January 2011 Archives

Understanding Electric Vehicle Charging

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Trading a gas pump for a plug is a wonderful thing. It's far more convenient, takes less of your time, and saves you from breathing toxic fumes and smelling like gas for hours after fueling. Charging is a different experience than pumping gas and understanding the subtleties takes time. I've been driving electric for over two years and I'm still learning. Potential EV owners might want to get a head start on the learning curve, and maybe save a bunch of money as a result.

Mostly, I'll relate how charging works for a Nissan Leaf, a four-door, five-passenger hatchback with a range of about 100 miles, but I'll also mention other plug-in vehicles. The Leaf is intended for typical daily driving, which for 78% of drivers in the US means 40 miles or less per day. Occasional longer trips are possible and understanding charging will help you evaluate whether an EV will suit your driving needs.

Level 1 Charging

Level 1 Charging - Standard house outlet
Level 1 Charging - Standard House Outlet

Level 1 charging is the technical jargon for plugging your car into an ordinary household outlet. For a Leaf, this means about 4.5 miles of range per hour of charging, or about 22 hours for a full charge. Wow, does that sound terrible! But there's a problem with thinking this way: you'll rarely need to do a full charge from flat empty to full. If you drive 40 miles per day and charge overnight, you'll be back to full in 9 hours. When you're sleeping, it doesn't matter if it takes one hour or 9 hours to charge.

But what if you have to drive a lot one day, say 80 miles? Sure, it would take 18 hours to get a full charge, but with a 9-hour overnight charge, you'll be ready for your normal commute the next day. If you drive less than 40 miles per day or charge for more than 9 hours, you'll work back up to a full charge over the next few days.

If you need to drive 80 miles on consecutive days, you'll need an alternative. Maybe you'll drive your other car, that gas-burner you keep around for long trips, or if there's public EV charging in your area, you can charge away from home while you're parked to do your shopping or other errands.

Level 1 charging at work could also be a supplement for people driving over 40 miles per day, or even a substitute for those who can't charge at home (because they don't have a garage or fixed parking place, for example).

Since it's easy to get 40 miles of range charging overnight from 120V, Level 1 is perfectly suited for overnight charging of the Chevy Volt, a plug-in hybrid with a 40-mile all-electric range.

Although Level 1 charging is generally too slow for a road trip, it can be helpful as destination charging. Cathy and I drove 90 miles to San Juan Island, charged for a few days in a friend's garage when not cruising around the island, and left with a full charge. That was great, but I wouldn't want to have to wait for Level 1 charging in the middle of a travel segment.

Beyond range issues, Level 1 may not be suitable for primary charging in all cases. In extreme climates, more power may be required to maintain proper battery temperatures. In these cases, Level 2 charging may be more appropriate (see below).

DC Fast Charging

The Blink DC Fast Charge Station
Blink DC Fast Charge Station
photo by ECOtality

At the other end of the spectrum is DC Fast Charging, the fastest type of charging currently available. It provides up to 40 miles of range for every 10 minutes of charging. These stations are expensive (up to $100,000) and require more power than your house, so you'll never have one of these in your garage.

They are going to start appearing as public charging stations in the next year, beginning in the Leaf target areas. If there's one conveniently located near where you drive, you can get back up to 80% of a full charge while getting lunch or drinking a latte. Charging this fast makes it far more practical to drive beyond an EV's single-charge range in one day. It's still not going to make a one-day 800-mile drive practical, but a 200-mile drive with a couple of charging breaks can be quite doable.

Level 2 Charging

ChargePoint/Coulomb Level 2 Charging Station
ChargePoint/Coulomb Level 2 Charging Station

Between the cheap Level 1 and expensive DC Fast Charging stations sits Level 2 charging. Level 2 supplies 240V, like what an electric dryer or oven uses. It goes through a box and a cord that improves safety by waiting to send power to the plug until it's plugged into an EV. Level 2 allows for a wide range of charging speeds, all the way up to 19.2 kilowatts (kW), or about 70 miles of range per hour of charging.

However, the charging stations being put in with federal grant money don't support the full range of Level 2 charging and max out at 6.6 kW or around 26 miles of range per hour of charging.

Both Level 1 and Level 2 charging stations simply deliver household electricity to the car. Electronics on board the car transform the wall power into the proper form to charge the battery. This bit of electronics built into the car also has a maximum power rating. The first model-year Leafs can only use 3.3 kW, about 12 miles of range per hour, or about 8 hours for a full charge from empty. The Chevy Volt's on-board charger is also limited to 3.3 kW, although its smaller battery pack gets full sooner.

Nissan recommends that you install a Level 2 charging station at home. That's a reasonable thing to do if you don't mind spending about $2,000, just consider it part of the cost of the car. Early buyers in the Leaf target markets may be able to get into The EV Project and get a free Level 2 charging station plus an allowance toward the install cost. Failing that, there's a 30% federal tax credit (up to $1,000) for installing EV charging, which can make it less expensive. Still, if you are planning to use your EV for a daily commute of 40 miles or less per day, you should at least consider using Level 1 charging at home. You can always add a Level 2 charging station later if you decide you need it.

There will soon be 20,000 public Level 2 charging stations (limited to 6.6 kW) installed mainly in the Leaf target areas. Even if you only have Level 1 charging in your garage, if you're in the early rollout areas, you should have access to convenient Level 2 charging available while your car is parked and you're doing something else. These charging stations will make it possible to drive 60 miles to a baseball game and pick up about 50 miles of range in 4 hours while you're having fun, thus easily driving over the single-charge range while always keeping a healthy reserve.

Charge Time and Battery Capacity

It's misleading that charging times are generally quoted as time for a full charge. While it does take about 22 hours (Level 1) or 8 hours (Level 2) to charge a Leaf from empty to full, you're not likely to do that often because  you will rarely arrive home with a fully depleted battery. It doesn't matter if you're driving a 40-mile Volt, a 100-mile Leaf or a 240-mile Tesla Roadster, if your commute is 40 miles, you'll only need about 9 hours (Level 1) or 3 hours (3.3 kW Level 2) to charge.

When we bought our Tesla Roadster, we got the high-power 16.8 kW Level 2 charging station, which can charge the car in 3.5 hours. After driving the car for a few months, I realized it's all but pointless to have such a big charging station in our garage. It's rare that I drive over 40 miles in a day. The 16.8 kW charging station can restore 40 miles in under 40 minutes. I want that charging speed when I'm making a long trip, not when I'm sleeping at home. In fact, I manually drop the power I pull from the charging station to about 7.5 kW because it's a little nicer to our electrical panel and the grid, and my typical overnight charge is still under 2 hours. Ignoring the fact that Tesla is still using the now-incompatible proprietary charging plug they picked before there was a chosen standard, most people buying a Tesla Roadster today would be well-served to buy a 6.6 kW charging station for home.

3 Roadsters Sharing the Charging Station at Burgerville
3 Roadsters Sharing the Charging Station at Burgerville

Level 2 Charging, Road Trips, and Charging Speed

Already, Ford has announced that the upcoming electric Ford Focus will support charging at 6.6 kW, and is making fun of the Leaf's 3.3 kW Level 2 charging limit. By the time Ford actually starts delivering the electric Focus, Nissan may have already upgraded the Leaf to 6.6 kW charging. I don't think it will be long before mainstream EVs are capable of even faster charging. The Tesla Roadster can charge at 16.8 kW, which combined with a larger battery pack makes 400-mile drives possible even without DC Fast Charging. Given that Level 2 charging costs 1/10 of what a DC Fast Charger does, I can imagine a lot of driving being supported by full Level 2 charging stations in areas that can't justify the investment in DC Fast Charging.

Personally, I'm disappointed we're spending so much money installing these 6.6 kW public charging stations rather than full-speed Level 2 chargers when most of the expense is usually just running the wires and buying the fancy box. A typical commercial Level 2 install runs around $10,000 for a charging station that's connected to a network and capable of billing the user. Cranking those charging stations up to the 19.2 kW limit would add a small incremental cost, perhaps 10%, and would allow for much faster charging. If you're a business owner installing a charging station and have to dig a trench and/or run conduit, even if it's just a for 6.6 kW unit, I strongly recommend planning for running 100A wire later without having to retrench or replace conduit so that upgrading to a 19.2 kW charging station will be much less expensive.

2008 Tesla Roadster Converted to J1772 Charging

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Cathy and I, with help from Dave Denhart and many others in the Tesla and broader EV communities, have converted our 2008 Roadster and Tesla High Power Wall Connector to use the new industry standard J1772 inlet and plug. This will allow us to charge without an adapter at the tens of thousands of Level 2 charging stations that will be installed in the US by the end of 2011.

j1772-charging-coulomb.jpgWhat we have is functional and completely reversible, but not ideal; we view this as a version 0.9 conversion. As there are very few J1772 charging stations currently installed, and the numbers probably won't take off until late spring or early summer, we have time to develop a better solution before it actually becomes compelling for Tesla owners to convert in significant numbers. I'm sure Tesla Motors could do a much better job of creating an integrated solution and I would prefer that to having the owner community develop a conversion solution.

We've hear rumors that Tesla is developing an adapter, but are still waiting for official word on what, if any, J1772 solution they will provide. While an adapter would give us a way to charge, we have heard from many owners who would prefer to convert their vehicles and charging equipment to the industry standard rather than leave an expensive adapter vulnerable to theft while charging.

Our effort started last summer when Cathy and I began working with Dave to figure out what it would take to build an adapter that would let a Tesla Roadster charge from the Level 2 J1772 charging stations. We discovered that SAE adopted Tesla's extension to the older J1772 communications standard, so a simple pass-through connector that converts Tesla's charge inlet to the J1772 inlet will allow charging to occur, although there is an issue, which is explained below.

Once we understood the protocol, Cathy and I built and tested a pass-through adapter. When I let the Tesla owner community know about our adapter in mid-September, I wasn't surprised to hear that lots of owners were thinking about those thousands of chargers, but I was surprised how nearly all who expressed an opinion agreed with us that the right way to do this was just to convert the Roadster to use the J1772 inlet. From what I'm hearing from new and prospective owners, it seems to me that many potential Roadster customers are put off by the Tesla plug and this is probably already becoming a barrier to sales.

In the absence of any word from Tesla Motors about a J1772 upgrade path, we've been slowly working toward doing a conversion ourselves. A few weeks ago, we finally obtained an ITT Canon 75A UL-approved inlet and plug pair from Clipper Creek. The plug cord is intended as a replacement cable for Clipper Creek's model CS-100, and carries the same power and signal wires as the TS-70 aka Tesla's High Power Wall Connector (HPWC, formerly the HPC). Clipper Creek also sells a holster for the J1772 plug that can be used to replace the holster for the Tesla plug.

With the necessary hardware in hand, we starting tackling the engineering challenges in getting the inlet mounted inside the Roadster's charge port: there's limited space to work with and the Roadster wasn't designed with the shape of the J1772 plug in mind, so getting the plug and cord to clear the body is tricky. It took a bunch of measuring, brainstorming, numerous experiments, a couple of laser-cut bracket prototypes, some Dremel work on the inlet cup, and then an adapter designed in CAD and printed on a 3D printer to get something functional.

This is what the back of the upgraded inlet port looks like. The blue piece is the mounting plate Cathy designed in CAD and we fabricated on a RepRap 3D printer at Metrix Create:Space.

inlet-adapter-plate.jpgHere's the work in progress just before installing the J1772 inlet and putting it all back together:

ready-to-assemble.jpgHere's the inlet mounted in the Roadster's charge port:

inlet-in-chargeport.jpgThe ITT Canon cord plugged into the Roadster's charge port:

Charging from our HPWC, now converted to J1772.

j1772-charging-home.jpgThe top of the inlet tilts back to angle the J1772 cord up. This works pretty well for the ITT Canon cord with enough clearance at the top of the port that it's easy to slide the plug in and engage the lock, easier than plugging in the Tesla connector in fact. The rubber strain relief on the cord barely rests on the body, plus our Roadster has the paint guard protection there, so I'm not worried about that minor contact damaging the paint.

itt-canon-cable.jpgIt's not quite as nice with the plug and cord used by the ChargePoint Coulomb chargers, but I think it's OK for use on the occasional road trip.

coulomb-cord.jpgIn addition to the cable clearance issue, there's another concern with our v0.9 conversion strategy that has to do with the largest difference between the Tesla and J1772 communication protocols.

The Tesla plug uses four contacts: two for power, one for ground and one for the pilot signal. The pilot signal is a low-voltage communications protocol that allows the charging equipment to tell the car the maximum amperage supported and allows the car to ask for the power to be turned on and off. The pilot signal is not connected to the car until the plug is connected and the locking switch is engaged. This switch plays a second role: if the driver tries to remove the plug in the middle of a charge, sliding the switch back interrupts the pilot signal which tells the car to stop charging. This happens very quickly so that the driver cannot get the plug untwisted and removed to break the electrical contacts while current is still flowing. It's important to prevent this because doing so can cause arcing, which would damage the contacts.

Instead of interrupting the pilot signal, J1772 uses a fifth wire for this purpose. Like the Tesla plug, the locking mechanism on the J1772 plug makes the proximity connection, so that when the driver wants to remove the plug and slides the lock it interrupts the proximity connection, thus telling a J1772 car to stop charging immediately (within a tenth of a second). Unfortunately, the locking switch on the J1772 plug doesn't interrupt the pilot signal.

With our v0.9 conversion (or a simple pass-through adapter), the driver can unlock the J1772 plug without the car knowing, and then pull the plug while power is flowing. Cathy and I need to make sure we don't do that. To solve this issue, we need to design a circuit that watches the proximity pin and interrupts the pilot signal when the J1772 plug is unlocked. I don't expect this to be difficult, but we haven't done it yet.

We have already made some improvements in the design. This is version 3 of Cathy's inlet mounting plate design, which we plan to print for our next revision:

bracket-front-v3.jpgIn addition to the improved mounting plate, our next steps are:

1) Hope that Tesla Motors provides an official conversion solution before it matters to most owners, thus saving us the remaining steps.

2) Design a circuit to monitor the proximity pin and disconnect the pilot signal when the J1772 plug is unlocked.

3) Test with other J1772 plugs and possibly work on a better solution for cable clearance over the body panel.

4) The 2010 and later Roadsters have the inlet cable assembly connecting to the PEM in a different location. There may also be other differences. We haven't looked into it yet and don't know if it will be more or less difficult to convert than the 2008 Roadsters.

5) Before recommending an unofficial conversion to other owners, we'll need to find out how this will impact our warranty. Tesla Motors has been cooperative with our efforts: they sold our group an inlet cable assembly so that we could do the conversion reversibly. We hope they will continue to be supportive rather than forcing us to wait until our warranties expire before being able to effortlessly access standard J1772 public charging stations.

About this Archive

This page is an archive of entries from January 2011 listed from newest to oldest.

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