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From: jmuchow@SPAMMENOTcamlight.com (John Muchow)
Subject: Re: Reducing contact resistance for low volt use?
Date: Fri, 13 Dec 2002 19:57:06 GMT
Organization: MindSpring Enterprises
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X-Server-Date: 13 Dec 2002 19:56:46 GMT
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>>>>Or one big mosfet, in parallell with the u-shaped copper bar, which is
>>>>adjusted during the test to give constant current.
>>>>Now we have a constant current test again, but we have solved the
>>>>problem with getting low enough resistance, and the mosfet only needs
>>>>to take a small part of the load.
>>>The test equipment could be in two parts, one mechanical unit, the
>>>copper bar and mechanical level to press it against the battery, the
>>>other part amplifier, mosfet and logging circuit.
>>>One part of the copper bar is used as a shunt, The voltage is
>>>amplified and used to steer the mosfet to keep the current constant by
>>>shunting the rest of the copper bar.
>>>So the u-shaped copper bar needs 3 terminals, at each end and
>>>somewhere in the middle.
>>>Different copper bar for different currents, like one for 30A another
>>>Machine the copper bar ends to fit tight over the battery terminals.
Wow, damn good idea. Sounds like it could work!
And I can "neck down" the copper bar in the middle to adjust the
resistance in case the bar is too long to be practical otherwise. It
will be air cooled (even fan cooled) so I can let the necked down area
get hot...as long as the battery doesn't get too hot.
Gotta put a physical layout and schematic together to work this out.
Might just be able to do this only with a ckt similar to John Fields'.
It would take a bit of calculating time to work out how to size the
MOSFET constant-current module to minimize the need to change copper
bars (balancing the size of the MOSFET module with the trouble of
replacing bars), but it would be worth it.
And the need to have discharge currents from 0.25A to 100A always
makes this device a challenge. But, I can size the MOSFET module to
handle a lot of the varying power load instead of making bunches of
Hmm...have to make sure the copper bar doesn't get too hot dissipating
120 watts of heat though. No problem if it doesn't reach the battery,
but copper is a pretty good conductor of heat. At 100A, the test only
lasts a very short time...might not be a problem. Heck, the battery
will easily reach 100-degrees C. with a 100A discharge anyway.
I'll be using a Basic Stamp and StampPlot software for the logging but
the idea is the same as your parallel-port setup.
I'm thinking of using a push-action toggle-clamp and a spring stop for
the two ends of the rig to hold the cell. The toggle clamp will lever
out a steel bolt that I can screw into a piece of delrin (or better
hi-temp plastic). Another piece of plastic would be bolted to the
face of the spring stop which is available with springs rated from
2-25lbs or 16-47lbs (initial to final end force). Place the cell
between the toggle clamp's sliding bolt and the spring stop, slide the
copper bar into place on both ends of the cell, and lever the toggle
clamp to press it all together against the spring stop whic would give
anywhere from 2-47lbs pressure depending on how I adjust the toggle
Need to work out the resistance for various copper cross-sections to
see if the bar can be made into a reasonable size.
Also need to work on the cell contact to copper bar interface
now...get that resistance down, down, down. :-)
Springy brushes, machined contact faces (hard to do with different
cell shapes though), conductive grease to fill in surface
irregularities (never coming between any metal that can be pressed
together instead)...lots of testing. :-(
Thanks for the great ideas!
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