Frigidity and the challenge of high-power coupling

It’s happening again — just like it always seems to as the warm summer fades and the dreary, hazy winter months begin to settle in.

Usually there’s a quick surge of energy as soon as mating occurs. But now? Things are turning frigid. The reaction after plugging-in is sometimes slow and tentative.

Of course, we’re talking about your electric car’s battery pack.

As each electron from the charging cable surges into a battery cell it causes a lithium ion (atom) to move from the cathode side of the cell over to the anode side.

Essentially all lithium ion batteries used in electric vehicles today have an anode side made of carbon graphite. The lithium ion “intercalates” or snuggles in among spaces within this graphite.

The chemical reactions underlying this process slow down at colder temperatures so the current (the number of electrons per second) used to charge the battery has to be reduced in order to lower the number of ions attempting to find places in the graphite at any one time. If the ions arrive too quickly at the anode they end up “plating” together on the surface of the anode and this can permanently damage the cell.

The battery management system knows about this temperature vs. charging current relationship and slows the charging rate to a safe level.

This graph from a study at the Innovative Vehicle Institute in Canada shows the Chevy Bolt EV’s charging rate at various battery temperatures.

The Chevrolet Bolt EV, as depicted in the graph above, can charge at its full ability when connected to a 125A 50 kW charger once it reaches 20C (68F) but at 8C (46F) it can only safely charge at around 60A or 20 to 25 kW. At colder temperatures the charge rate is even slower, although it isn’t shown on this graph. The Bolt can charge at up to 150A at 68F or warmer when connected to a more powerful charger.

In order to speed charging in cold conditions the Bolt EV has a dedicated battery pack heater. However, even with a battery heater it can still take dozens of minutes to bring the temperature up high enough to allow faster charging because the battery pack is almost 1,000 pounds of cold mass.

The Bolt doesn’t typically turn on the battery heater during normal driving when the battery isn’t being charged because that would be inefficient. The battery is capable of discharging adequately at cold temperatures even though it cannot charge back up rapidly. So, the car just disables or reduces its regenerative braking when driving.

The battery will naturally warm itself somewhat when it discharges but even an hour of driving at freeway speeds in winter conditions can leave the pack below 68F.

This is a general problem with today’s typical lithium-ion batteries and is not specific to the Bolt EV.

Some cars, although not the Bolt, have ways of sharing heat generated by the motor and its power inverter with the battery in order to help warm it.

In fact, some car designs go so far as to skip having a dedicated battery heater altogether.

The new Hyundai Kona Electric can share waste heat among its powertrain components including the battery so in milder climate regions Hyundai plans to eliminate the battery heater to reduce costs.

The Tesla Model 3 follows a similar strategy but Tesla figured out how to use its motor to emulate a battery heater by sometimes intentionally generating waste heat.

If a Kona does have a dedicated battery heater installed the owner can enable a “Winter Mode” which uses the heater to help proactively warm the battery pack. Software updates to other car models could add a similar feature someday.

See also: Hyundai’s Kona EV one-ups the Chevy Bolt EV, except…

There are some useful strategies to mitigate the need to fast charge a cold battery.

On a long distance trip, try to stay at a hotel that has overnight 240V charging. Not only will this fully charge your car but it will also warm the battery as a side-effect so fast DC charging the next day can actually be fast. Use your car’s charging controls, if possible, to have the charging finish just before you begin driving again in the morning. This also works well when charging at home in the winter.

If you are staying overnight without 240V charging then try to charge the car before arriving for the night rather than trying to charge a cold battery the next morning.

As a last resort in an emergency you might try driving with a series of rapid accelerations to warm the battery from the large power discharges. This could be especially hazardous in slippery winter road conditions and shouldn’t be attempted when other traffic is nearby. Bjorn Nyland tried this in one of his YouTube videos.

Alternative battery technology may improve cold charging performance.

Toshiba sells SCiB batteries which use a different kind of anode based on lithium titanate instead of graphite. One reason these cells are not used much for EVs is due to lower energy density but a new generation of cells from Toshiba may improve that.

Future so-called solid-state batteries may do away with using graphite on the anode side of the battery and always plate lithium metal. The solid instead of liquid electrolyte can render this safe and eliminating the graphite makes the anode more energy dense.

Even battery cells with graphite anodes based on today’s typical chemistry could be dramatically improved during winter charging by building a heating mechanism directly into each cell.

See also: Researchers create self-heating battery cells for speedier winter charging

Categories: Charging

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