The Electric Motorcycle: A DIY Primer: Page 2 of 5

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The finished electric motorcycle.
The finished electric motorcycle—road-ready for miles of fun.
1984 Honda VF500F Interceptor donor bike
This 1984 Honda VF500F Interceptor donor bike has its frame stripped and primed, ready for paint.
This 1984 Honda VF500F Interceptor donor bike.
This 1984 Honda VF500F Interceptor donor bike has its frame stripped and primed, ready for paint.
AC brushless motor.
AC brushless motor.
PMDC brushed motor.
PMDC brushed motor.
BLDC hub motor.
BLDC hub motor.
The controller, contactor, and fuse.
The controller, contactor (lower right), and fuse (upper right), with everything neatly laid out, will be hidden under the old fuel tank.
Charger and the controller.
Both the charger and the controller generate some heat, but airflow around the aluminum mount will keep things cool. Note the two chargers: one is a 72 V charger for the main pack. The second, smaller one is a 12 V charger for this bike’s separate 12 V system battery.
Battery mounts.
Battery mounts are another place where hefty design and redundancy are wise.
Motenergy ME0709 motor.
An example of a basic motor mount with a Motenergy ME0709 motor. The mount bolts to the frame at the top and bottom, and a bar counters the rear pull of the chain.
Testing component placement.
Using duct tape to test component placement.
Battery mount.
This mount has three sets of hold-down brackets, able to handle the 90 pounds of batteries.
Two 12 V SLA scooter batteries.
Two 12 V SLA scooter batteries, wired in parallel to give enough capacity for lights and a horn.
The Cycle Analyst display.
The Cycle Analyst display tracks vehicle speed and the propulsion batteries, including voltage, power consumption, and remaining capacity.
Klll switch.
The kill switch operates the main battery contactor to cut power in an emergency.
The throttle.
The throttle, which is electrically connected to the controller, is used to increase and decrease the power that goes to the motor.
The newly converted electric motorcycle.
The author’s son Tyler takes a spin on the newly converted electric motorcycle.
The finished electric motorcycle.
1984 Honda VF500F Interceptor donor bike
This 1984 Honda VF500F Interceptor donor bike.
AC brushless motor.
PMDC brushed motor.
BLDC hub motor.
The controller, contactor, and fuse.
Charger and the controller.
Battery mounts.
Motenergy ME0709 motor.
Testing component placement.
Battery mount.
Two 12 V SLA scooter batteries.
The Cycle Analyst display.
Klll switch.
The throttle.
The newly converted electric motorcycle.

The Controller

The controller is the “brain” of the propulsion system. Using pulses of current, or pulse-width modulation (PWM), the controller allows a twist-grip throttle to precisely vary the motor’s speed and power. Pulse-width modulation (PWM) looks like small on/off bits of full power, and is how any DC motor speed control works. AC motor controllers manage the timing of the pulses to synchronize with the AC motor’s rotation. A controller can also turn the motor into a generator during deceleration (regenerative braking), feeding the generated current back to recharge the batteries. Controllers are generally programmable and, in some cases, can also handle data logging.

Usually, the controller and the motor are chosen together. In the case of a PMDC motor, there are some that work better with certain motors. With BLDC or AC motors, the controller must be matched to the motor. Very often you can buy the controller and motor as a matched set. If you can’t, follow the motor manufacturer’s controller suggestion.

The Contactor

A motorcycle’s electrical power is potentially dangerous. It’s usually 48 to 72 VDC, and between 250 to 400 amps. A safety switch—one designed handle the bike’s voltage and current—is needed. This contactor is a high-current electromagnetic relay, and is standard equipment on virtually every type of EV from golf carts to cars. It allows turning on and off the main pack power, using a small, lower-current switch—a safety cutoff switch that mounts on the handlebars.

The Batteries

Many think that an EV’s power comes from the motor, and a bigger motor gives you more power, but the power really comes from the batteries. How fast the batteries can deliver current, and the battery type and performance will be a big factor in determining how powerful and fast the bike will be. Simple lead-acid batteries can’t deliver large amounts of current for long periods of time. Batteries like lithium-polymer (Li-po), used in radio-control cars and planes, can deliver high current for extended periods—and current equals motor power.  The motor, as well as the controller, must be able to handle the power the batteries can deliver. If you have small, low-discharge batteries, you can run a smaller motor. If your batteries are the big, high-discharge lithium type, you’re going to need a big motor to handle the heat and strain.

A battery’s Ah capacity determines the bike’s range. For a motorcycle, the minimum would be about 20 Ah. At 72 V, my 22 Ah lead-acid batteries lasted about 12 miles at normal speeds. The Brammo Enertia production motorcycle, for example, has 40 Ah of batteries with a range of about 40 miles.

On the one extreme, you can set up a bike with 72 V of comparatively inexpensive lead-acid batteries and it will be heavy, but the batteries will only provide a limited amount of current. On the other, you can set it up with lithium-ion batteries for a lot more money, have about half the weight and bulk, and get a discharge rate that will melt even the most robust motor if it’s not handled properly. Lithium battery technology is remarkably “power dense,” yielding light weight and small size—but is also dangerously volatile.  Modifying lithium with the use of lithium-ion technology takes full advantage of lithium chemistry, but is considerably more stable, safe, and reliable. These batteries still, however, require special handling, charging, and management to provide the best performance and lifespan, compared to lead-acid types.   

To get a little more familiar with specific types of batteries, let’s take a look at the two extremes. You can set up a bike with 72 V of lead-acid batteries and it will be heavy, but cheap, and the batteries will only be able to provide a limited amount of current. Or you could set the bike up with the highest-performance lithium-polymer batteries for a lot more money, at about half the weight and bulk, and get a discharge rate that will melt even the most robust motor if it’s not handled properly. Or you can do something in the middle of the two extremes. There are several variations of lithium-ion battery technology that are moderately expensive, light, and have very good power density, like lithium iron phosphate (LiFePO4) batteries, which are increasingly common for electric vehicle applications. 

With lithium-ion batteries, you’ll need a management system (BMS); and need to pay attention to high- and low-voltage limits; and balancing cells—all issues that, because of the cost and volatility of lithium chemistry, have to be handled properly. 

Consider starting with lead-acid batteries, with the idea that you’ll eventually replace them with a lithium-ion system. As for the rest of the system, the battery type doesn’t matter, as long as you’re supplying the required voltage. The system is, by nature, “battery agnostic.”

Comments (2)

jerryd's picture

The one thing missed and the biggest factor in EV MC range is aerodynamics. A standard MC has the aero of a brick!! And that at over 25mph just sucks power.

But fairly simple aero mods can double range. Also lowering the seat as much as you can helps lower frontal area.

Even a box behind the rider with curved front corners wider than the rider and gently curving back inward before being chopped off cleanly plus going down to the axle level can seriously cut drag and give lockable space for shopping, etc, by cleaning up the airflow, thus cutting drag.

I'm building a complete aero cabin on my MC to make it a long distance cruiser at 70 mph. Just got the chassis running on it's own power as should be finished by mid Feb with the aero cabin.

Ben Root's picture

Awesome points you make about aerodynamics. Once, back in my bicycle racing days I heard that, at 30 miles per hour, a cyclist is using 90% of their energy just pushing the wind, and only 10% moving themselves and the bike. (or, if there was a 30 mph headwind, they'd be using 90% of their energy just to stay upright). I'm guessing that an MC will have a similar aerodynamic profile...if not worse.

Another thing to remember is that (like the wind turbine guys say), the power in the wind is a cube of its velocity. I assume that it's the same ratio working the other direction...pushing through the wind. So as your speed goes up, the energy that it takes to push through the air is going up at a cube of that. IE going from 20 mph to 30 mph is a 150% increase in speed, but a 337% increase in required power (1.5 x 1.5 x 1.5 = 3.375). So, reducing speed is the best way to reduce required energy. Albeit, who want to slow down on a motorcycle.

We'd love to see you completed aero-bike.
Ben

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