“Why does my range drop when it’s cold?” is one of the most common questions we hear.
While some drivers fear frigid winters and others have to navigate fiery summers, nearly all EV owners have questioned the effect of temperature on their vehicles. This article is designed for a general audience, but we want to include some technical bits for our more technical readers. We’ll be publishing some Battery 101 guides soon for people who want to learn more.
What does cold weather do to EV range?
In short, cold weather reduces EV range. AAA tested the range effects of 20F degree weather on several popular EVs and found that temperature alone could reduce range by 10-12%, while the use of in-vehicle climate control could amplify range loss to 40%. Idaho National Labs reported that cold weather can increase charging times by almost threefold, as seen in this chart by AutoBlog.
Why does the cold affect lithium ion batteries?
Cold weather slows the chemical and physical reactions that make batteries work, specifically conductivity and diffusivity, leading to:
- Longer charging time (increased impedance)
- Temporary reduction in range (lowers capacity) which is why some vehicles have auxiliary batteries to support A/C and heating systems that would otherwise pull charge away from the primary battery
Even though diminished range effects are temporary, you should charge your EV with caution when it is below freezing outside. When you must charge in freezing weather, make sure to warm your battery above freezing first or use very slow “drip” charge. Be particularly vigilant to avoid fast charging or high voltage charging when the battery is below freezing.
One important thing to remember about battery temperature is that most vehicles do have some sort of temperature regulation in their battery management system (BMS). This means that if your vehicle is turned on or plugged in, energy will be drawn to keep the temperature in a healthy range. The two outliers for this are Nissan Leaf, which only has thermal regulation kick on when the temperature is below 1C (33.8F), and Tesla, which will activate thermal management even if the vehicle is off or not plugged in. This temperature regulation protects your battery health, but can also cost you some range.
Charging in cold weather can pose long term risks: lithium plating
If charging occurs at sub-freezing temperatures (below 0 C/ 32 F), it is possible for long term damage to occur from lithium plating. Lithium plating is a metallic build up at the negative battery node (the anode, where the energy goes during charging). In short, cold weather makes ions flow through the battery more slowly, causing lithium to build up outside the node and turn into an inert metal. This metal disrupts the future flow of energy and uses up some of the lithium that is supposed to power the battery.
If you’re interested in a little more detail: battery anodes are made of materials, like graphite, that have lattice-like structures. This is important because when a battery charges, lithium ions move from the cathode into the anode and are stored in this grid-like structure. This process is called intercalation. Force (in this case, current) is needed to push the ions into the anode and lodge them in the grid. If this process happens when it is cold out, the ions enter the anode more slowly and the build up of lithium outside can form a metallic plating. Some of these ions will gradually enter the anode over time, but some will remain plated outside, permanently reducing capacity and increasing internal battery resistance.
On the other hand, it seems fine to use an already charged lithium ion battery when it is cold out. Short term reduced range will be noticed since the cold weather inhibits ion flow, but there is likely no long term damage.
What does heat do to my EV battery?
Temperature is known to have a big influence on the rate of Li battery degradation. One of the main reasons that it is so critical is because temperature affects the rate and efficiency of chemical reactions inside a battery. Higher temperatures, (or higher voltages - but that’s another article), generally lead to faster reactions. This often means that the “unwanted” chemical reactions that make batteries degrade happen faster at higher temps.
If you want to get into the nerdy specifics, high temperatures negatively affect the Solid Electrolyte Interphase (SEI) - a layer of inactive lithium that forms on the anode surface. The SEI is made up of lithium salts that react with the electrolyte solvent and become inert. Unlike lithium plating, this layer is a necessary evil: while it does use up some of the lithium available to the battery, but also makes the electrolyte stable enough to last and helps protect the anode from corrosion. This protective layer forms quickly when the battery is first used and its formation explains the sudden drop in capacity seen in new batteries. To clarify: this layer is not the same as lithium plating. The SEI is permeable to the lithium ions that make the battery work but impermeable to the electrolyte.
The problem is that high heat can affect the composition and organization of the protective SEI layer, triggering reactions that use up too much active lithium or creating inert compounds that prevent ions from flowing freely.
The exact way that the heat degradation mechanism works is different for batteries at rest, batteries charging, and batteries being cycled. The heat effects also differ between specific battery chemistries. In most cases, though, higher temps will lead to faster degradation.
What happens when I store my EV in the heat?
A process known as “calendar aging” refers to battery degradation that happens regardless of use. It is largely due to growth in the protective layer around the anode (SEI). At high temperatures, reactions happen faster and its protective structure can degrade. The physical reason that heat causes faster degradation is because ions and other battery particles move through this layer via diffusion, which is directly correlated to temperature. Heat increases the rate of chemical reactions so much, secondary reactions may also occur at high temperatures, creating new inert compounds and increasing impedance, according to studies by Vetter et al.
What happens when I charge my EV in the heat?
Another way that high temperature damages batteries is during charging. High temperatures increase the effective force of the electric current that drives lithium ions from one node of the battery to the other, causing physical stress and damage on the receiving end. The higher the temperature - or the higher the current - the more stress fractures and damage the battery node experiences. All these little fissures and cracks become surfaces for secondary reactions, using up available lithium and creating compounds that hinder the free flow of energy.
Again, we can be a little more technical for those who are curious. Intercalation, discussed above, is necessary to make a lithium ion battery work, but does cause physical stress to anode. When you charge at higher temperatures, the lithium ions intercalate more forcefully, often generating small cracks and fissures. This creates new surfaces for chemical reactions between the anode and the available lithium, mainly new areas for extra SEI growth. Any additional reactions use up lithium and lead to capacity fade. Additionally, if all the ions can’t find storage within the anode grid, they may get stuck outside the anode and undergo chemical reactions that create inert substances, including lithium plating. Both of these effects can increase the internal resistance of the battery and lower its available power.
Monitoring Range Fluctuations
Recurrent's range reports for EV drivers make it easy to monitor the effects of climate and temperature on your vehicle. Each month you receive a recap that analyzes short term fluctuations and long term battery degradation. Here's a sample report for a BMW owner.
Temperature analysis is built into the report and monthly prompts remind you to follow trends. Connect for free to monitor your EV range.