My last answer, of course understanding what is said is important:
Battery capacity varies with temperature due to changes in the battery’s internal resistance and the rate of electrochemical reactions. At lower temperatures, the battery’s internal resistance increases, which hinders the movement of ions and slows down electrochemical reactions, thereby reducing the battery’s capacity. Conversely, higher temperatures can increase the rate of these reactions, potentially increasing capacity but at the cost of reduced battery lifespan. For example, a battery’s capacity can decrease by up to 20% at 0°C and increase by up to 10% at 40°C.2 However, high temperatures can also accelerate the degradation of battery components, leading to a shorter overall lifespan.
Talk to anyone with an electric car that has a wide temperature range environment (such as northern USA). They will tell you about much lower travel range in winter because of the cold weather - for that temperature range typically of the order of half to two thirds the range to the range in summer.
Li-ions certainly provide less energy when cold, I don’t think anyone disagrees on that. It’s only the question of why. My original understanding was also that the cold just slows the chemical process, making it less efficient. After the idea was brought of there being excess energy when the battery cools which it can no longer hold and that can be damaging, I searched a bit about that and wasn’t able to find any mention even, iirc.
The problem is that the charge you did put into a battery at a certain temperature, can’t be maintained if you lower the temperature.
What happens to the excess charge? Where does it go?
The “Law of conservation of Energy” says that the energy cannot go lost. It is transformed.
But the energy doesn’t have to go anywhere. A battery at 100% state of charge doesn’t have any “more” stuff in it than a battery at 10% state of charge. It’s just in a different chemical state. It requires energy to be applied in order for that chemical process to get into a “charged” state. The battery releases that energy as that chemical process reverses.
The only reason for the reduced “capacity” when cold is that the chemical change that causes the release of energy simply cannot happen as quickly when the battery is cold. But no energy has to go away as the battery cools down, other than a bit of heat energy.
At lower temperatures, the internal chemical process that generates the power happens more slowly, reducing the amount of power the battery can produce. But if you heat the battery back up, it would be right back to the original performance. The energy didn’t go anywhere.
A battery swells because gas is created within the cell as the various chemicals degrade and break down. This gas causes pouch cells to expand.
Also, no matter what the state of charge a battery has, performance will drop as temp goes down. It’s not like it’s a drop in max capacity. It’s just a reduction in performance, in general. If it was a matter of energy having to go away as the battery cooled down, this would happen as a battery cooled down, no matter the state of charge.
I don’t know. But my guess is, with the lower efficiency of the chemical process at cold temperatures, that as you attempt to use power from the battery the difference in energy is lost as minor heating of the cell. An alternative is the energy was lost in the actual cooling of the cells. As heat, the motion of atoms, is energy. It could also be a combination of things, or something else which doesn’t come to mind immediately.
You’re right that energy has to go somewhere. I just haven’t seen anywhere that says a li-ion is damaged by just moving it to a cooler location if the battery is full or otherwise if the state of charge is just too high to accommodate it. Certainly open to any published research from li-ion manufacturers that shows this.
I agree. If there is something going on in a battery that causes a breakdown of the materials to create gasses to swell the battery if it cools down, and this for some reason only happens when the battery is fully charged, I’d love to see it. Honestly.
I agree on that. But as with the C-Rate value (which is rather a marketing thing), we don’t get the details for “reasons”.
As a RC plane guy, I tend to deal with batteries quite a lot. Got about 50, and these are no “low” power batteries, but high end devices that we actually drive to the limit of its capabilities. That’s where you see these happening.
Thing is, I never saw an empty batter (or unloaded to “storage” capacity → 3.8V) swelling.
Mostly, my LiFePo batteries I use as backup-power batteries / connected in parallel to the ESC.
The ESC being configured to provide 7.8V as the cells provide 7.30V - this way, when the ESC can’t deliver enough power to the servos, the battery compensates.
The thing is, this backup battery rarely gets used. This also means that the cells I had charged at max at 28⁰C in summer went back to my basement which is 10⁰C cooler.
It took one season for these cells to swell in such a way that they became unusable.
This is what my reasoning is based on (analysis is my day to day work. I look for patterns, and that is one pattern I have seen). And I tend to not believe all what manufacturers say and pretend their devices can do. Because most of it is Marketing BS.
And of course battery manufacturers will not tell you that, because it would force them to put a sticker on it: Do not put into the oven/microwave to heat the battery up for better performance. Cause that’s also the reason they have a warning in the car handbook to not drink the battery liquid, while 20years ago they explained how to fine-tune the valves and how to perform an oil change or change the light bulbs.
Regardless of temperature, simply being stored at 100% charge will absolutely cause a battery to degrade more quickly, and potentially swell over time. When fully charged, the chemicals in the battery are in a more volatile state and they will break down or “age” more quickly. As the chemicals inside break down, they can create gasses internally which cause the pouch to swell. It is very well known that spending an extended time at 100% charge is very hard on a lithium ion batteries, regardless of temperature. This is the whole reason for the battery life extender mode Framework added to the BIOS. It’s also the reason for the battery charge limit that has been in place for years. It’s also why many phones and laptops are implementing algorithms to limit the amount of time the battery spends at 100% charge. Some electric cars do the same thing.
Ever since getting some of the first available lithium batteries for RC planes and cars probably 20+ years ago, it was always the recommendation not to store the batteries at 100% charge. You want to store them at a lower charge level, but not so low that they might drop below a safe voltage level before the next use. Pretty much every RC battery I’ve had swell over the last couple decades was inadvertently left fully charged, and they were always charged and stored at the same temperature.
RC batteries are just more volatile in general, so I always charge/discharge them to 3.85V immediately after use