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From: jmuchow@SPAMMENOTcamlight.com (John Muchow)
Subject: Re: Reducing contact resistance for low volt use?
Date: Fri, 13 Dec 2002 03:37:28 GMT
Organization: MindSpring Enterprises
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X-Server-Date: 13 Dec 2002 03:37:04 GMT
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Most of the folks I know were doing BattleBots...much higher
performance robots there. But, RobotWars does give you $2,000 and
pays all expenses to London for 2 weeks (invitation only)!
It's incredibly fascinating and I've learned a TON of stuff in the
past two years I've been involved. Much more than I've learned during
the few years I've been involved with R/C plane and cars. Not many
R/C flyers and drivers have to worry about their packs being shorted
out due to their motors being shredded by a 25-lb. blade spinning at
>>>The process is so slow so I guess you can do the data logging
>>>manually, I guess that is how you do it.
We've paid others to do automated testing before but this discharger
is supposed to change that.
Otherwise, yup, we manually recorded the voltages...which we all
absolutely hated doing and would *really* hate doing for the several
dozens of tests we want to do in the near future with this discharger.
>>>It you need even lower resistance loads you could consider a purely
>>>mechanical discharger, without Mosfets and wiring details.
>>>You could simply short the battery with a hefty bar of copper, and
>>>record the voltage manually every 10-20 seconds or so.
>>>That would lower the load resistance to very low values.
>>>Choose the dimensions of the copper (silver) bar so you get the
>>>milliohm resistance you need.
>>>Use a mechanical switch like pressing a u-shaped copper bar against
>>>the battery poles with a mechanical lever system, lockable.
I like that idea but unfortunately we need the constant-current
functionality. (see below)
>>>You probably have no real need for many different resistance values or
>>>for keeping the Amp value constant.
>>>You could keep it simple.
>>>Just compare batteries by discharging through a known low resistance.
>>>Record the voltage over the discharge time period.
>>>The Amp value is easy to know from the cell voltage if you use copper
>>>u-bars of exactly 10mOhm or 5mOhm or even lower if needed.
>>>This mechanical method needs no electronics circuits.
>>>Only a voltmeter.
A very, very, VERY attractive solution but we need to spec the "true"
AH rating of these cells by using a constant-current load. It's how
the cells are rated by most manufacturers and third-party testers and
the users of these cells are accustomed to interpreting and comparing
We also do need to test from 0.25A to 100A as we'll be testing several
different size/capacity cells. Some are used for the R/C receivers
and <1-lb. Antweight robots (low current) and some are used for drive
motors (high current).
We have a couple of nice units that I put together (up to 20A) that
use constant-load discharging through fixed 50W and 100W power
resistors for 6V to 36V battery packs. This is fine for conditioning
the cells and discharging the packs before long-term storage, but for
testing and comparing individual cells, we need the constant-current
It's all very frustrating as these cells are all used in constant-load
applications, but spec'd using constant-current tests.
>>>Using batteries in series is acceptable if you can assume that the
>>>cells are fairly similar, but if you short the battery with a hefty
>>>copper bar, no mosfets, there is probably no need for that method.
I agree. If only we could use that method.
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