liveforphysics
100 TW
auraslip said:
The film that develops is because and ONLY because the electrolyte breaks down... It is a film made from decomposed electrolyte. The anode and cathode and separator could call care less if it's freezing or 100c, they just know the lithium moves much more freely when it's warmer.
And once again, you can get a range of electrolyte options from things that break down at below freezing to things that as stable and happy as can be at >100c. It all depends on what the manufacture decided to spec for the electrolyte, which depends on what the cell will be used for. To just pick a temperature and say this is the bad temp is like saying "engines get damaged over 5,000rpm" or something as silly as that. Some engines might be happy as a clam at 5,000rpm, or even 10,000rpm for the whole service life of the machine, and some big diesel engines would explode way before getting to 5,000rpm.
What I'm trying to say, is to just pick a temp number for a battery makes no sense at all. If you were writing a super generalization to cover all batteries, you might make a statement like, "exceeding 50C will accelerate the failure of your batteries", just like somebody might make the statement, "most passenger car engines will pre-maturely wear out if normally operated at over 5,000rpm." It's accurate enough to give somebody guidelines that you could consider 'generally-good-advice', but ultimately the matter comes down to the specific design of that engine (or cell electrolyte in this case), some engines designs are built to barely notice 5,000rpm and wouldn't even be stressed a bit, where some others would be rapidly on their way to the junkyard.
auraslip said:
I wrote this because you said something about other ill effects from over temp (aside from electrolyte break-down), implying that there was some other worry or concern or possible catastrophic failure mode or something that your batteries are going to experience if they get warm. The answer is, no, their isn't (on modern cells).
auraslip said:
No. This if a common misconception. If you're causing lithium movement in the cell (charging or discharging), the least damage occurs to the cell if it's warm. Ideally warm right up to the point where the electrolyte starts to break-down (which again totally depends on the electrolyte choosen.) Charging is the most damaging to do to a cell when cold (it also can't accept a charge as efficiently since it's an endothermic reaction and it needs to draw thermal energy form somewhere to charge. Generally the Ri*I^2 loss can supply this, but I've got a number of charge graphs where temperature drops rapidly on recharge with good LiPo.)
Nobody who is anywhere humans are able to survive the temperature needs to be wary of cell heating if you picked an appropriate pack. If you live in a super warm area, store your pack in a cold room or something if you're not going to ride for long time (and at nominal voltage). But while riding, the pack is going to want that heat, and happily take all the heat you throw at it, and just perform more efficiently and with less stress/damage to the pack.
If you DIDN"T choose an appropriately spec'd pack for your needs, then you can create cell overheating issues that could be addressed by cooling designs. But again, this just means you're using the wrong setup and should spend efforts/money trying to correct that rather than trying to band-aid a way to make the wrong pack work.
If you want to focus on thermal designs in a pack, it's very important, but the focus is all about how to keep the outside cells from getting cooler than the inside cells, and how to evenly get a cold pack warm (and warmed evenly) so you can charge it and operate it with the minimum damage. Every well engineered EV (like Tesla, Leaf, Volt) has sophisticated thermal systems with heaters inside the packs and insulated pack designs and heat-spreaders (or liquid for the Tesla) to keep the cells at a uniform and warm temperature. They also have sophisticated chargers that do not enable charging the pack until first the energy from the charger goes into the heating elements and the pack warms up.
We are done with the days of putting fans and spacing out cells in packs. This was the band-aid to try to get by using the wrong cells for the job, or undersized packs for the job, or whatever. Now the world has appropriate options for EV cells, and they are affordable enough to size the pack correctly for the job.
