Battery-Powered Electric Motorcycles

Are battery-powered motorcycles the future of motorcycling? Not yet, we say. Here’s why, along with what they can do superbly and what they can’t do at all.

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If you talk with Neal Sakai, the man who began Zero Motorcycles, it's hard not to catch his enthusiasm for electric motorcycling. Internal-combustion engines, he likes to point out, are less than 30 percent efficient. Electric operation—where losses in the vehicle can be held to as low as 10 percent—is the better answer. Bring up the issue of limited range on current electric bikes, and he waves it away as a passing phase. "They're getting quicker to recharge and quicker to have more energy per cell. In the next couple of years, we'll have motorcycles with a couple of hundred miles range. You won't have to charge them every day, just once a week or twice. The battery companies are taking care of the problems."

Bill Dube, the energetic and creative force behind the most successful—and record-setting—electric drag-racing motorcycle would agree. He routinely runs his Killacycle through the quarter-mile in the 7.8- to 8.0-second range, with a 0-to-60 time of 1 second and terminal speeds as high as 175 mph, all on a few cents of electricity per run. “An electric-powered vehicle could be the quickest thing out there,” he says. “The batteries will support it. We just don’t have the motors designed for the application.”

As you start hanging out in the electric-vehicle world, that’s a common theme—almost all the parts being used for electric motorcycles and racing cars were originally designed for something else. No one has yet built a dedicated, optimized production motor for an electric bike—or a lightweight, short-fused, 5000-horsepower motor that would be perfect for running against Top Fuel cars.

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Killacycle
Bill Dube’s Killacycle is perhaps the best example of the kind of performance that can be obtained from electric bikes for short distances: 7.8 seconds and 175 mph in a quarter-mile. It uses high-power A123 26650 cells originally designed for DeWalt 36-volt cordless tools; it takes 1210 of them to make up the 200-pound pack in the current Killacycle.

But the batteries are progressing. Dube, sponsored by Boston-based A123 Systems, a specialist in high-power cells, has been given a gift by his corporate benefactor: a small pallet of A123’s extremely limited-production Formula 1 battery cells, intended for use in F1 kinetic energy recovery systems (KERS) when such things were allowed. They’re the highest-power cells on the planet that aren’t locked in a laboratory, and a 50-pound pack of them will readily support the 550-hp-or-so his hot-rodded industrial motor is capable of on the dragstrip. With their help, Dube plans to take 150 pounds off the Killacycle and recover his very recently lost title: owner and rider of the quickest electric motorcycle on the planet. All the while, he believes it’s only a lack of resources preventing him from being able to omit the adjective “electric” in that phrase.

With all this optimism, and demonstrated performance to boot, what's to stop electric bikes from taking over? Well, there's the little issues of range and recharging time. Many gasoline-powered bikes will go 150 to 200 miles on a tank of fuel, and replenishing those five gallons might take all of five minutes. Recent electric bikes, such as those made by Zero or Brammo, have both had limited performance (top speeds less than 70 mph) and limited range. An enthusiastic rider could easily push a first-generation Brammo Enertia down to a 20-mile operating envelope, and Zeros are only marginally better. Plus, a refill for either is measured not in minutes but in multiple hours. Both companies are offering new bikes with more speed and more range.

But, the problem with electric bikes is clear: They offer superb performance possibilities, as demonstrated by the Killacycle and recent electric roadracers such as the MotoCzysz E1PC or the Chip Yates SWIGZ electric racer, but with very limited range and slow recharging. To understand the nature of this dilemma, it’s necessary to retreat back to a high-school physics course and consider the difference between power and energy.

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Zero Motorcycles
The 2011 Zero S street model has a 4.5 kW-h lithium-ion battery pack, good for a 45-mile range in suburban riding. The pack, also used in the new Zero MX model, is put together with patented interconnects between cells that make it capable of sur-viving the kind of high-impact events an off-road bike can experience.

Energy is what’s used in doing work. It takes a certain quantity of energy to heat a pound of water 100 degrees or to travel for an hour at 70 mph, fighting wind resistance and rolling drag the entire way. It’s not something motorcyclists typically think much about, because with gasoline, they almost always have enough. If you start running short, you just find a gas pump. You could easily think of energy as similar to the amount of money you have in your bank account: Just as with energy, it takes a certain quantity of money to do certain things.

Power is something else; it's the rate at which you can use energy. Have enough power, and you can accelerate quickly and go very fast; have enough money, and you can buy the machines that enable you to do so. A low-power engine is like having an ATM/credit card with a very low limit: You might have a lot of money at the bank (i.e., fuel in the tank), but if you can only withdraw $20 a day, you're not going to have the same possibilities—for better or worse—than if you could get it all more quickly. Conversely, lots of power with little energy is like being in Vegas with a no-limit ATM card but just $200 in the bank: You're soon going to be standing by the side of the road with your thumb out.

Electric vehicles work on the same principles but differently. In an electric vehicle, the batteries are the energy storage device, not a combustible fuel. The batteries release energy through a reversible chemical reaction as electricity that is capable of directly powering an electric motor. But a battery’s design and size set limits on both energy and the rate at which it can be released—that is, batteries are both power and energy limited. Generally, the motors on electric vehicles are sized to the limits imposed by the batteries; there’s no point in having a 100-hp motor if the batteries can’t release electricity quickly enough to achieve that level of power.

For electric vehicles, and particularly in reference to battery capabilities, power is almost always expressed in kilowatts, not horsepower. It’s merely a unit change, however, since 1 kilowatt is equal to 1.34 horsepower. And when dealing with electricity, energy is generally expressed in terms of kilowatt-hours, or kW-h, rather than in horsepower-hours. The point of all this is that we can use those same units to compare batteries and gasoline.

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Mission Motors
The latest Mission R racer proves that electric bikes don’t have to look weird. The chassis was conceived with the aid of RADD designer James Parker and is packaged around a 14.4 kW-h battery pack and a 141-hp AC motor. Its broad torque spread allows it to have a single-speed gearbox, and it is capable, says Mission Motors, of accelerating from a stop to 160 mph without its rider having to operate either gearshift or clutch.

It turns out gasoline is very energetic, indeed. A single gallon of gasoline, when burned with the oxygen freely available in the air, can release 36 kW-h of energy in the form of heat. But, because internal-combustion engines aren’t terribly efficient in converting that heat into work, perhaps only as little as 6.7 kW-h of that is actually usable. Measured against weight instead of volume, gasoline, as processed into energy by a typical engine, is good for about 2.3 kW-h per kilogram (and we’re using metric here because almost no one talks about battery specific energy in any other units than kW-h/kg).

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The bad news about batteries is that compared to gasoline, they’re almost off the chart in specific energy. Lead-acid batteries, at 0.04 kW-h/kg, are pathetic; in terms of riding range, almost 370 pounds of lead-acid batteries would be needed to match what an engine can do with a gallon of gasoline. It’s only when you get to the latest lithium-ion batteries, a battery type that has only been commercially available for about 15 years, does it get much better, and even then it’s only a matter of degree. It takes 60 lb. of the latest, greatest and most-expensive lithium-ion cells to equal what a gallon (6 lb.) of gasoline can do. And in an actual motorcycle, those cells will have to be encased, protected, monitored by additional electronics and have some attention paid to controlling their temperature. The hardware necessary to do all that will add 10 to 20 percent to the pack weight.

What’s more, although lithium-ion cells have been gaining energy density ever since they first became commercially available, the rise is linear and not exceptionally rapid. It’s going to be a long time before batteries catch up to the specific energy of gasoline. That will almost certainly require a step-leap upward with an unpredictable breakthrough in getting an exotic cell chemistry to work someplace other than in theories.

Why electric vehicles weren’t happening before, though, was the cost. Until recently, a rule of thumb was that a completed lithium-ion battery pack for a vehicle cost about $1000 per kW-h. That would have placed a pack that matched the usable energy in a gallon of gasoline at north of $6000—and that’s the manufacturer’s cost. It would have added $12,000 to the sticker price, and unless the manufacturer offered an unusually heavy discount, a replacement pack would have retailed for around $20,000.

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Chip Yates
The most recent electric motorcycle to make the news is the Chip Yates/SWIGZ racebike that made the podium in a club race against Open-class Superbikes. With an 11 kW-h battery pack and a 190-hp electric motor, it can out-accelerate and out-speed club-level Ducati 1198s, but not for long: Range is 25 miles at best. It also looks like the product of a mad inventor in a home garage.

Fortunately, even as carmakers have begun to embrace some degree of electrification, the entire consumer electronics industry beat them there, with lithium-ion batteries essentially standard in every laptop computer and cell phone made. The resultant volume increases have started driving prices down; current costs are perhaps half as high as before and still falling. Indeed, the ubiquitous 18650 size cell (18mm in diameter, 65mm long, just a little bigger than an AA alkaline), which can be found in most Windows laptop computers, has fallen to the point that if you aren’t buying cutting-edge energy capacity in each cell, a manufacturer could put a pack of those together for about $350 per kW-h.

Curiously, as General Motors looks to drive lithium-ion battery-pack prices down for the Volt and its other hybrids by using large-format prismatic cells that will simplify pack construction, California start-up Tesla is riding the consumer-electronics learning curve and building its pure-electric cars around batteries consisting of literally thousands of those tiny 18650 cells. Its next car, the Tesla S sedan, may actually use 8000 cells in its optional high-capacity battery pack.

Why? Because Tesla expects to be able to put packs together for $200 per kW-h by 2012 using the cells that every battery company in the world seems to be making, with the competition driving prices down faster than the seemingly more-rational GM approach. Both Toyota and Panasonic, a major lithium-ion cell supplier, have invested in Tesla. The company looks to solve the range problem by brute force: get the battery cost low enough that you can afford to buy a really big and heavy one.

Indeed, the amount of research going into battery technology right now is unprecedented, and electric vehicles are interesting because of some of the results. Remember earlier when we said that batteries are limited in both power and energy capacity? Well, it’s possible to optimize for one or the other, so there are both “power” cells and “energy” cells. American battery company A123 has specialized in the first, and its cells, while having less than half the energy of the highest-energy cells of its competitors, have exceptional power, as much as 20 kW/kg (12 hp/lb.) for its Formula 1 cells.

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Brammo Enertia
Brammo’s first bike, the Enertia, has a 3.1 kW-h battery pack and will carry its rider a little more than 30 miles before needing a charge.

As long as you’re willing to live with a gallon of gasoline’s worth of energy capacity, it’s possible to put together a complete electric powertrain—motor, controller and battery pack—that either matches the weight of sportbike and racing engines or is only slightly heavier. Try to match five gallons, though, and the complete powertrains get piggishly heavy. What’s more, the power cells are capable of being charged exceptionally quickly—as long as you’ve spent several thousand dollars to have a high-rate, 220-volt charger installed in your garage. A standard 120-volt outlet is limited to 1.5 kW, making a high-power home circuit essential for fast charges.

But what does all this mean for electric motorcycles? In the near future, there’s going to be a complicated trade-off between power, range and cost. If a manufacturer chooses a relatively large battery pack, the price, weight and range will increase, particularly if power and performance have stayed relatively modest.

This will likely be the strategy of Brammo and Zero for their urban commuter and off-road models. For ultimate performance, high-powered electric bikes priced like Bimotas (or more) may soon be able to hang with or even better internal-combustion-powered sportbikes, even on the racetrack, but only for minutes; the ones with longer range will be too heavy. Electric roadracers over the next several years eventually are likely to set some impressive lap times. but it’s going to be only for race lengths much shorter than a 50-mile AMA Superbike race or a 45-minute MotoGP race, at least until batteries improve to the point where a 35 or 40 kW-h pack can be part of a 375-pound motorcycle.

In fact, the most interesting applications of electrification are for bikes that already have tiny gas tanks, such as motocross or trials, and that also only spend a small fraction of their time at full power. With only the expected improvements in battery performance, it should be possible within the next several years to build an extremely competitive electric motocrosser capable of finishing 20-minute motos and being recharged between heats. This should open up entirely new possibilities of quiet urban motocross on courses built on vacant lots in industrial neighborhoods. And drag racing, with its defined quarter-mile commute, is ideal for high-power, low-energy applications.

But what will really make or break electric motorcycles will be how current motorcyclists react to them. With seamless performance, no gear shifting and the loudest sounds being only motor whine and the annoyingly loud whir of a chain (no longer masked by exhaust note), it may simply be that electric motorcycles aren’t as engaging as internal-combustion models.

For that, only time, and tastes, will tell.