The focus of my previous instructable was on repurposing R/C hobby, namely model aircraft, parts for vehicle propulsion. It will also make up a substantial portion of this one. Why on earth
would you use parts designed for things which aren't technically supposed to be touching said earth when they're working correctly?
R/C aircraft components are the highest power-to-weight electric power systems
easily available to the average consumer.
In the quest for ever-increasing flight durations, model sizes, and acrobatic performance, the R/C aircraft world has seen most components such as motors and motor controllers pushed to the limit of realizable power densities. The performance of the average brushless airplane motor greatly exceeds that of an average ferrite-magnet, DC brush scooter or bike motor - typically by three or four times. In the size and weight of a 500 watt electric scooter motor (about 3 inches diameter, 4 inches long, and weighing about 4 pounds), the example motor shown at the bottom can handle 2 - 3000 watts
(i.e. 3 to 4 horsepower) of power throughput, and up to 6,000W peak.
The advent of mass produced, high performance lithium ion batteries is another factor. A modern lithium polymer (more on the distinction between different lithium flavors later!) battery the size of a U.S. house brick (about 1.3 liters) can store up to 300 watt hours, at the usual lithium polymer energy density of 240 Wh/L, and discharge at several kilowatts for a few minutes. No traditional lead-acid, or even nickel-chemistry battery system can come close to that. Granted, there are downsides to having such a dense energy source, and those will be addressed, but the truth is there in the numbers.
Part of my engineering interest is taking advantage of these increased power densities to create ever more compact but still practical vehicles. For instance, only with brushless technology is the miniature hub motor
I've been working on a possibility...much less electric inline skates using them
. Electric scooters are still imagined by most people to be large, low-performance, lead-acid battery powered monstrosities that are reserved for kids with back yards too big to run around efficiently in. While this may be true for most commercially sold ones due to cost reasons, if you're building one yourself, there's no reason to not expect better.
R/C aircraft components are the best power-to-price electric power systems, period
You'd notice I left the fine print "commonly available" up there, because there are things which are more hardcore than a cheap brushless motor in terms of power density. The problem is, you can't buy
them. Not easily, and definitely not cheaply. However, because R/C aeromodelling has become so prolific, parts are readily available over the internet and extremely cheap. Most R/C parts these days are manufactured in China and other East Asian countries and sold by dealers directly from those areas.
Even three years ago when I wrote my previous piece, this was not as true as it is now. The electric flight market, for the better part of the last decade, was dominated by high-market European manufacturers, and so was extremely
expensive and also exclusive. The American market had two major motor manufacturers in this era which were well established: Astroflight
and Aveox (which seems to have left the hobby market), and probably also Neu Motors
. While the most hardcore of aeromodellers still stick with the highest performance European and American brands and companies, the majority run some inexpensive East Asian power system.
For a custom small vehicle, it's hard to find parts which have better price numbers. Depending on your power needs, a motor that has enough wattage to propel a vehicle will run between $40 to $100. A controller that can run the motor will be around $50 to $150. Batteries are the big cost breaker for EVs, still, but typical model aircraft lithium polymer packs price out to between $0.40 to $0.70 per watt-hour
. A scooter may have around 150 to 200 watt hours of battery onboard, so expect about $70-100 in commonly available batteries. As I will show in more detail, you can assembly a roughly 1500 watt electric power system for something like $200 to $300 - and that is for everything
, even including batteries, and a charger if you're good at shopping.
All of this might seem like cheap airplane parts are the way of the future. However, they also have their downsides.
Unfortunately, R/C aircraft components are usually very rudimentary and somewhat fragile
Let's face it - airplanes are, in the purely mechanical sense, pretty easy loads to handle. The torque and power required increase directly with speed, so there's no static or locked-rotor (stall) conditions to worry about, unlike in vehicles where maximum torque is required at zero speed in order to accelerate from standstill. Airplanes are never supposed to hit things, bound over rocks and sidewalk cracks, or be jostled by suspension movements. What matters more in aeromodelling is light weight.
As a result, model motors are usually made as lightweight as possible, using thin metals and plastics, and undersized bearings and shafts. Substantard metal alloys (such as soft architectural aluminum) are common in "cheap" motor construction. R/C power controllers are usually single-PCB affairs which pack power semiconductors and logic right next to eachother using the minimum amount of support components possible by design, and only capable of reaching their ratings when placed in a constant airflow (such as, you know, the draft of a propeller), and are rated to just under the peak power handling capabilities of the semiconductors. One of the major themes I'll hit on later is that you must derate everything
. The ratings given for aircraft parts, especially cheap ones, are generally unrealistic for EV use.
Even worse is the fact that the average aircraft lithium ion pack doesn't have a hard shell. Lipo batteries are little squishy bags of volatile electrolytes and active alkali metals. I don't know who's
great idea it was to make a battery without a shell, but the abuse of a vehicle dictates that batteries must be properly mounted and proofed from shock, impacts, and weather. It makes me cringe a little even recommending using lithium batteries to the public because of those reasons.
You've been warned.
Overall, though, I think my point is clear: R/C model aircraft components are a good economical choice for those looking to begin experimenting with electric vehicle technology
. It may not be the best solution for someone looking for a reliable, maintenance-free, long-life commuting vehicle. Many R/C components are definitely not
manufactured to vehicle specifications and may fail or become finicky over a period of time. There are definitely commercial EV solutions available which are even plug-and-play - but I'm assuming if you are reading this, you have a little sense of adventure.