Subject:
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RE: Battery Status
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Tue, 4 May 1999 12:58:23 GMT
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Original-From:
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fthompson9@aolSTOPSPAMMERS.com
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Viewed:
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1018 times
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I have quite a few battery discharge curves lying around, and one
thing I can tell you is that they are all different. Generally they start at
a high voltage, quickly discharge to a normal operating voltage, hold that
voltage for a majority of their life, and finally roll off around a "Knee."
There is a slope in the normal operating voltage region of the curve, but it
is very small especially compared to the other operating regions. It is easy
to tell when the battery has reached its knee, but once the knee shows up you
have very little battery life to work with. The big catch with all this is
that these battery curves are all made under constant current conditions. (I
have a device called "The Battery Tool" which places a programmed current
load on the battery, and measures the voltage every second. The computer
program on the controlling computer, plots the curves, and computes useful
information.) A robot uses everything except a constant current.
I get the impression that you would like to know the status of the
battery so that your robot can do something about the battery (seek a
charging station) when the battery reaches a certain level. I think that you
will have a hard time doing this with the Handy Board as it is currently
made. Measuring the battery terminal voltage will certainly tell you more
than you currently get (nothing). But you will have to jump through some
extra hoops to get useful information out of if. The first thing you will
need to do is to find a way to "idle" the robot while you make your voltage
measurement. You don't have to remove the load, just make it the same for
each measurement. Second you will need a history of typical behavior.
Finally you will need a way to update that typical history as the battery
ages. You could probably do the last two parts in the same algorithm if you
can set the initial conditions right.
I personally feel that the right way to go is with a smart power
source. This is where your NiCads are packaged with a microcontroller and
current sensor. The Micro keeps track of the charge taken (and the current),
the last time and amount of charge, and can respond to a host (serial or I2C)
clearly stating the expected remaining battery life.
It would be nice to hear from the satellite people. They have to be
real battery experts. (You can't just go up and change a battery.)
Later,
Pherd
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