HERE IS AN EXCELLENT EXPLANATION REGARDING EV CHARGING AT 32F OR LESS
Explanation by Expert
'Cold temperatures' is awfully vague. First, let me actually specify some real, hard numbers.
Do not charge lithium-ion batteries below 32°F/0°C. In other words, never charge a lithium-ion battery that is below freezing.
Doing so even once will result in a sudden, severe, and permanent capacity loss on the order of several dozen percent or more, as well a similar and also permanent increase in internal resistance. This damage occurs after just one isolated 'cold charging' event, and is proportional to the speed at which the cell is charged.
But, even more importantly, a lithium-ion cell that has been cold charged is NOT safe and must be safely recycled or otherwise discarded. By not safe, I mean it will work fine until it randomly explodes due to mechanical vibration, mechanical shock, or just reaching a high enough state of charge. See URL
Now, to actually answer your question: why is this?
This requires a quick summary of how lithium-ion batteries work. They have an anode and cathode and electrolyte just like any other battery, but there is a twist: lithium ions actually move from the cathode to anode during charging and intercalate into it. The gist of intercalation is that molecules or ions (lithium ions in this case) are crammed in between the molecular gaps of some material's lattice.
During discharging, the lithium ions leave the anode and return to the cathode, and likewise intercalate into the cathode. So, both the cathode and anode act as sort of a 'sponge' for lithium ions.
When most of the lithium ions are intercalated into the cathode (meaning the battery is in a fairly discharged state), the cathode material will expand slightly due to volumetric strain (because of all the extra atoms wedged in between its lattice), but generally most of this is intercalation force is converted to internal stresses (analogous to tempered glass), so the volumetric strain is slight.
During charging, the lithium ions leave the cathode and intercalate into the graphite anode. Graphite has is basically a carbon biscuit, made of a bunch of graphene layers to form an aggregate biscuit structure. American biscuit structure.
This greatly reduces the graphite anode's ability to convert the force from the intercalation into internal stresses, so the anode undergoes significantly more volumetric strain - so much so that it will actually increase in volume by 10-20%. This must be (and is - except in the case of a certain Samsung phone battery anyway) allowed for when designing a lithium-ion cell - otherwise the anode can slowly weaken or even ultimately puncture the internal membrane that separates the anode from the cathode, causing a dead short inside the cell. But only once a bunch of joules has been shoved into the cell (thus expanding the anode).
Ok, but what does any of this have to do with cold temperatures?
When you charge a lithium-ion cell in below freezing temperatures, most of the lithium ions fail to intercalate into the graphite anode. Instead, they plate the anode with metallic lithium, just like electroplating an anode coin with a cathode precious metal.
So, charging will electroplate the anode with lithium rather than, well, recharging it. Some of the ions to intercalate into the anode, and some of the atoms in the metal plating will intercalate later over 20+ hours, if the cell is allowed to rest, but most will not. That is the source of the capacity reduction, increased internal resistance, and also the danger.
If you've read my related answer on stack exchange to the question 'Why is there so much fear surrounding lithium-ion batteries?', you can probably see where this is going.
This lithium plating of the anode isn't nice and smooth and even (like chrome plating). It forms in dendrites, little sharp tendrils of lithium metal growing on the anode.
As with the other failure mechanisms which likewise are due to metallic lithium plating of the anode (though for different reasons), these dendrites can put unexpected pressure on the separating membrane as the anode expands and forces them into it, and if you're unlucky, this will cause the membrane to one day fail unexpectedly (or also immediately, sometimes a dendrite just pokes a hole in it and touches the cathode).
This makes the cell vent, ignite its flammable electrolyte, and ruin your weekend (at best).
However, you might be wondering, "why do below-freezing temperatures cause lithium metal plating of the anode?"
And the unfortunate and unsatisfying answer is that we don't actually know. We must use neutron imaging to look inside functioning lithium-ion cells, and considering there are only around ~30 (31 I think?) worldwide active research reactors (nuclear reactors that act as a neutron source) that are actually available for scientific research at a university rather than used for medical isotope production, and all of them booked 24/7 for experiments, I think it is just a matter of patience. There have only been a few instances of neutron imaging of lithium-ion batteries simply due to scarcity of equipment time.
The last time this was used specifically for this cold temperature problem was 2014 I believe, and here is the article.
Despite the headline, they still haven't really solved exactly what it is that causes plating rather than intercalation when the cell is below freezing.
Interestingly, it is actually possible to charge a lithium-ion cell below freezing, but only at exceedingly low currents, below 0.02C (a greater than 50-hour charge time).
There are also a few exotic cells commercially available that are specifically designed to be chargeable in cold temperatures, usually at significant cost (both monetarily and in terms of the cells' performance in other areas).
Note: I should add that discharging a lithium-ion battery in below freezing temperatures is perfectly safe. Most cells have discharge temperature ratings of -20°C or even colder. Only charging a 'frozen' cell has to be avoided.
See section Charging Electric Vehicles During Freezing Conditions in URL
CHEVY BOLT CATCHES FIRE WHILE CHARGING ON DRIVEWAY IN VERMONT
THETFORD; July 2, 2021 — A fire destroyed a 2019 Chevy Bolt, 66 kWh battery, battery pack cost about $10,000, or 10000/66 = $152/kWh, EPA range 238 miles, owned by state Rep. Tim Briglin, D-Thetford, Chairman of the House Committee on Energy and Technology.
He had been driving back and forth from Thetford, VT, to Montpelier, VT, with his EV, about 100 miles via I-89
He had parked his 2019 Chevy Bolt on the driveway, throughout the winter, per GM recall of Chevy Bolts
He had plugged his EV into a 240-volt charger.
His battery was at about 10% charge at start of charging, at 8 PM, and he had charged it to 100% charge at 4 AM; 8 hours of charging.
Charging over such a wide range is detrimental for the battery. However, it is required for “range-driving”, i.e., making long trips. See Note
NOTE: Range-driving is an absolute no-no, except on rare occasions, as it would 1) pre-maturely age/damage the battery, 2) reduce range sooner, 3) increase charging loss, and 4) increase kWh/mile, and 5) increase the chance of battery fires.
Charging at 32F or less
Li-ions would plate out on the anode each time when charging, especially when such charging occurred at battery temperatures of 32F or less.
Here is an excellent explanation regarding charging at 32F or less.
Fire in Driveway
Firefighters were called to Briglin’s house on Tucker Hill Road, around 9 AM Thursday.
Investigators from the Vermont Department of Public Safety Fire and Explosion Investigation Unit determined:
1) The fire started in a compartment in the back of the passenger’s side of the vehicle
2) It was likely due to an “electrical failure”. See Note
NOTE: Actually, it likely was one or more battery cells shorting out, which creates heat, which burns nearby items, which creates a fire that is very hard to extinguish. See Appendix
GM Recall of Chevy Bolts
In 2020, GM issued a worldwide recall of 68,667 Chevy Bolts, all 2017, 2018 and 2019 models, plus, in 2021, a recall for another 73,000 Bolts, all 2020, 2021, and 2022 models.
GM set aside $1.8 BILLION to replace battery modules, or 1.8 BILLION/(68,667 + 73,000) = $12,706/EV.
Owners were advised not to charge them in a garage, and not to leave them unattended while charging, which may take up to 8 hours; what a nuisance!
I wonder what could happen during rush hour traffic, or in a parking garage, or at a shopping mall, etc.
Rep. Briglin heeded the GM recall by not charging in his garage. See URLs
- Cost of replacing the battery packs of 80,000 Hyundai Konas was estimated at $900 million, about $11,000 per vehicle
- EV batteries should be charged from 20 to 80%, to achieve minimal degradation and long life, plus the charging loss is minimal in that range
- Charging EVs from 0 to 20% charge, and from 80 to 100% charge:
1) Uses more kWh AC from the wall outlet per kWh DC charged into the battery, and
2) Is detrimental to the battery.
3) Requires additional kWh for cooling the battery while charging.
- EV batteries must never be charged, when the battery temperature is less than 32F; if charged anyway, the plating out of Li-ions on the anode would permanently damage the battery.
See section Charging Electric Vehicles During Freezing Conditions in URL
Charging Electric Vehicles During Freezing Conditions
A 3-layer tape (cathode, separator and anode) is wound on a core to make a battery cell.
An EV battery pack has several thousand cells. The cells are arranged in strings, i.e., in series, to achieve the desired voltage
The strings are arranged in parallel to achieve the desired amps.
Power, in Watts = Volts x Amps
EV Normal Operation at 32F and below: On cold/freezing days, EVs would use on-boardsystems to heat the battery, as needed, during daily operation
EV Parking at 32 F and below: When at home, it is best to keep EVs plugged in during periods at 32F and below, whether parked indoors or outdoors.
When parking at an airport, which may not have enough charging stations, it is best to fully charge EVs prior to parking, to enable the on-board systems to heat the battery during parking, as needed.
Charging at 32F and below: Li-ion batteries must never be charged when the batterytemperature is at 32F or below. Do not plug it in. Turn on “pre-conditioning”, to enable the battery heating/cooling system (which could be a heat pump) to very slowly heat up the battery to about 40F. After the battery is “up to temperature”, normal charging can be started, either at home, or at a fast-charging rate on the road.
If the battery does not have enough charge to heat itself at about 40F, it needs to be heated by an external heat source, such as an electric heater under the battery, or towed/driven to a warm garage. All this, while cumbersome, needs to be done to safeguard the expensive battery.
Pre-conditioning can be set to:
1) Preheat the cabin and/or seats
2) Defrost windshield wipers, windows, door handles and charge port, etc., in case of freezing rain conditions; newer Teslas have charge port heaters. See URL
3) Pre-heat the battery, before arriving at a fast charger.
Power Outage, while parked at 32F and below: During a power outage, partially charged batteries, connected to dead chargers, could use much of their remaining charge to keep the batteries at about 40F.
If the power is restored, and the EV is plugged in, charging must never begin, unless the battery temperature is 35 to 40F
During charging, Li-ions (pos.) are absorbed by the anode (pos.) at decreasing rates as the battery temperature decreases from 32F
Any excess Li-ions arriving at the anode will plate out on the anode and permanently reduce the absorption rate.
The plating is not smooth, like chrome plating; it is roughish and may have dendrites, which could penetrate the thin separator between the anode and cathode, and cause a short and a fire.
A similar condition exists, if charging from 0 to 20% and from 80 to 100%; the more often such charging, the greater the anode resistance to absorbing Li-ions, and the greater the likelihood of plating.
The plating condition is permanent, i.e., cannot be reversed.
Also, frequently charging from 0 to 20% and from 80 to 100%, increases the charging percentage, increases kWh/mile of travel, and reduces range.
- EV batteries have miscellaneous losses to provide electricity to on-board systems
- On cold/freezing days, an electric bus should be ready for service as soon as the driver enters the bus
- On cold/freezing days, the bus driver would need at least 70% charge, because travel would require more kWh per mile
If the battery temperature is less than 40F or more than 115F, it will use more kWh/mile of travel
The best efficiency, charging and discharging, is at battery temperatures of 60 to 80F.
Batteries have greater internal resistance at lower temperatures and at high temperatures.
Pro-bus folks often point to California regarding electric buses, but in New England, using electric buses to transport children would be a whole new ballgame, especially on colder days. See URLs
EV Electricity Supply: Where would the electricity come from, to charge and protect from cold, expensive batteries during extended electricity outages/rolling blackouts, due to multi-day, hot and cold weather events, with minimal wind and solar, as occur in New England throughout the year?
Would charging electricity be supplied by emergency standby diesel-generators, or emergency standby batteries?
Tesla reported WORLDWIDE deliveries that totaled 241,300 EVs for the third quarter of 2021, up from 201,250 in Q2 and 184,800 in Q1.