Re: Battery

[Tim Vanderhoek <vanderh@ecf.utoronto.ca>]

On Sun, Jan 09, 2000 at 08:43:08PM -0500, Cindy Wong wrote:
> 
> Anyway, how important is it that our batteries maintain a stable discharge
> voltage at high rates of current discharge?  I have no idea what this
> means, so, you guys may be able to help me.

The question of a "stable discharge voltage at high rates of current
discharge" applies primarily to the batteries that supply the motors.

In a real voltage source, especially DC batteries, as the current
drawn from the source increases (and remember that motors draw more
current when they are stalled or when loaded) the voltage provided by
the source decreases.  This can be modelled using the concept of
internal resistence of a voltage source (remember ece150?).

For electronics, it is important to maintain a stable output voltage.
If the output voltage drops below a certain level, the electronics
(especially comp) may cease to function or, worse, may function in
unpredictable ways.

The current through motors can vary tremendously.  The current drawn
by the interface+comp system is fairly constant.

For motors, nothing particularly bad happens.  Because of the
wheel-speed feedback system, the computer can easily compensate for
the decreased voltage --- merely increase the duty cycle.

It is possible to determine the maximum current draw of a motor by
stalling the motor and measuring the current.  Except for possible
wierd-ass effects caused by the motor acting as an inductor, current
for the motor will at no time exceed the stall current.  It is
possible to create a linear current vs. torque vs. rpm (for some
combination of two :) graph.  The Q4D link in weblinks.html has an
essay explaining how to do this.

HTH.


-- 
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