How do batteries lose capacity in winter?

Question

One problem of electric vehicles is that the battery capacity often shrinks under low temperatures. Some batteries can lose as much as 50% of capacity in winter.

What I don’t quite understand is how the capacity shrinks. Does the low temperature prevent the batteries from being fully charged or fully discharged or both? If batteries can’t be fully charged under low temperatures, what would happen if an already fully charged battery is cooled to a low temperature? Because the energy is conserved, the excessive energy needs somewhere to go, which may pose a serious safety risk. If cold temperature limits the extent of discharge, would an already fully discharged battery have difficulty to be charged under low temperatures because it’s overly discharged under the new environment.

P.S. I am not sure if it’s more appropriate to pose my question in chemistry stackexchange because the charging and discharging of batteries are chemical reactions.

Answer 1

It is true that battery performance is reduced at colder temperatures.

This is because temperature has an effect on chemical processes within the batteries, especially lithium-ion batteries, used most frequently in electric vehicles. See this "Temperature effect and thermal impact in lithium-ion batteries: A review.", wherein it is concluded that:

At low temperatures, the degradation of performance is mainly caused by the reduction of ionic conductivity [of the electrolyte solution] and the increase of charge-transfer resistance.

The problem of cold temperatures and battery performance is one of chemical dynamics and reaction rates etc., and not related to things like the effect of temperature on current flow (resistance) through wiring etc., which seems to be suggested. In fact, higher temperatures increase resistance (especially in metal conductors).

Answer 2

A deeper, physics/chemistry approach to the question:

A battery (whatever chemistry it uses) invariably contains solid electrodes and liquid electrolyte (there are batteries that have it the other way round, but the problems are pretty much the same).

The dust-sized particles that the electrode is made of, contain some substance in their entire volume, but the chemical processes of charging and discharging happen only at the interface between the solid particle and the liquid electrolyte.

No matter if you charge or discharge the battery, the surface of the particle gets enriched in the reaction product and depleted of the substance yet to be processed. The process that transports the product to the inside of the particle and the unprocessed substance to the surface is called diffusion.

The diffusion is profoundly temperature-dependent. Every few degrees C double its speed.

It is also concentration-dependent - if there is more unprocessed mass inside the particle, the diffusion will go faster.

If you force the battery to do its job faster (use higher charge or discharge current) the diffusion may fail to keep up. This manifests as a non-linear electrical resistance of the whole cell, limiting the usable power.

If we combine this with the above inherent diffusion properties, we see that the available power depends on both the temperature and the state of charge of the battery. If the temperature is low, the available power capability of the battery falls sooner below whatever usable power one needs.

E.g.

• at 30C the battery may be happy at its design capacity.
• at 15C the battery may be usable between 5% and 95% state of charge, giving us ~90% of usable capacity
• at 0C the battery may be usable between 20% and 80% so we have ~60% in between. ... and so on.

When charging the battery in a cold weather, we have the luxury either use some extra energy to heat the battery up so it could charge at a high rate all the way to 100% - or - use lower charge rate at the end of the process, reaching 100% e.g. overnight.

On the other hand, when discharging the battery, e.g. driving an EV car or talking on the phone, the power demand is generally dictated by the load and if the battery cannot meet it, it is considered empty even if it could give off e.g. 30% more energy at lower rate.

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Two more interesting battery properties, also related to diffusion:

• The electrolyte generally has the same diffusion-related behavior, but it happens at different timescale (e.g. seconds). This is why a starter battery (found in cars) can give off 1-3 kW for few seconds (needed to start the engine) but needs some "rest" if the engine fails to start and one needs a second attempt. The needed "rest" also depends on the temperature and in sub-freezing temperatures a minute or two is advisable.

• The problem with the solid-state diffusion (and the temperature dependence of the usable capacity) gets worse as the battery ages. This is because the particles in the electrodes tend to get bigger with time and the diffusion has to carry the substances over greater distances.

Answer 3

In addition to the charge in the battery itself, you also need to consider increased HVAC usage.

Unlike an internal combustion engine vehicle, which produces waste heat as a byproduct of combustion and pipes that heat into the cabin, electric motors are extremely efficient and thus produce very little waste heat. So to keep the occupants warm, EVs need to purposely create heat. Often cabin heating is accomplished using an electric resistive heater, similar to a plug-in portable space heater you might have in your home. These use a noticeable amount of electricity but are simple to implement. Newer EVs like the Tesla Model Y use a heat pump, which compresses thermal energy and works almost instantly, with the added benefit of being much more efficient, thus less impact on driving range. No more waiting 15 minutes for your car to warm up enough to defrost the windshield!

In some EVs, again like Tesla, the coolant system for the batteries runs off shared componentry with the cabin HVAC system. Not only does the battery need to stay cool in warm weather and when under load, but it also needs to be warmed up if it's too cold. Basically, the thermal management system for the battery also uses electricity to keep the battery itself warm.

Answer 4

The charge in a rechargeable battery is literally a charge, i.e. a quantity of electrons that have been moved. Charge is conserved; temperature change does not alter it.

It is normal to stop adding charge to a battery that exhibits a voltage rise to some level, because that voltage rise indicates depletion of the electrode being oxidized, and of the electrode being reduced. At low temperature, it is possible that internal resistance and normal charging current can cause a voltage rise that stops the automated charge, before the electrodes are depleted.

And, under use, a voltage drop due to cold-electrolyte resistivity may cause a 'zero capacity' indication before all the available charge is used.

So, a battery condition monitor may indicate lessened charge because it has sensitivity to temperature effects, such as reading 'full' when the battery has not achieved full chemical charge state, or 'empty' even if the battery has available chemical energy.

For a battery electric vehicle, in particular, battery charging is done with the battery capable of being warmed, or cooled, to optimum temperature for the process. There should be no problem in charging or using the vehicle in the cold.

In operation, too, a motor vehicle battery may be kept in a useful temperature range, rather than always at ambient temperature. The temperature limitation that matters, is the parasitic circuit element of cold-enhanced resistance inside the battery, not the chemical state of full/zero charge available.

In short, battery charge isn't lost if you cool it, because charge is conserved. A charge/discharge cycle, though, is intended to never apply to frozen batteries.