Cold is not
what kills your battery.
Your phone dies at the bus stop in January and gets worse every winter, and the two things are not the same fault. Cold hides capacity and gives it back. Charging in the cold takes it for good. Here is the cell drawn open, the liquid the ions have to swim through with a temperature slider on it, the V that says where your charger should live, and the one rule for a frozen phone.
Reflux Student builds and runs process models like this one, in your own Aspen Plus, from a sentence. Try it free on Aspen Plus →, or jump straight to what actually kills it.
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Reflux Student drives your own Aspen Plus V14 from plain English. You type what you want, it opens the case, makes the change in Aspen, runs it, reads the result back and tells you what it verified. Aspen does the math. You keep the judgment. Windows, your own licence, and free to start is a trial with a limit on it, not a free product.
Enter your email on the next page and the download link lands in your inbox, so you can open it on the Windows machine Aspen lives on. Windows will say it does not recognise the app the first time: click More info, then Run anyway. The certificate is new and Windows trusts it by reputation, which takes downloads to build. Nothing is wrong with the file.
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A tiny chemical reactor, cut open.
A lithium-ion cell is a roll of five thin layers: copper foil, graphite, a porous separator, the cathode, aluminium foil, wound around a mandrel and sealed in a steel can. The whole thing is soaked in a liquid electrolyte. Below, the same cell as in the reel, cut down its length, with the cold side drawn on the left and the hot side on the right, because the two halves age by two different mechanisms.
To charge it, every lithium ion has to make the same trip: swim through the liquid, cross a thin skin that forms on the graphite the first time the cell is charged (the solid electrolyte interphase, SEI), and slot in between graphite layers. Intercalation is a good word for it and a better picture is a book sliding onto a shelf.
Cold thickens the liquid
What actually kills it: charging in the cold.
Graphite sits at only 0.05 to 0.2 volts above lithium metal. Push ions at cold graphite faster than they can slot in and the anode voltage dips below zero volts against lithium, and at that point the ions stop being ions. They deposit on the surface as lithium metal: a fuzz that never comes back into the graphite, and that can grow into a short.
Every plated ion is capacity the cell never gets back, and the deposit is rough, so the next charge plates a little more on it. That is why fast-charging a frozen phone is spending its life rather than borrowing from it.
Ageing is V-shaped.
Heat does the slow version. When the cell is warm, the skin on the graphite keeps growing, and every extra layer locks lithium away for good. So there are two mechanisms pulling in opposite directions: plating on the cold side, skin growth on the warm side, and a floor between them where the cell ages slowest.
The number that surprised me most is inside one cell. A single pouch cell with one end three degrees warmer than the other aged about three times faster than a cell held uniform, and cooling it at the tabs, where the current enters, beat cooling it across the surface.
A pack is not a coin cell.
In a cold test of five electric cars, cabin heat on, the cars lost 41 percent of their range at 20 F (minus 7 C). Heat off, 12 percent. The heater is most of the loss; the cold chemistry is the rest.
A coin cell in a lab holds one temperature across its entire body. A pack in a car does not: cells in the middle run warm, cells at the edge run cold, and the coldest cell in the pack is the one that plates. Which is why the cooling system is designed for the gradient, usually to hold every cell inside 15 to 35 C, and not for the average.
A heat balance stitched onto a reaction model.
Everything above is one model with two halves. A heat balance says what temperature each cell reaches: heat generated by the current, heat carried away by the coolant, heat conducted to its neighbours. A reaction model says what that temperature does: an Arrhenius rate for the skin growth, and a plating condition on the anode voltage. The temperature the first half produces is the temperature the second half needs.
That is the kind of model Reflux builds the layer for. Describe the cell, the pack and the duty in plain English; Reflux sets it up in the simulator, runs it, and reads the result back with what it checked. The judgment stays yours.
Reflux Student builds the model and reads it back to you.
It drives your own Aspen Plus V14 from plain English: opens the case, makes the change, runs it, and tells you what it verified instead of reporting a converged result and moving on. Free to start, on your own licence.
Sources: 13 percent at minus 10 C, graphite at 0.05 to 0.2 V against lithium and plating below 0 V from the 2022 low-temperature lithium-ion review; the V-shaped ageing map with minima near +5 C at 0.2C and +35 C at 1C from the Journal of the Electrochemical Society; the 3 C gradient and tab cooling from this pouch-cell study; 41 and 12 percent range loss from AAA, 2019; the coupled model and the 15 to 35 C window from this electrochemical-thermal modelling paper. Cold does not kill a battery permanently, and ageing does not double every ten degrees; both are popular and both are wrong.
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