29Nov 2017

Congratulations, you just purchased a new Tesla model S electric vehicle (EV). You also committed an extra $2,000 to install a level-2 charger on a wall in your spacious garage. A level-2 charger will deliver 6 kW of power at 240 V to charge your big car battery overnight. Better yet, you are even considering investing an additional $20,000 to install solar panels on your roof and live a life with zero carbon. You might be cringing by now and thinking: “Wow, this is for the rich, not me.

So let’s consider instead a more socially responsible scenario. You leased a much more affordable Chevy Bolt that promises to give you 200+ miles of electric driving. You don’t have a garage. Perhaps you live in a large city so your car may be parked on the street. You are scratching your head: “How will I charge my car battery?” You might be lucky to charge your car during the day at work instead of overnight at home. But what about the weekends? No quick and easy answer.

As the adoption of electric vehicles becomes more widespread especially in congested urban geographies, questions about the charging infrastructure become prominent. Tesla leads in the deployment of their Supercharger network with over 1,000 charging stations installed worldwide, especially near major transportation corridors and highways. But the Tesla fast charging network is not compatible with other electric vehicles. Imagine that you can refuel your present vehicle at only one brand of gas stations, say at Shell only but not Exxon. No practical!

The buildup over the coming decade of a charging infrastructure that is publicly available to all electric vehicles is a must if EVs are to become a real alternative to vehicles powered by gasoline (or diesel). A fundamental requirement for charging is the availability of fast charging, more specifically, charging that can provide at least half-a-tank (or ¾ of a tank) in about 10 minutes.

Let’s do some simple math. An electric vehicle with a 200-mile range equates to a battery size of approximately 60 kWh. Half-a-tank is 30 kWh (or 100 miles). Charging 30 kWh in 10 minutes equals to 180 kW (or 3C effective rate). By the time we factor inefficiencies, the charging station needs to deliver a minimum of 200 kW. To put that in perspective, that is the amount of power used by an entire residential block! These chargers are big, expensive and hence have to be shared among dozens if not hundreds of vehicles.

But the infrastructure for fast charging is only half the problem. The elephant in the room remains: Can the battery itself charge at such a fast rate without being damaged?

The data suggest otherwise for the time being, unless we add a lot more intelligence to how we charge the battery.

The following chart shows the results of charging a battery at a slow rate compared to fast charging the same battery 30% of the time (or about once every three days) and 50% of the time (every other day).

The green curve shows how the battery retains its charge with slow charging. After 700 charge cycles (or about 130,000 miles of driving), it still retains 90% of its original charge. In other words, you can still drive 180 miles in what used to be a driving range of 200 miles. That is good!

The blue curve shows what happens if you charge 30% of the time. The capacity retention drops to 80% after 600 charge cycles. That is a rapid degradation. After 100,000 miles, your driving range is now 160 miles. It might be acceptable to some EV buyers but just barely. The resale value of your car has depreciated substantially below the average value.

The red curve spells major trouble. If you fast charge your electric vehicle every other day, your battery capacity drops to 75% of its original charge after only 300 cycles. That means that your driving range drops from 200 miles to 150 miles after about 50,000 miles of driving. What this graph does not show is that this battery is failing rapidly and has now become a serious safety hazard because of the presence of lithium metal plating. This is a serious problem!

So, if you own an electric vehicle such as a Tesla, and you are tempted to use the Supercharger network frequently, consider an alternative charging solution !!