Range Confidence: Charge Fast, Drive Far, with your Electric Car

By David Herron

When will electric cars charge faster than it takes to fill-up an equivalent gasoline car?

Replicating the gasoline car experience may be required to bring over some folk who won't otherwise drive an electric car. We collectively have 100+ years experience with a quick refueling experience, gaining 300+ miles gasoline range in about 5 minutes. The current best (early 2017) in the electric vehicle space is 300 miles range in about an hour, at the Tesla Supercharger stations. While that's less range per hour than gasoline refueling, it's enough that people routinely take Tesla Model S and Model X cars on coast-to-coast road trips.

Will the batteries take it?

Lithium-ion batteries store electricity/energy in chemical bonds. Reformulating those chemical bonds is not an instantaneous process, especially as the battery pack is closer to being full. I once heard Elon Musk give an analogy of filling the seats in a conference hall. When the conference hall is mostly empty it's easy for people to find a seat, and they just sit down. As it fills, the people have to search for a seat before they can sit down. Likewise, in a battery pack, electrons are looking for an atom/molecule to land in, and as the battery pack fills up the electrons have a harder time finding a place to land.

Batteries have two ratings of interest to this discussion. Each of the ratings are measured in "C" units, so lets talk about what "C" means first.

Batteries are rated for energy capacity in "amp-hours", and 1 amp hour is 1 amp of consumption for one hour. It's known that the energy quantity you can extract from a battery depends on the discharge rate. Typically the higher the discharge rate, the less total energy you can extract. Therefore the amp-hour rating is measured at a standard discharge rate.

A 1C rate is the rate equal to the rating of the battery. Charging or discharging 10 amp-hour battery charging at a 10 amp rate is described at 1C. At a 20 amp rate, it is described as 2C, or at a 5 amp rate it is described as C/2.

When the Tesla Supercharger recharges the car in one hour, that is a 1C charge rate. To recharge an electric car in five minutes requires a 10C charge rate. Er... 12C actually.

Typically lithium-ion batteries can't charge that fast. A 1C charge rate is thought to be the fastest lithium-ion can be recharged without damaging the pack.

But, the actual fastest-safe-charge-rate depends on the precise chemical formulation of the battery pack. Some can charge faster than others.

Implementation of higher charge rate

Our discussion of the electrical code requirements for recharging electric cars should have made it clear the higher charge rate requires a thicker cable. See Electric car charging within electrical code and power outlet limits

The Tesla Supercharger at 120 kiloWatts gives about 170 miles range in 30 minutes. Giving 300 miles range in 5 minutes requires 12x that energy, or about 1.4 megaWatts. Unless I got confused with the scribbles on the back of this napkin.

The Supercharger cable is nice, supple, thin, and delivers energy through a very nicely shaped easy to handle charging plug. That's very much different from the experience with, say, CHAdeMO charging cords that are ungainly. But, to deliver 12x the electricity will require a much larger cord. Unless Tesla Motors is able to, as Elon Musk once suggested, make headway with liquid cooled charging cords.

Beyond the charging cord is the sheer impracticality of delivering a megaWatt for each car at a charging facility. What electrical utility will be able to deliver that much energy to such a facility?

A charging facility might have co-located energy storage and solar panels, to mitigate the energy consumed from the grid. But is it practical to install a several megaWatt solar array and several megaWatt-hours of energy storage at each charging facility?

The likely result

The forces behind the Combo Charging System have begun demonstrating a 350 kiloWatt charging rate. That would give about 170 miles range in 10 minutes, a big improvement over the 30 minutes currently required. It's unknown when this will be widely deployed, and whether every DC fast charging station will be upgraded to this rate.

This points us towards the likelihood that charging rate will improve, but is unlikely to get to 300+ miles delivered in five minutes. Can we collectively grow the patience required to wait a little longer for a recharge?

Even if a different energy storage method were available, the impracticality of a 1+ megaWatt charge rate is likely to keep us from a 5 minute recharge. For example, a hybrid battery design could be used with supercapacitors handling the ultra-high charge/discharge rate, and using lithium-ion or lithium-sulfur or lithium-air for bulk energy storage. Supercapacitors can discharge/charge at an extremely higher rate, and could handle the 1 megaWatt charge rate, with a charge controller then shuttling energy over to the bulk energy storage unit at a safe charge rate.

But -- will the powers-that-be ever let regular people handle charging cables capable of 1+ megaWatts? Will they ever allow charging facilities running at that rate? That's unlikely.

Battery swapping?

If we reformulate the question, it becomes - What will it take for electric cars to gain energy at a rate allowing 300+ mile range in five minutes like gasoline cars do?

The act of "gain energy" can be performed by swapping a depleted battery pack for one that's fully charged. Multiple companies have demonstrated battery swapping in under five minutes. Clearly we could implement a battery swapping system right now and electric car drivers would enjoy the same recharge time as gasoline car drivers.

But, there are many problems with the whole battery swapping idea. See Wouldn't battery swapping be preferable over waiting to recharge?

Is the five-minute recharge even required?

The average driver travels 40 miles or less per day. How often do we take road trips where the goal is covering as many miles as possible? When is a five-minute recharge time actually required?

In the vast majority of recharging scenarios, we plug in the car then walk away and do something else. The time commitment is less than a minute to set up the charging. The rest of the time for the charging session doesn't count, because we're not waiting around.

It's when we're on a long road trip with multiple recharging sessions per day that the recharge time becomes significant. How often does the typical person take such trips? A couple times a year? Is it reqlly necessary to optimize the charging system for such an infrequent need?


Range Confidence is Copyright © 2016-17 by David Herron

When will electric cars charge faster than it takes to fill-up an equivalent gasoline car?

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