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From: jmuchow@SPAMMENOTcamlight.com (John Muchow)
Subject: Re: Reducing contact resistance for low volt use?
Date: Wed, 11 Dec 2002 23:44:08 GMT
Organization: MindSpring Enterprises
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X-Server-Date: 11 Dec 2002 23:43:35 GMT
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>>>[snip] Lowest resistance I could get was 0.8milliohms.
>>>That's 0.4 milliohms/contact point. That doesn't
>>>include any of the wire or connection clamps. Multiple contacts
>>>with independent springs should reduce the resistance
>>>Here's a picture of the jig.
Whoa! Thanks for doing that test! Hmm. maybe there's hope yet. :-)
I need to set up some way to measure low resistances...standard DMM's
just don't cut it. Time to search the Web.
That's great setup you've got with that jig. It's giving me a couple
>>>With such low voltage margin for the load, you
>>>might have to resort to using a hall-effect current probe in
>>>the feedback to set the current for a passive
>>>load...well not passive, just not using a KW of power to create the load.
I think I understand you...
I could use a Hall effect probe as the input to an amplifier (if
needed) whose output would vary the gate voltage of my load MOSFET's
to linearly control the current through the MOSFET's?
I could then avoid having an inline current-sense resistor.
>>>More volts would help. Can you discharge two or
>>>more at once, but just measure across one cell? That would make the
>>>load design a lot easier.
I've thought about that but the inter-cell connection resistances
would need to be taken into account (wouldn't they?). Don't know if
that's a real problem. I could use 2-4 cells if the extra voltage
helped more than the interconnections would hurt.
>>>What characteristic of the cell are you using to
>>>determine it's health?
Not so much determining the health of the cell but what current levels
they're capable of for how long. The combat robot and R/C plane and
car builders/users go crazy looking for just a couple of tenths of
extra cell voltage under load, or just a couple of seconds of extra
run time before the voltage drops too far.
The tests will determine true AH-ratings of each cell at different
current levels (the voltage of the cell will be plotted over time).
The discharger will also be used to test (essentially prove/disprove)
the effects of "zapping" cells under certain circumstances, show the
effects of low-current and high-current conditioning, deep
discharging, and overcharging.
A future upgrade will put the discharger under control of a Basic
Stamp to vary the load over time (perhaps a 5A baseline load with
pulses of varying duration up to 100A to simulate combat robot and R/C
>>>Could you use a pulse measurement? That would
>>>help the heat problem a lot.
Oh man, I'd love to. But most of the tests involve running the cell
at particular current level until it drops to 0.9V (typically).
>>>Might be interesting to put a cell on a curve
>>>tracer and see if you can learn anything.
That would be pretty cool for determining the pulse current vs.
terminal voltage of the cells...if I remember what a curve tracer
does. It's been a longggg time since I used one...25 years!
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