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From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: What is the noise voltage of a 1.5V cell
Summary: Gas backpressure may complicate things
Message-ID: <13749@milton.u.washington.edu>
Date: 4 Jan 91 04:15:19 GMT
References: <1991Jan3.001303.483@ae.chalmers.se> <4285@kitty.UUCP>
Organization: University of Washington, Seattle
Lines: 28

In article <4285@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes:
	(replying to query on low-frequency noise from dry cell)
>
>	The essential noise component of a simple electrochemical primary
>cell is thermal noise (Johnson Noise, for Johnson fans). 
>
>	If you were drawing some serious power from the alkaline cell,
>I would consider some other factors in evaluating noise (like temperature
>shifts, gas generation, etc.)

	Some researchers here who needed a steady voltage tried out
dry cells and found that some gave 10 mV sawtooth output voltages
(about 1Hz frequency).  Gas generation was their explanation; a small
pressure buildup in the cell eventually pushed past the seal on
the battery.  The cell voltage, of course, is depressed when the
gaseous byproduct has significant backpressure.  Even at very low
currents, the leakage current in a dry cell is guaranteed to generate
some gas.
	The folks found that their best bet (Zeners and IC references
were too noisy) was a stack of standard cells (which have no gaseous
byproduct).  They bought a few dozen for their 90V bias battery.
It was expensive.  Then they put the cells in a temperature-regulated
box (chilled, I believe).
	Different battery systems have different stabilities; get
a few different types and test 'em.  The only ones I KNOW won't work
are carbon/zinc.

	John Whitmore