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From: hull@hao.UUCP (Howard Hull)
Newsgroups: net.consumers,net.physics
Subject: Re: Applicability of rechargeable batteries
Message-ID: <1250@hao.UUCP>
Date: Sun, 11-Nov-84 12:32:53 EST
Article-I.D.: hao.1250
Posted: Sun Nov 11 12:32:53 1984
Date-Received: Mon, 12-Nov-84 07:17:23 EST
References: <1789@garfield.UUCP> <488@aluxe.UUCP> <392@ihu1h.UUCP> <4603@fortune.UUCP> <371@crystal.UUCP>
Organization: High Altitude Obs./NCAR, Boulder CO
Lines: 31

Many battery-operated devices contain *switching regulators* which make up for
low supply-side voltage by drawing more current.  This is in accordance with
the formula:
		Power at the Load = Source Voltage X Source Current

Since, for any given instance of a fixed load power, the source voltage will
slowly decrease as the battery discharges, the source current will gradually
rise.  At some point, the source current will increase rapidly and then become
so great as to exceed the maximum current specification of the regulator parts
and the regulator will burn out.  Some manufacturers protect such regulators
by means of a fuse; however, replacing the fuse every time the battery gets too 
low is inconvenient for most consumers.  Using a low-value resistor in place of
the fuse is another technique, but that makes the regulator inefficient - and
efficiency is the primary reason for using a switching regulator in the first
place.

Instead, most manufacturers prefer to rely on the discharge characteristics of
the carbon-zinc battery.  For such batteries, the resistance is low during most
of the battery life, but increases under heavy current draw - and does so to a
much greater extent at the end of battery life.

With Nicad batteries, resistance is uniformly lower during most of the battery
life.  As has been pointed out, the voltage is uniformly lower as well.  This
causes a uniformly higher current draw than would be experienced for a zinc-
carbon battery under the same load conditions.  Then, near the end of a nicad
battery's life, the voltage sluggishly rolls off while the resistance becomes
only moderately higher.  The switching regulator is thus subjected to a rather
longer period of greater input current stress, during which time it is more
vulnerable to even brief load surges.
							    Howard Hull, HAO.
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