Subject:
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Re: Power Usage of RCX
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Newsgroups:
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lugnet.robotics
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Date:
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Thu, 12 Aug 1999 05:15:17 GMT
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Viewed:
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668 times
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In article <01BEE41B.A7DED0C0.pritchard.james@virgin.net>, James Pritchard
<pritchard.james@virgin.net> wrote:
> I was astonished to find that when the RCX is just on (NOT running a task)
> with the light sensor, both switches, and both motors connected that the
> Battery voltage drops very quickly. 12 mV a second or more. Is this to be
> expected.
As you draw power out of a battery, its voltage will start to decay.
Usually this is specified as some sort of discharge curve of battery
voltage vs. time under a constant load. Each type of battery has
different characteristics, but assuming you were using Alkaline cells...
There is usually a very steep portion of the curve near the "fully
charged" state, where even a slight current will cause a rapid drop in
voltage. You will usually see voltages over the nominal cell voltage
(1.5V) during this portion of the curve. Very quickly, however, the cell
starts to plateau. During this time the voltage drop is slow and fairly
linear. A battery spends most (but not all) of its useful life in this
state. Finally, near end of life, the curve has a "knee", and voltage
drop vs. time gets pretty steep again.
I measured current of 48mA in this situation. 39mA if the light sensor is
either not connected or inactive. This will probably give you upwards of
30 hours of "idle" time off of new alkaline cells(1).
The rapid voltage drop you saw could've been because the batteries were
either at the start or the end of their effective life (where the curve
gets pretty steep).
Another factor is that the battery readings get sampled every so often,
and there's some sort of averaging going on. When you first turned on the
RCX, the battery had been "resting", and this typically results in a
slightly higher voltage for the cell. This "recharge" vanishes very
quickly, however, so it is possible that the RCX took one battery reading
in the "over voltage" state, then over time averaged in more accurate
readings of the battery under load, so the moving average was declining
even though the cell had already plateaued.
Bottom line - don't panic. If you think the RCX is draining batteries too
fast, then try to make a current measurement to tell for certain. If you
don't have a meter to do this, then setup the RCX and take voltage
readings every minute or so for a 10 minute period. The readings near the
end of the 10 minutes should be pretty stable. If you do an experiment
like this, post the results and I'll double-check with mine.
As usual, I'm sure I posted a lot more info than anyone is really interested in.
Dave Baum
(1) I haven't been keeping up with battery technology, so I have no idea
how dense alkalines are getting these days. I assume 1500mAH for a AA
cell isn't too aggressive, though. Anyone know for sure?
--
reply to: dbaum at enteract dot com
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Message is in Reply To:
 | | Power Usage of RCX
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| I was astonished to find that when the RCX is just on (NOT running a task) with the light sensor, both switches, and both motors connected that the Battery voltage drops very quickly. 12 mV a second or more. Is this to be expected. Is the RCX watch (...) (25 years ago, 11-Aug-99, to lugnet.robotics)
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