Li-ion Batteries in Winter

After listening to my coworkers whine about their cell phones going dead in the cold, I decided to look up an explanation for why it happens. (Keep in mind that I work outside a lot of the time and this has been one of the coldest winters in a generation. Temperatures hovering around 0° F every morning is getting monotonous.) It turns out that my habit of keeping my phone in an inside pocket, where it won't get lost, also protects it from an issue arising from the chemistry and construction of lithium-ion batteries.

Lithium-ion (Li-ion) batteries are becoming ubiquitous; they're what power most cell phones, small electronic devices, and common cordless tools. Chances are that your rechargeable flashlights, radios, etc. all have lithium-ion batteries in them and they are becoming common as replacement batteries for motorcycles and ATVs. Lighter and having a higher power per pound than lead-acid or nickel-cadmium batteries, they are also the choice of most electric car companies, but after looking into the issue of phones “losing” 70-90% of their charge when exposed to temperatures below freezing, I don't expect to see very many electric cars this far north. I know the cars have a heating system to keep the batteries from freezing, but since it uses power from the battery to produce the heat, that will drain the battery and reduce the range of the car even further.

The problem comes from the way all batteries work, with a slight twist on the lithium-ion system. Batteries work by storing energy in the form of a chemical reaction that is reversible; when you charge a battery, you are forcing electrons into it to move ions from the cathode (positive post) to the anode (negative post). In Li-ion batteries, the ions actually squeeze themselves into the spaces between the molecules of the anode and cathode instead of chemically reacting with the materials (bumping other ions off and taking their place). Incidentally, this is why some Li-ion batteries will swell as they age -- the graphite that the anode is made of doesn't contract to its former size once the Lithium ions have left during discharge. Samsung has a bad reputation as a battery maker because they ignored this tidbit of information.

Cold temperatures slow down all chemical reactions. In the case of Li-ion batteries, as the temperature drops the Lithium ions may follow an alternate reaction and “plate” out on the surface of the anode as metallic lithium during charging. This “plating” is not fully reversible, and the battery will lose some of its charge capacity because of the lack of free ions to be moved around. The layer of metallic lithium will also create a barrier to the free flow of ions which increases the internal resistance of the battery, causing heat. A much bigger problem is that you now have a layer of pointy, conductive, metal crystals on the anode, and if they puncture the insulating barrier, being forced against it by the normal swelling of the anode during charging and the thermal expansion from the added heat, they will cause an internal short-circuit which can be very energetic.

“Energetic chemistry” is a euphemism for an explosion. For this reason: NEVER CHARGE A FROZEN LI-ION BATTERY. It is theoretically possible to charge a frozen Li-ion battery safely, but the charge time would be measured in days instead of hours. Always warm the battery up to as close to room temperature (70°F) as you can before trying to charge it. A much more technical explanation can be found here.

A secondary problem with the slowed-down reaction inside a cold Li-ion battery is the fact that the battery won't be able to produce the voltage that a cell phone expects to see, with the circuitry inside the phone seeing the reduced voltage as an indication that the battery has lost its charge. Once the battery has had a chance to warm back up, the phone should read the charge more accurately. This is what is affecting my coworkers who keep their phones clipped to the outside of their coats, and once they let their phones warm up, they usually show most of the charge that they should.

The main reason I keep my phone in an inside pocket is because I have had them fall out of pockets in my outerwear without my noticing. When I'm wearing four layers of clothing, I lose some of the cues that something is missing. Keeping it warm is a side effect that ensures I have a working phone when I need it. If I'm carrying a radio for communications and it uses Li-ion batteries, you can be assured that it will be kept warm as well.