Using phase change material to cool li-ion cells. Organic, salts, bad idea ? - Page 1
Forum section: electronics / general

author: Hydrochloride
author_rank: Contributor
timestamp: September 11, 2025, 08:21:57 pm
content: Hello, everyone. I hope you can guide or illuminate me on this topic.Well, I'm thinking of a very economical way to keep the temperature of some lithium-ion cells very stable at a certain value in degrees Celsius. Let me explain. I have a charger that can operate in ambient temperatures up to 50 degrees, and I'm seeing that my cells are heating up 10 degrees above that, so they would be charging at 60 degrees. After searching online, I've seen these phase change materials. In order to cool or retain cells temperature.The way this material works is by using the solid-to-liquid phase change to remove heat from the element it's in contact with. So, I could embed the cell with this to keep the temperature from exceeding 50 degrees for a time. Obviusly you have to adjust the material you use, the quantity of it, the charging time, etc., so you don't saturate all the material and have it all turn to liquid, which would cause it to lose its effect. Also only cool the object touching it, if the ambien temp increases, the PCM temp increases too.My battery pack is quite compact. Two 21700 cells in series. In a plastic case.My question is whether this is a common thing to use. From what I've been looking up, it seems so in EV and e-bike environments. (I'm attaching a couple of reference photos).I'm having a hard time finding information in practical videos—lots of papers and simulations in ANSYS or CFD—but nothing that clearly shows how the material behaves (how it liquefies, solidifies, obviously the material will contract and expand with the phase change...).I'm thinking of ordering paraffin from Rubitherm or CrodaTherm. Also looking up to non-organig PCM salts from same brands.I'm still new to these materials. I need to order samples to see how they behave, but if you know anything about the topic, I would be very grateful.If you have already gone this route with battery packs or the like, I would appreciate hearing about the pros, cons, and details to keep in mind.

author: thm_w
author_rank: Super Contributor
timestamp: September 11, 2025, 11:02:53 pm
content: You'll need to do some calculations, how much heat is put out and how much heat the PCM can absorb.It seems like a huge amount of effort to just go from 60C to 50C, which does not sound like much heat in the first place.I would compare it to passive cooling, with say an aluminum enclosure. Or active cooling, if your charger stationary then you can add a fan onto it, like many cordless battery tool chargers have.

author: kite31
author_rank: Regular Contributor
timestamp: September 11, 2025, 11:16:09 pm
content: Alloy heatsink. Fan FTW.

author: Hydrochloride
author_rank: Contributor
timestamp: September 12, 2025, 07:13:48 am
content: The thing is, according to the cell's datasheet, to maintain specifications, it needs to be charged at 45°C. Since it heats up by 10°C, that limits me to environments with a maximum of 35°C (many places will reach that ambient temperature inside maintenance cabinets).I've updated the charger's software to allow the cell to be charged up to 50°C, which is a 40°C ambient temperature. It charges, but it gets an intermittent over-temperature warning. The charger stops, and once the warning clears, charging restarts. However, it takes a long time to resume because of thermal inertia.I'd like to remove heat to be able to charge at a 50°C ambient temperature.Is it a problem to charge NCA cells at 60°C?Due to the temperature increase during charging, it will rise to this temperature. Clearly, there will be a loss of life cycles, but I want to know if it could be a safety issue. That's why I was thinking about something to remove heat. If possible, something compact without the need for active refrigeration. I'll give the heat sink idea another thought. But for the commercial product, it seems like a bulky solution because it would be sold as an accessory product, not embedded in the charger/battery. And if it needs to be embedded, the mold will have to be redone to implement the solution.

author: Simmed
author_rank: Frequent Contributor
timestamp: September 12, 2025, 09:51:21 am
content: heat pipe made with Cyclopentane inside ( about 50C boiling)https://www.engineeringtoolbox.com/boiling-points-fluids-gases-d_155.htmlor peltier

author: Hydrochloride
author_rank: Contributor
timestamp: September 12, 2025, 09:59:01 am
content: Nah, I think I'm ruling out the PCM. It would be fine to make a prototype as a hobby at home, but for a commercial product, as you say, it needs extensive analysis and there are many edge cases. So inconveniences I've been seeing while searching online and reading manufacturers' datasheets.Organic PCMs are flammable. Like anyone with common sense will think, coating a cell with a flammable material is not a good idea, because in the case of thermal runaway you have made the system less safe.And not just a little bit. A paper in MDPI have conducted failure tests to evaluate this solution for controlling or reducing thermal runaway. They found that using organic PCMs worsens the flame, creating a small inferno in the chamber.adding image. Check stage IIpaper ref :https://www.mdpi.com/2313-0105/9/10/513Okay, then let's check salt types. They are less flammable (but still do ignite) and have more heat storage capacity. But they have their own issues too. They corrode metals, so now I need to make sure the protection circuit module and also the battery's metallic case elements are not in contact with this liquid/wax (it will change phase). Moisture is also a variable now. Battery cases can be sealed, but it looks like to use these PCMs you need a dedicated container in contact or have a really good seal if directly applied inside your container. I also don't know how the PCM is applied during the production phases to avoid moisture absorption during assembly lines.I did the math using the Rubitherm datasheets. I measured the energy in Wh given off during charging when the cells were in the 50ºC to 60ºC range. This showed that to keep the cell's temperature around the PCM's melting point (around 50ºC), I need to dissipate approximately 8 Wh of energy.- With a quick calculation, if the SP50gel (salt inorganic type) can retain 60 Wh/kg, I need to use 133 grams.- If I use an organic option like RT54HC to have a less corrosion, I need 140 grams.(It melts at 53-54ºC, so the required mass will be a bit less than calculated, but to keep it simple, let's use 50ºC). You still has the flammability risk in thermal runaway.It seems to be used in two ways. Injecting the material into the holder, but I imagine it won't faithfully follow the datasheet's behavior then. Or using it in a compartment adjacent to the holder. In both cases, there's a lot of process to do it well and in a controlled way. In an EV, it makes sense because of the number of cells and especially with high charging currents. But for my application, it's not viable at scale.Too many things that I don't control. As said fun for a project in free time.

author: jpanhalt
author_rank: Super Contributor
timestamp: September 12, 2025, 10:20:10 am
content: Look up Glauber's salt (Na2SO4 dot 10H2O) melts at about 32°C. That temperature can be modified.My first experience with it was about 60 years ago when I got a nice crystalline product from an organic synthesis. My clue was the yield was about 200%. My advisor recognized it immediately. It is non-flammable.Despite that, I don't think your approach is very practical. Use a water bath or large heatsink to dissipate the heat.

author: Chesthair Ascot
author_rank: Newbie
timestamp: September 12, 2025, 04:59:21 pm
content: QuoteI wouldn't recommend generalizing all cells of one category in this way. Each cell model will have a datasheet that lays out the safety requirements. Generally they'll give you a table of charge/discharge current that is allowable at various temperature points.Instead of taking an educated guess on temperature rise during charge, a safer way would be to add thermistors in various spots and then feeding those temperatures into a BMS that can open contactor if something gets too hot. For example, if you're doing pgroups with more than one cell and a bond to a parallel cell breaks, you can get a lot more heating in the remaining cells, especially during charge. You'll also get a temperature gradient from the outside to inside of the pack.One other note is that in 18650 cells, the jellyroll is terminated in the bottom of the can, so you're going to get a lot more thermal conduction through the base of the cell than through the sides. On the Alta Redshift pack, that's why the cells were epoxied down to an annodized aluminum heat-spreader plate. That said, that approach is also hard since you have to use an exotic epoxy with spacer microbeads in it to get the required electrical isolation, since relying on the annodizing isn't enough.

author: jpanhalt
author_rank: Super Contributor
timestamp: September 12, 2025, 05:47:38 pm
content: Epoxy filled with BN (hexagonal boron nitride (h-BN) is electrically insulating, and thermally conductive. BN is relatively cheap and not exotic in my opinion. I keep a small bottle handy. (It can also be used as a dry lubricant like graphite.)

author: thm_w
author_rank: Super Contributor
timestamp: September 13, 2025, 01:43:27 am
content: Quote from: Hydrochloride on September 12, 2025, 07:13:48 amYou're not going to get good passive cooling without bulky heavy materials. Which is why most products end up with fans.

author: kite31
author_rank: Regular Contributor
timestamp: September 13, 2025, 02:54:08 am
content: Quote from: Hydrochloride on September 12, 2025, 07:13:48 amI did not spot mention of this being a commercial product for sale, in the opening post. Is this for manufacture or are you modifying an existing device? While fan cooling is a great multiplier of heatsink effectiveness, as I implied, I have no idea of the size, proportions or constraints of the device about which you are asking. The discussion thus becomes rather hypothetical. More information? Pictures?

author: chen liming
author_rank: Newbie
timestamp: September 30, 2025, 01:58:41 am
content: Chervon,A Chinese power tool manufacturers.Phase change materials were once widely used in EGO FLEX SKIL power tool batteries.However, the phase change material will absorb water vapor in the air and eventually turn into liquid at normal temperature, eventually causing a short circuit.FLEX SKIL's latest battery has eliminated phase change materials.