Batteries and heat

Desertprep

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I have been doing my homework about diy battery building. I keep running into references to heat killing batteries. All of the solutions I have seen so far talk about cooling from the outside of the battery pack - fans, fins, etc. That would depend on heat transferring from within the battery pack to the outside, where it would be carried away. Is there any way (or is there a need) to manage heat from within the battery pack?
 
Perhaps some of the best examples can be seen in OEM automotive applications where the battery cooling circuits are designed to deal with high and low temperature issues.

I think Tesla uses a glycol filled cooling ribbon snaking between the cells. From memory it requires safety features for isolation if the battery is flooded. GM use a glycol system with heat transfer plates pressed against the prismatic cells.

I suppose that for a DIY solution outside cooling poses less risk in the event of failure.
 
I started a discussion thread here:
https://endless-sphere.com/forums/viewtopic.php?f=14&t=55074

But in general, yes heat + high states of charge + long periods of time will decrease the life of your battery.

That's basically Arrhenius' equation - Arrhenius equation.JPG

The degradation reactions happen twice as fast for every 10'C increase in temperature.

If you can soak heat away however you can, usually by homogenising the heat within a battery pack, and then using conduction and convection to wick it outside of the battery pack, you're doing better than nothing at all.
 
Building a battery that required active heat management would be my last preference. I do wonder what compromises you made that forces you to require active cooling.

I've only had one battery that could benefit from cooling, and it was being drained at 8C (10c rated) in a very confined space. My compromises were for price and space. If I had more space and money, the battery wouldn't need cooling.
 
Here is an interesting scholarly paper on the subject of Li-Ion capacity degradation and temperature.

Here is a link to the paper Effect of Temperature on the Aging rate of Li Ion Battery Operating above Room Temperature

And the most interesting graph - Charge (Q) curves for a number of cycles vs temperature

It seems the short answer is, under 35°C LI-Ion batteries do not suffer appreciably, by 55°C the capacity loss is double the capacity loss at 35°C, and triple that at 25°C.

[EDIT] added degree symbols
 
I have some 18650's on a 4-battery discharge-charge tester that charges at 0.5 or 1 amp per battery. They are running 31.9°C at 500ma in a 26°C room, and 35°C at 1000ma charge current in a 26°C room. Some of the heat may be coming from the charger, but this is giving me the idea that batteries may run 10°C above ambient in free air under quite small currents. More experiments on real world battery packs coming soon.

[EDIT] Add degree symbols
 
llile said:
It seems the short answer is, under 35C LI-Ion batteries do not suffer appreciably, by 55C the capacity loss is double the capacity loss at 35C, and triple that at 25C.
Wow ! Careful with your units there !
When talking about batteries 35C ..55C can be interpreted in more than one way !
 
Yes the degree symbol can be hard to type here. I use the apostrophe for degrees Celsius. I guess we should all use Kelvin :)
 
Journalistic practice is to write out the full word once, then use abbreviations. That makes it clear in most cases. To avoid confusion in this post, I just avoided using the term c rate.

I take a practical approach to hot battery, since I live in the desert. This is for typical bike use, not racing, or stationary applications.

Firstly, I cant do shit about it being hot out there, 40 degrees centigrade out there is the reality.

Secondly, if my batteries heat past that in use, then I have a problem to solve. Usually by making the battery bigger, lowering the discharge rate, so the battery no longer gets any hotter than it starts out. Or, switching to a better battery that can stand the discharge rate of my bike.

Thirdly, store as cool as you can conveniently. I'll never bring my battery inside to be cooler again, so now I'm storing them in an insulated box. It still gets hot in there, sometimes 30c but never as hot as the days high temp.

But a stationary battery, large and expensive, it would make great sense to climate controll it. If nothing else, inside very very thick insulation.
 
tesla cranks the temperature into the 50's (celsius) when using ludicrous mode. it also takes about 5~10 minutes to heat the pack to this temperature. that lowers the internal resistance (IR) so the pack den deliver more power. it peaks around 1450 Amps in that mode.
but in short: for lifetime i would try and keep the cells below or in the low 40's. this can be simply obtained by reducing the load per cel, aka: more cells/bigger pack.
 
Yes I've done a 300 A to 250 A discharge test (a little over 1 C) on a Tesla module and the cells with thermistors most definitely get up to about 55'C. They cool down fairly quickly though. I read that the Model 3 cooling system is better still, however the packing density is crazy and hard to get right...
 
Referring back to the original post, you asked if there was any need to actively cool the pack.

The answer could be yes or no. As posts above point out, in some situations you might choose to heat the pack. Like to win a drag race. Or if you are hammering your battery into dogshit, you should try to at least cool it off when you are done, and its way over 40centigrade.

So to really answer your question we need more info what you want your battery to do, last forever, or win that race.

To last longer the general practice is to undercharge, under the discharge rate, and keep it at least below 40 c. 40c =104 farenheight. When storing fully charged, keep much cooler than 40c. My practice in use is to store no more than 90% full, then top off to ride. In winter I may store fuller, but in summer I try to store less than half charged, since they may be above 30c day or night, in the outdoor storage.

Out on the road on a hot day wind cools the battery enough to keep it close to ambient temp, but do try to park in the shade when not riding. A box in the sun in my climate will get its contents to food cooking temps after a few hours, way above 60c. you sure don't want that. Just shade will keep a battery no more than 40c in my climate.
 
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