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From: rpw3@rigden.wpd.sgi.com (Rob Warnock)
Newsgroups: sci.electronics
Subject: Re: Charging Nicads
Message-ID: <113790@sgi.sgi.com>
Date: 28 Jun 91 03:00:38 GMT
References: <51309@ut-emx.uucp>
Sender: guest@sgi.sgi.com
Reply-To: rpw3@sgi.com (Rob Warnock)
Organization: Silicon Graphics, Inc., Mountain View, CA
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In article <51309@ut-emx.uucp> CCEB001@utxvm.cc.utexas.edu writes:
+---------------
| The discussion of charging nicads makes it sound as though a simple
| power supply regulated to about 1.4 volts would be an excellent
| battery charger.  Is that correct?
+---------------

NO! When NiCd's get to full charge, they heat up. When they heat up,
their terminal voltage *drops*! With a fixed-voltage supply such as
you suggest, this causes thermal/current runaway, and the NiCd fries!
(...or boils or explodes, depending on configuration.)

Almost all fast-charge NiCd chargers use internal cell temperature to
know when to stop. And almost all fast-charge NiCd batteries have an
internal thermistor which connects to the charger, so that the measured
temperature is that of the cells inside, and not the battery case (which
may be warmer/cooler, depending on ambient airflow). This is why such
batteries have more than two contacts; the additional one(s) give(s)
access to the thermistor. [The other side of the thermistor may or may
not be connected to one of the battery terminals, and the battery terminals
themselves may or may not come out to multiple contacts.]


-Rob

-----
Rob Warnock, MS-1L/515		rpw3@sgi.com		rpw3@pei.com
Silicon Graphics, Inc.		(415)335-1673		Protocol Engines, Inc.
2011 N. Shoreline Blvd.
Mountain View, CA  94039-7311