had one fail on me yet from getting handled roughly on nitrous bottles.

 

[regmeister]

deleted

 

[grindz145]

There has traditionally been a energy to power density tradeoff that is slowly going away. Tesla uses extremely energy dense cells that are cheap, and have a relatively high internal impedance.

 

[auraslip]

These are from a 2002 report on sony li-ion 18650 cells. The chemistry is different than lipo and lifepo4, and technology has progress, but this should give ya'll an idea of what I am talking about.

You can see even at 45c(113f) a substantially decrease in cycle life. 55c(122f) and 55c(133f) are even worse. It's kinda insidious because the immediate effect is small. However, at 300 cycles the difference between room temp and 55c is 1.6 ah compared to 1.3 ah. The 55c cells don't even make it to 500, while the RT cells make it to 800 with a 1.2 ah life or %60 capacity.

This might be more interesting for those interested in high power draws. Once again the difference between 45 and 50 Celsius is quite large.

The date is pretty clear. Above 113f, or 45c, bad things happen to the life span of your batteries.

Now, will your battery get there? Lets looks at a pack of 2s2p pack of 5ah 10s 45-90c nanotech lithium (80v10ah). Lets say we want to draw 40 amps, or 3c.

40amps x 80v = 3200watts / (40 cells ) = 80 watts per cell.

Heat produced is (I**2)R or 80 squared time resistance in this case:

These cells are 0.8mOhm each.

61.25 * 61.25 * .0008 = 5.12 watts of heat per cell. There are 40 cells in the pack, for a total of 204.8 watts of heat. If we run until the pack discharges in 15 minutes it will have produced 184300 joules.

How much does that heat the pack?

The amount of heat energy (q) gained or lost by a substance is equal to the mass of the substance (m) multiplied by its specific heat capacity (Cg) multiplied by the change in temperature (final temperature - initial temperature)

q = m x Cg x (Tf - Ti)

Lipo has a specific heat capacity of 1010 j/kg/Celsius. Weight is 5780 gm. Joules is 184300. Initial temp is 35 celcius (a 95f texas day).

Tf-35 = 184,300 / (5780 * 1.010) = 66.57C

DISCLAMER - I'VE NEVER DONE THIS SORT OF WORK BEFORE. I MIGHT BE REALLY OFF - DISCLAIMER

This little test assumes:

no heat escapes
a full 15 minutes burn at 40 amps
this assumes no SUN. most bags are black, and attract a LOT of heat from the sun in the summer.

Those with batteries in trunk bags that are insulated for groceries, or in wooden box should take heed.

 

[auraslip]

The more >I< read about Lithium batteries, the more I realize that chemistry, above all else, determines available output.

That is the truthiest thing I've read here. All of this is dependent on the recipe and process used to make the battery.

 

[liveforphysics]

auraslip- Your numbers are extremely extremely wrong. lol

In your example at 40amps on a 10ah nano-tech pack, it's 0.32watts of heating per cell over a 15minute period. Or, 288joules of heating per cell.

This means you get a total temperature increase of 1.90degC on your nano-tech pack discharging at 4C.

 

[auraslip]

oops
forgot to divide the 3200 watts by 4c.

(((3200 watts / 4c)/40)**2)*.0008 = .32 watts per cell or 288 joules per cell. That's 12.8 watts - 11520 joules for the pack.

tf-35 = 11,520 / (5780 * 1.010)

1.9 celsius gain is like 37 celsius.

Damn that is a good battery.

What happens if it's a 20ah and we go for half an hour though?

(((3200 watts / 2c)/40)**2)*.0008 = 1.28 watts per cell or 768 joules per cell. That's 51.2 watts - 30,720 joules for the pack.

tf-35 = 30,720 / (5780 * 1.010)

That's a 5.26 celsius increase, and you'd be running at 40 celsius by the end.

 

[jag]

I'm looking at these kits:

http://www.quadrikits.com.pk/temp.html

I figure I can rig up some switches to switch temp probes, and monitor several different temps.

First you want to determine if you have a problem. Just measure your Ping battery temp after a run and see how much it rose.

My take on battery life and temp:
I store my batteries cold. This can increase life substantially.
I use my batteries (reasonably) warm. (25-40C). This increases performance.
Above 40C would not be good for many battery types (there are extreme heat tolerant batteries used in for instance missiles and similar military applications), but like already mentioned if the batteries get very hot they likely have a too high internal resistance.

Before plunging for the 72V 10Ah headway pack, check what the internal resistance and compare that to the 20Ah ping you have. Real experimental data would of course be best, so if some volunteer would measure their headway pack at say 20 or 30A that would be better than using manufacturer data. Then do the same with your ping.

 

[auraslip]

hey ya'll I've taken the formulas and created a spreadsheet you can use - it's really sloppy, but usable. I'll clean it up tomorrow. hopefully we can find a way to add cooling into the mix the spread sheet is available to any one - feel free to edit it....i'm sure you guys can add some good stuff to it.

https://spreadsheets.google.com/ccc?key=0AuLYJj0UJIOndHlaejFwLVRzVTdxenBKNHJTRjROUlE&hl=en&authkey=COLxqNYI

One of the interesting things I've found is that at 1c the pack has more time to warm up. For example the hypothetical pack above discharged for an hour at 1c heats up by 8 Celsius. Of course it would be pretty easy for that to radiate off with air flow. However, I think that with pouches, you'd have a hard time cooling the packs in the middle.

Still though, even on a hot day the pack wouldn't get into the danger zone. Of course this is with the latest greatest battery. It's gonna be a lot harsher with cheaper batteries. I need to find out the specific heat capacity of lifepo4.