Subject:  Re: Electric Car
Date: 1997-02-20 01:58

Here are some details on what I envision for the Electric Car project.

     (1) deep-cycle marine 12V battery under $50 -specified by Lee

We'll use a conventional lead-acid deep-cycle battery, weighing 40-60 lbs.
Deep-cycle batteries survive hundreds of deep discharges. Regular car
batteries are ruined after a dozen or so deep discharges.

By standardizing on a particular battery, we are defining the total amount
of energy available. It's like limiting the amount of gasoline in your gas
tank. you'll focus on economy, not drag racing. (By the way, a 50 lbs
battery has the energy of 1 cup of gasoline.)

Most batteries use "flooded" cells, which means the acid can spill if the
battery is tipped over. This is the only kind Fleet/Farm (and most other
battery stores) carry. So I am looking for batteries with "gelled" or
"sealed" cells, which won't leak in any position. They cost more, but I
have a couple leads on suppliers that I think will give us a price break.

     (1) automobile heater motor P/N 26102 for $20

The most suitable motor we found at Fleet/Farm is this replacement heater
fan motor. It is typical of the type of motors found in every car, for
radiator fans, windshield wipers, power seats, etc. I expect it to be
around 1/10 to 1/6 HP; that's enough to reach 3-5 mph.

I think we should supply at least one workable motor, but encourage the
students to try others. Allow them to use other motors, or more than one
motor as they see fit. More, or bigger motors will increase speed, but at
the expense of range.

     (1) pulley for motor shaft, to be specified by Lee, for $10

It won't be nearly this expensive. One of the simplest, yet most effective
drive methods is to simply rub the motor shaft against the tire. Arrange a
door hinge so the motor's weight (or a spring) holds it against the tire.
The rubber grabs pretty good on the metal shaft. The bigger the shaft, the
faster the wheel goes. So the trick is to find the optimal diameter tube or
pulley for the motor shaft. (A great lesson in torque, speed, and reduction
ratios lurks here). My goal is to find some tubes or pulleys that fit the
motor and provide a range of sizes.

     (1) 90-3HWP High 3-wheel kit chassis for $125 <--actually $128

This looked ideal. It is a 3-wheeled cart intended for farm use. It has a
steel frame about 2' wide by 3' long, two 24" bicycle wheels on a 3' long
axle, and an 8" diameter wheel that pivots like a chair caster. Very
rugged, with ball bearings, etc.

Steering: You need to rig up a way to steer the caster wheel. This could be
a shaft and steering wheel, rope and pulleys, attach a bar and steer it
with your feet, etc.

Brakes: You must have some form of brakes. Speeds (and skill levels) will
be low here, so anything goes as long as it works. Bicycle caliper brakes,
a hinged board that presses against the tire(s), or an anchor (cement block
on a rope) are fine by me!

The kids could build a body out of wooden crates, aluminum siding and pop
rivets, plastic wading pools and duct tape, chicken wire and paper mache',
fiberglassed cardboard boxes, etc.

     (1) circuit breaker to be specified by Lee for $10

Like the battery, this is a part that must be used. It is for safety, to
prevent any damage when kids short the batteries or miswire things. Just as
the battery sets the energy, this circuit breaker also sets the peak power
available. This is equivalent to limiting horsepower in a car.

I used a 30 amp breaker for "Sparky", which was much heavier. So this
should be more than enough here. At present, I think we should try
providing a 20 amp breaker. It is easier to find wire and switches for 20
amps. We can increase this limit if there is a need to go faster, climb
hills, etc.

Note: UL considers voltages less than 30 volts to be intrinsically safe;
i.e. there is no shock hazard from a 12 volt battery. The main risk is that
a short circuit can cause a wire to get hot; maybe hot enough to burn
someone or something. The circuit breaker's job is to prevent this.

Wiring, motor speed control, switches, etc.

In such a low powered vehicle, simple on/off switching is adequate.
Ordinary house wiring switches are fine. They should be rated at the same
or more current that the circuit breaker. The main "throttle" switch should
be a pushbutton, or have a spring return so it automatically turns off if
the driver lets go of it.

A double-pole, double-throw switch (called a "4-way switch" in electrician
jargon) can reverse the leads to the motor to make it go forward or
reverse. If there are 2 motors, an elegant speed control is to arrange
switches to connect the motors in series or in parallel. This provides two
speeds.

I would encourage the kids to have fun with the wiring. Encourage them to
add lights, horn, radio, or anything else that they find appealing. The
rule should be "as long as you use the supplied battery and circuit breaker
as the only source of motive power, anything else you want to add is fine."

The Race

The course should be paved, and as flat as possible. The vehicles will get
more complicated if they have to climb hills or go on rough terrain (a
transmission or much bigger motor needed).

I think the race should be based on who can cover the most distance in X
amount of time. This penalizes excessive speed (the batteries will die
quickly), or excessive slowness (These vehicles are easily capable of going
for several hours and up to 10-20 miles on a charge.

Should we do a long relay race, where each team member gets to drive a
number of laps; or a number of short races with different drivers?

I'm getting excited; the possibilities are here for some serious learning,
and real fun. Feel free to call or write if I can be of any help. Comments
would be greatly appreciated!

Lee Hart