10 March 2012

Scooter: Part I, the Battery and Powertrain

One of my dreams or goals is to design and build wheeled vehicles, and what better way is there to start than to build an electric scooter, especially with the large community of hackers at MITERS? With that being said, I set off to build a scooter about a year ago, with steady progress over the year.

As with any project, I need to elaborate on the goals:
  • Top speed of at least 24kmph
  • Range of at least 8km
  • Weigh under 10kg
and macroscopic design choices:
  • I chose to work on a scooter because it's the smallest easy-to-control conventional vehicle that exists (but rest assured, a go-kart is coming in the near future). 
  • The scooter to be modded is a Razor A5. The gain of a larger frame and ride comfort greatly offsets the cost of an marginal increase in weight and cost.
  • Electricity is ideal because MITERS has an arbitrarily large supply of A123 batteries, which are conveniently donated by an MIT startup. 
  • I chose to go direct drive and build my own hub motor using Charles' excellent write-up, rather than using an indirect drive setup with an off-the-shelf motor, because it would give me a substantial introduction to mechatronics and machining techniques. In addition, direct drive powertrains are simpler because they eliminate the need for anything connecting the motor and the wheel (whether it be a belt, chain, or transmission).
To date, I have completed the battery and some of the powertrain, which I will elaborate below.


As previously mentioned, the battery pack consists of A123 cells in a 2x10 configuration (that's two parallel groups of 10 cells in series). Each cell has a nominal voltage of 3.3V and a nominal capacity of 2.5Ah, as outlined in the product page. In this setup, the batteries can output a peak of ~5HP (!!); it will also go a long way if I don't constantly floor it. Each of these cells weighs around 125g, so 20 cells will be approximately 2.5kg.

16 of the 20 cells

Next in discussion is the packaging. One critical characteristic of the Razor is that the deck is very low, which I would like to maintain. Thus, the batteries need to be lying on their sides. The scooter deck is wide enough for four cells lying in parallel and long enough for four of those groups, so I'll have 16 cells on the bed of the scooter and four in the steering column.

To build the package for the deck, I worked with the cells upright to simplify the soldering job. I soldered them in pairs on one side using aluminum ribbon. Then I took pairs of pairs and soldered the bottoms of the cells because the cells need to fold out into a flat layer in the end. Finally, the batteries were connected in parallel.



The most central part of a hub motor is a stator, which provides some number of legs onto which coil is wound. I'm using a giant stator because the wheels on my scooter are huge -- they're 200mm in diameter. I used a DLRK (distributed LRK)-style winding (I forgot the exact winding). After manipulating 18-gauge wire for an hour, here is the wound stator:

Finished Stator

Next up comes the can and the magnets. The size of the can was chosen to minimize the air gap between the magnets and the stator, which conveniently turned out to be a 5.5" (OD) x 1.5" (thickness) steel can. Before installing the magnets, I had to do whatever mechanical modifications to the can as needed, which was just drilling holes for the endcap screws.

Drilling holes on the mill

Speaking of endcaps, they're not too particularly interesting -- just polycarbonate disks. They're not yet complete because I haven't drilled the holes for securing to the can or have the proper cavity in the center for the bearings.

Choosing magnets was especially tricky because there was no exact formula for calculating sizes. Thus, I had to try different sizes and configurations (16 or 20 are optimal for a 18-tooth stator) while CADding. I settled on a 16-magnet configuration, where each magnet is actually a pair of N45 1" x 1/2" x 1/8" and N42 1" x 1/4" x 1/8"neodymium bar magnets from magnets4less.com, where the N numbers denote the strength of the magnet.

With magnets in hand, I needed some way to secure them to the motor can. I printed a support with notches for each pair of magnets using Makerbot, a 3D printer. Then, for each pair of magnets, I inserted them into the holder, making sure that they were facing the same direction with respect to polarity, and glued them to the stator with superglue. I also made sure that adjacent pairs were oriented with opposite polarities (something which I failed to do the first few times).

Printing the magnet holder

All magnets in place

Unfortunately, the opposing magnetic forces overpower the force of the superglue, thus requiring a stronger compound: epoxy. This wasn't any traditional epoxy; I manually confected the mixture from hardening, resin, and cancer, which is a white powder.


The result was a viscous yellow-green matter, which I hand-applied between and around the magnet pairs on the stator. At one point, one group of magnets overcame the superglue and merged with another group, which was remedied when the great Charles Guan stuck in a ziptie as a separator between the two groups.

The finished product, for now

To be continued!

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