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NNTP-Posting-Date: Wed, 11 Dec 2002 09:35:13 -0600
From: jfields@texas.net (John Fields)
Newsgroups: sci.electronics.basics,sci.electronics.design,sci.electronics.misc
Subject: Re: Reducing contact resistance for low volt use?
Date: Wed, 11 Dec 2002 15:13:06 GMT
Organization: Austin Instruments, Inc.
Message-ID: <3df7482a.151850701@news.texas.net>
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On Tue, 10 Dec 2002 23:01:08 GMT, jmuchow@SPAMMENOTcamlight.com
(John Muchow) wrote:
>I've been moving forward with designing a constant-current battery
>discharger (thanks to a lot of help from this group) but realized that
>I'm going to run into problems unless I can reduce the resistance
>between the contacts of the discharger and the battery cell terminals.
>
>One of the discharger's functions is to discharge individual NiCd and
>NiMH cells at levels from 0.25A to 100A (and feed voltages to data
>logging software for plotting). Keeping the resistance of the
>discharger low enough to get 100A from a 1.2V cell is one heck of a
>challenge.
>
>I don't think that merely pressing spring loaded (or screw-tightened)
>metal contacts against the cell terminals will work well enough. And
>soldering 4AWG wire to each cell being tested is a huge PITA (and can
>easily fry the cell if not done properly).
>
>I was wondering if an electrically conductive compound might be the
>solution? I've seen silver-filled conductive epoxies, but not a
>"grease" or similar non-adhesive compound.
>
>Does anyone have any conductive compound recommendations or ideas as
>to how I might be able to reduce the resistance between the discharger
>contacts and the battery terminals?
---
The challenge here is to provide as large an area as possible (as
many contact points as possible) for current to flow from the cell
terminals into the discharger contacts. I haven't seen your circuit
but I'm assuming you're using a shunt somewhere in the circuit to
derive a voltage proportional to the current from the cell, and then
using that voltage to control the load resistance to the point where
it will allow the desired current to flow out of the cell. Such
being the case, the total resistance in series with the 1.2V cell
when it's delivering 100A will be 12mV, part of which will be the
contact resistance between the cell terminals and the discharger
contacts. Ostensibly you'd like to keep that resistance as low as
possible, so that most of the power being dissipated will be in the
load and in the cell's internal resistance. In order to provide the
largest possible area between the cell and the discharger contacts,
they would have to be in intimate electrical contact over the entire
suface of the cell terminals, and that means intimate mechanical
contact as well, So what I'd do would be to machine the contacts to
provide a snug-to-tight fit on the battery terminals, use a _small_
quantity of a heavily silver-loaded grease between the terminals and
the contacts, and use as much pressure as possible between the
terminals and the contacts. If possible, it would also be a good
idea to put the cell in a lathe and remove a _small_ amount of
material from the + and - terminals in order to make the contacts
flat and parallel to each other so the contacts will seat properly.
If you can, it would be a good idea to make the contacts out of
silver bar stock. Silver is pretty soft, has the best electrical
conductivity of any material, and is also the best thermal conductor
(except for diamond) AFAIK. What might also help would be to rotate
the cell as the contacts were being clamped down. This would allow
the silver particle in the grease to jam up against one another and
get stuck in the microscopic spaces between the terminals and
contacts, firther decreasing the resistance. If you use a set of
contacts like this it would be an easy matter to provide the Kelvin
contacts you need to measure the cell voltage; just drill and tap a
small hole in each of the contacts, crimp some lugs down on a couple
of wires, screw the lugs down on the contacts, and there ya go!
---
John Fields
Professional circuit designer
http://www.austininstruments.com
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