The Electric Motorcycle: A DIY Primer: Page 3 of 5

The Charger

The charging system has to be tailored to the battery system since every battery chemistry has its own charging profile. Also consider the charger’s size—will it be mounted on-board, or will it sit in the garage? The charge rate usually dictates a charger’s size and cost. A fast, powerful charger is going to be larger, heavier, and more expensive, and may not be practical to carry on the bike.

Don’t wait to purchase your charger—when your batteries arrive, you’ll want to fully charge and test them. Especially in the case of lithium-ion batteries, you’ll need to make sure they are balanced, and then, if possible, cycled a few times to break them in gently. You can do this while working on the rest of your bike project, if you have the charger in hand.

Tools

Besides the basic tools, you’ll need fabrication tools like a drill press, a power metal saw, a big, sturdy vise, and other basic metalworking equipment. Here’s a list of tools that will make the project go smoother.

• Soldering iron. Get a powerful iron rated for the gauge of wires you’re working with. There’s nothing as frustrating as trying to make a good connection with an underpowered iron.
• Multimeter. A digital display makes it easy to get fast, precise readings. Top-end meters aren’t necessary, but good ones are nice to work with, and quite versatile.
• Cable crimper. Assembling your battery pack requires cutting large cable and solidly affixing appropriate lugs—a good crimper is essential. They range from $14 for hammer-type tools to about $60 for fancy manual ones. The basic ones work fine for low-volume production. 
• Heat gun. Every electrical connection on the bike should be insulated with heat-shrink tubing. The best way to work with heat-shrink is to use a heat gun.
• Die grinder. For sanding, grinding, removing corrosion, and even cutting and shaping, these tools are a joy to use. They can save an enormous amount of time.
• Angle grinder/cutoff tool. This tool will let you tackle just about any cutting or grinding task.

Safety first: All of the tools that will be used on a bike that has “live” power should have rubber-coated handles to keep from coming into contact with live, high-voltage connectors. Wear eye protection and gloves—not only are you working with machinery and cutting tools, you’re working with a lot of electrical power.

Building the Bike: The Plan

We started with a 1984 Honda VF500F Interceptor, a common bike to convert. This one had a clean title and a seized engine. The Motenergy ME0709 motor has long been the standard of the light EV world, and the controller that matches it is the Alltrax 7245. Highway speed is the goal, so we’re aiming at 72 V—the higher the voltage, the faster the motor can spin. We used sealed lead-acid (SLA) “mobility” 22 Ah batteries. That should provide about 10 to 15 miles of range, a top speed of about 65 mph, and a battery pack of less than 100 pounds. This will be a good beginning project, and provide a fun bike to ride.

The Motor Mount

The motor and battery mounts require some fabrication to withstand the weight and torque stresses safely. Recruiting some professional help for this is wise if you don’t have the necessary skills—especially where welding is concerned.

The fabricated motor mount usually attaches to the frame using the rear motor mounts the gas engine used. If possible, locate the electric drive sprocket with the same center as the original internal combustion sprocket, since the travel of the rear suspension pivots near that point and affects the chain tension. You can move it a little forward or back in the frame, but up or down (relative to the pivot point of the swing arm), will create chain wear and safety problems.

The stresses on a motor mount are large. First, there’s the motor’s weight. Then there’s movement—the motor takes stress from every direction as the bike hits bumps, accelerates, turns, and brakes. Then there’s the motor’s torque and a strong pull on the shaft to the rear caused by the chain trying to pull the motor backward.

If the motor is a long, AC-type motor, its face and rear both need to be supported. Shorter PMDC motors, such as used in this project, only need to be supported at the face. Most common EV motors use a standard mount pattern, called a NEMA-C face mount so they can easily be switched with four bolts.

I made a CAD drawing of the mount, printed it, cut it out of cardboard, and then tested the fit. After a few tweaks, I e-mailed the template to a fabricator friend who made the mount out of 1/4-inch steel for $100. The mount fit into the frame like a glove, and static testing showed it to be strong and rigid.

The Battery Mount

The “duct tape method” is common for testing where to locate the batteries or other components. There are challenges to designing a battery mount. First, it has to carry the weight of the batteries securely, in a way that protects them against damage and accidental electrical discharge. You’ve got to think past normal use, too. Consider the stresses the mounts may be subjected to—include laying the bike down while riding, and possibly more severe crashes.

Another challenge is the design flexibility. Suppose you want to start with SLA batteries, but plan to switch to lithium-ion later. You’ll need a mount that will allow you to swap out batteries, or at least change mounts.

A common solution for lead-acid batteries is an angle-iron frame and shelf that can be bolted into the frame. We created a bracket solution that allows the batteries to be clamped into place.