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From: jmuchow@SPAMMENOTcamlight.com (John Muchow)
Subject: Re: Reducing contact resistance for low volt use?
Date: Thu, 12 Dec 2002 19:48:47 GMT
Organization: MindSpring Enterprises
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X-Server-Date: 12 Dec 2002 19:48:19 GMT
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>>>Let me ask you about the loads which are used.
>>>What resistance do these motors have?
>>>Do they really have inner resistancies in the milliohm range?
>>>Do you have more specifications of such a motor?
The Magmotor is very popular and has a resistance of 50-60 milliohms:
Follow the "Robot Motors" link (darn frames). Most combat robots
using these motors will use 4-6 of them (sometimes more) all powered
from 1-6 battery packs in parallel.
Many other motors are used typically used at twice their rated voltage
to get 4 times the power....lots of current. They only need to go for
a 3 minute fight, so destructive overheating can be avoided. There
are a lot of liberties taken with the "standard" specs for the
components being used...they only need to last a few hours total, if
not just a few minutes.
The motors I'm using in a combat robot draw 40-amps at wheel spin and
120 amps at stall. They're 12V starter motors overvolted to 24V. I'm
using a 24V, 3000mAH NiCd battery pack for each motor and I'm lucky to
get 5 minutes from them if I spend a lot of time pushing against
another robot. And that's only for a 60-lb Lightweight robot. The
Heavyweight and Super-Heavyweight classes (almost 400lbs) use
extraordinary amounts of power.
>>>What is the typical discharge curve of a battery when it is used for
>>>maximum current in real use?
>>>For example if you take out 50A, for how many seconds does the battery
Most of the batteries drop to about 1.0V and last for, maybe, 3-5
minutes under full load before dropping off in classic NiCd/NiMH
style....depending on the motor used, line resistance, # of cells in
Here's a 30-amp discharge curve:
The discharger I'm hoping to build will give much better data for the
higher discharge current levels.
>>>As I understand it we are talking about the size of cells which are
>>>used in consumer electronics, like R14 and R20. (?)
>>>They cannot last for long at those kinds of currents.
The Sanyo Cadnica N-3000CR (R14 size, but NiCd...KR-26?) is a very
popular cell and is often used at 40+ amps continuous discharge...gets
hot though. :-)
Yea, they don't last long. But they don't need to for these
applications. Sealed lead-acid batteries are often used to save money
but their efficiency is lousy and the capacity drops to 1/2 the
AH-rating because of the high-current levels.
Some of the R/C car racers only use their cells for one race and throw
them away. They're sponsored so they can afford to, but it's also due
to the damage to the cells from just one race.
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