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.
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.
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.”