By Reg Nicoson
Basics for charging lithium batteries in cold weather
Lithium batteries contain no water, so temperature limitations based on the freezing temperature of water are misleading at best. The REAL freezing point of a lithium battery would be associated with the electrolyte freezing point which is less than -60°C.
A lithium battery, like all other types of batteries, have reduced performance and service life when operating at temperatures below room temperature. Performance reductions are in the form of reduced power (lower cranking amps), reduced capacity (less amp-hours stored), and slower charge times. Reduction in service life is NOT imminent but rather measured by months (i.e. 60 month service life versus 72 month service life).
Some online statements read like, “lithium batteries (LFP) cannot be charged below freezing” and this is an inaccurate statement. EarthX batteries can and will charge below 32 Deg F. The point is that charging current is reduced or should be reduced. Because the internal resistance of all batteries increases as temperature decreases, this will slow the charge rate. During charge, with higher internal cell resistance causes a temperature rise that helps compensates for the cold temperature. Moreover, for an engine starting battery, like EarthX batteries, during the first crank process, this will warm up the battery >10°C prior to the charging from the vehicles charging system. Example: if it is 0°C, it will be 10°C (or 32°F will be +50°F) after the engine starts.
Other common inaccurate online statements read like “permanent damage will result from charging below freezing.” Stating that permanent damage will result is like saying landing an aircraft will result in permanent damage to your tires. It is more appropriate, and less provocative to say landing will result in normal or expected wear.
More Technical Explanation of charging lithium batteries below freezing
Generic statements about cold temperature limits are too broad for there are many variables that determine the operating temperature range like cell chemistry, cell geometry, separator material, electrolyte chemistry, etc. For example, the electrolyte or electrolyte additives can vary greatly. Low temperature electrolytes like the one used in an EarthX battery can be found in many aerospace batteries. The low temperature formulation improves the ionic conductivity thus reducing the internal resistance (increasing cranking power and charge acceptance) and enabling capacity retention down to −30 °C (> 95% charge retention). Other consumer-grade lithium-ion batteries on the market show a capacity retention as poor as 50% at -30°C.
Charging at low temperatures is slowed but it is NOT stopped. The slowdown is associated with the slow diffusion of lithium ions within electrodes. At low temperatures the lithium-ion intercalation (or diffusion) into the anodes during the charging process slows down, thus lithium-ion cathode material is deposited on the surface of the anode (a process called plating). EarthX LiFePO4 batteries formulated for cold weather performance can achieve a near 1C charge rate at -30C which is 2X better than a lead acid battery. And at this high charge rate, there is very good intercalation, thus the high charge retention already mentioned.
Summary
Operating a battery at room temperature with charge and discharge at a minimum will get the best overall service life, well beyond the rated life. However, this is not realistic. A battery used in the real world, within the manufacturers operating limit should provide the rated life. And a battery operated outside the operating limits would provide less than the stated service life. But to use the tire analogy again, to get the best overall service life, you would not accelerate fast , you would not make rapid stops or you would not drive on hot days but this is not realistic. Tires, just like batteries, are used in the real world in not ideal environments which will dictate their life and the when they need to be replaced.