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December 10, 2015


Electric Vehicles
Electronics & Systems
Fast Charging
Nadim Maluf

Before I start this post, I encourage all new readers to go back to the early posts if they desire to learn more about the basics of lithium-ion batteries and their operation.

This post is dedicated to deciphering the growing complexity of charging stations for plug-in hybrid and pure electric vehicles (xEVs), especially as their popularity grows among drivers. If you own an electric vehicle, a Tesla, a Nissan Leaf, or other models, chances are you are using one or more of these charging methodologies…and if you so desire to fast charge your Tesla or electric vehicle, this post will give you the insight to know what type of charging station you may need.

The charging of xEVs, whether at your home or at dedicated charging stations, is usually governed by a set of standards agreed upon by a vast number of contributing organizations, such as the automakers, electric utilities, component makers and many others. Several organizations including SAE International, ANSI and IEEE have led the  coordination and development of such standards — they are numerous — covering, for instance, the types and interoperability of connector designs and charging power levels (SAE J1772), communication and signaling protocols  (SAE J2931/1) between the xEV and the charging station (also known as EV supply equipment — EVSE), wireless charging (SAE J2954) as well as many others related to diagnostics, safety, DC charging…etc. Today’s post addresses charging from the perspective of the SAE J1772 standard and its competing standard CHAdeMO.

So let’s start with understanding what charging levels are and how they are defined by the various standards. There are 4 levels of charging:  two levels using conventional AC charging, and two additional levels using higher-voltage DC  charging. They are summarized in the next table:

Let’s start with AC levels 1 and 2. Level 1 is what you get using the charging cord supplied to you by the car maker if you own an xEV. It plugs into the standard 120V household outlet and delivers, in theory 1.7kW. Some of you are probably tempted to multiply 120V by 20A and realize that’s more than 1.7kW…if you are that person, remember that this is an AC current, so you need to multiply by 0.707. This is the maximum power delivered to the connector plugged into the car. It is not the power delivered to the battery. The battery’s power is delivered by an on-board battery management system (on-board charger) that has to convert the voltage to a level appropriate for the battery. In reality, the car battery receives a best-case power of about 1.2 – 1.3kW to account for the electrical efficiency of the system — taking an average consumption of 250Wh per mile, that is equivalent to about 5 to 6 miles for each hour of charging…ouch!

Reality is a little worse than that: standard household power plugs are limited to 15A (instead of 20A) thereby decreasing the power delivered to the battery to a measly 900W. At this power level, a Nissan Leaf’s battery (nominal 24kWh / effective 20kWh) takes 22 hours to fully charge. Yikes! Now one can begin to understand why xEV owners do not line up near a Level 1 charger…but it does get crowded at a Level 2 charger.

AC Level 2 uses a 240V single-phase mains. The lowest current level is 20A corresponding to a maximum power (again at the output of the connector) of 3.4kW. The typical public charging stations, such as the ones managed by ChargePoint, provide 6.7kW. However, there is a catch. The on-board charging circuitry in your xEV must be able to use that power. Early Nissan Leaf models had 3.3kW-circuitry — in other words, regardless of the maximum power at the charging station, the maximum power the car is willing to accept is 3.3kW.  Newer xEV models, e.g., Nissan Leaf, Ford Focus Electric, Chevy Volt, have on-board chargers capable of up to 6.6kW. Again, this means if the charging station were to provide 19.2kW, your car cannot accept more than 6.6kW…this is by far the most common charging level as dictated by the presently deployed infrastructure. It equates to about 25 miles for each hour of charging. Once again, using the Nissan Leaf as an example, its battery will fully charge in 3.5 hours with a Level 2 charger (6.6kW). That’s not fast charging, but it sure is a heck-of-a-lot faster than Level 1.

Fast charging with DC gets more complex because there are competing approaches. Of course, we are also now talking about insanely high power levels, and consequently very expensive charging stations  and associated installation costs ($50,000 to $100,000 each).

SAE has the J1772 Combo DC standard. CHAdeMO has another competing standard. Tesla has its own proprietary fast-charging using their network of Superchargers (though not DC). But what they have in common is that they all seek to provide high power levels to the vehicle…up to 120kW. This infrastructure is still relatively scarce — Tesla is the only one aggressively deploying fast charging Superchargers along specially designated highway corridors.

Naturally, charging a car battery at such high power levels begs a new series of questions on whether this creates any significant and permanent damage to the battery. The brief answer is: YES, damage does occur…but super fast charging is so rare that no one is really paying attention to this question, at least not for now. Besides, with the exception of Tesla, your average xEV cannot charge faster than 6.7kW, so having a fast charging station is a moot point.

A final word on fast charging the Tesla batteries. At 120kW of input power, this equates to a charging rate of 120/85 = 1.4C — this is guaranteed to cause serious damage to your Tesla battery if you were to charge on a regular basis. But then again, if Elon Musk and Tesla Motors are willing to cover you under their warranty, do you really care?

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