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From: jmuchow@SPAMMENOTcamlight.com (John Muchow)
Subject: Re: Reducing contact resistance for low volt use?
Date: Tue, 07 Jan 2003 03:13:34 GMT
Organization: MindSpring Enterprises
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X-Server-Date: 7 Jan 2003 03:15:25 GMT
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>>>>>>BTW, the load resistance of the motors often used is only about
>>>>>>50mOhms or so....pretty close to that dead short you mentioned. These
>>>>>>current levels I need will give me real data for real applications.
>>> Not after the battery makes it's way through the wires and circuits
>>>of the controller, and then the wires leading to the motor.
>>> THAT load is a series of series resistors. Each wire, and each
>>>connection. THEN the motor. You might see 100 A pulses or surges,
>>>but it is quite doubtful that any such current would show up as a
>>>continuous duty figure.
I agree, the load in a robot (or car or plane) is a series of
resistances. But, they have much higher voltages available to push
against those resistances and give them 100A discharge current levels.
The motor load number was merely mentioned to show that the resistance
levels are much lower than most folks think they are.
Almost none of the discharge current levels seen in our applications
are continuous. But, testing with fixed loads is a great start for
comparing cells as they are often used at almost-continuous levels at
up to 50A. Another great test is with varying loads that can simulate
actual conditions encountered by these cells. A great idea that will
have to wait a bit until we can upgrade an unit that still needs to be
The continuous-current discharge curves are valuable for comparison
because each builder doesn't have to wonder if the variable-load
simulation really approximates the way they will discharge their
battery packs. For combat robots, a spinner draws almost the same
current until it hits something, then a big spike of current as the
motors start up again. A wedge/pusher will have current levels that
are almost never the same for one moment to the next as it scoots
around the arena.
The continuous-current numbers are a worst-case scenario for builders.
They know, due to the varying current needs during an event, that the
cells will last longer than the test number show. But now they can
select a cell that will last the longest at their typical current
levels (often "averaged" out as even a typical level varies a lot)
with the lower current curves.
>>> I find it difficult to believe that you actually think a two inch
>>>bar of copper equally represents the loading presented by several feet
>>>of copper in a motor winding. Use some common sense.
Wow, I wish you'd do the same and read the posts in this thread. I'm
begging you now.
I *never* said that my load is a bar of copper. The load is a MOSFET
(part of a constant-current discharger circuit).
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