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Saturday, June 3, 2017

Bigger Battery Allows Slower Charging

Tesla Model X slowly charging from a standard US wall outlet 

Daniel Gross has written an article at Slate titled The Bolt Still Needs a Jolt. In the article, he makes the point that GM and other EV makers need to invest in charging infrastructure. Although I agree with most of his points (many of them are things that we've said here before) there are two things that he gets wrong IMHO.

First, he overlooks DC fast charging and the need for standardization. Just stating that the OEMs need to get involved is not enough. There are already three fast charge methods on US roads today. If Porsche follows up on their announcement to make their own, there will be a fourth. That is being "involved" but I don't think it helps the EV community. We don't need VW, BMW, and others to do the same. Today, you can drive a gas-powered car into any gas station and fill up. You don't need to look for stations that accept Fords or Toyotas... EVs need to strive for the same universal goal.

The second (and bigger) thing that I think Gross gets wrong is when he bemoans the slow charging speed that you get from a standard 120V US outlet. I have been driving a 250-mile range EV for 6 months now and I'll spend the rest of this post explaining how the meager 120V outlet is great for many people, most of the time, and how a bigger battery makes it more useful, not less.

Here is the quote from Gross: “You just plug the cord that comes with the car into a three-pronged outlet. But unless you install special equipment, the batteries sip their juice very slowly, just a few miles of charge per hour. Plug in at night and you’ll return in the morning to find that the car’s range has expanded by only about 30 miles.

He is right the charge is slow, but that does not mean it's not useful. Here are the specifics for how I've been using a 120V outlet for my long range EV. Seven months ago we purchased a Tesla Model X. The car has 257 miles of range and a 90 kWh battery pack. Our range is similar to the 238-mile range of the Chevy Bolt that Gross was talking about.

The Tesla now parks next to our Nissan Leaf. We have a 100% electric home fleet. The Leaf has a degraded 24 kWh battery pack. We have one charging station in the garage. We were considering buying a second charging station for the Tesla, but we wanted to see how it would fit in the garage and how we'd arrange things before we placed a charging station on the wall.

Each of the cars came with a trickle charger. So we can use the charging station for one of the cars and the trickle charger for the other. Initially, I assumed that the Tesla, with its bigger battery, would be the one that needs faster charging. To my surprise, it has not turned out that way.

On most days, both cars are driven. My wife drives one of them and I drive the other. They are used for commuting and errands; typically 20 to 40 miles per day, not a lot of miles. With its short range, the Leaf needs to be recharged frequently. When the Leaf is the kid-duty car, after the morning school drop off it's plugged in and charged up for an afternoon of errands and the school pick-up run. Being able to recharge at Level 2 is important when adding range mid-day.

For the Tesla, on the other hand, it has plenty in reserve. It does not need mid-day charging for normal daily use. This means that it will be charging overnight. If it is plugged in from 7 PM till 7 AM, that is 24 miles of range. This is enough to for my 20-mile commute and to run the heater and still break even. However, it is not really important to break even. If I used 30 miles of energy on a given day but only replenish 20 miles, then the next morning the car is a few miles short of full when it is unplugged, but there is still plenty of range in reserve.

Any accumulated mid-week deficit is made up for on the weekend when the vehicle is usually in the garage for 20 hours.

We also bought the CHAdeMO adapter for the Tesla. This allows us to charge up at any of the West Coast Electric Highway spots. We have not had to use it yet, but there are several CHAdeMO stations in our region.

Now that we've had the Model X for awhile and I know that backing in on the left side is where/how we're parking the X, I'll likely get a Tesla wall connector. But before I do, I thought I'd share this unexpected (but retrospectively obvious) learning that 120V charging actually works very well on a long range EV.

Final thought: this bigger battery enables slower charging comment only applies to at home charging where the car is generally being used for short trips such as less than 40 miles per day. This applies to about 80% of people but certainly not everyone. And this certainly does not apply to EV road trips.

Charge on!

EV Road Trip