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From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: mixing bettery types. Why not?
Summary: Reverse-biasing dead battery cells
Message-ID: <19082@milton.u.washington.edu>
Date: 27 Mar 91 00:25:34 GMT
References: <77759@bu.edu.bu.edu>
Distribution: usa
Organization: University of Washington, Seattle
Lines: 26

In article <77759@bu.edu.bu.edu> jdubb@bucsf.bu.edu (jay dubb) writes:
>
>   I was wondering. I often see warnings on electrical equipment to
>not mix battery types. Can anyone explain to me why this is so (or,
>maybe it is not so)? I have racked my brains trying to think of a
>reason why the current flowing between the cells should care 

	All batteries have a total charge limit; when the number of
amp-hours the cell is good for is exhausted, the cell voltage
drops (and impedance rises).  Mixing battery types puts several
series-connected cells together, BUT always the current flow from
each of the cells is the same (Kirchoff's laws, for series
connection).  So, if you have a 1.2 amp-hour NiCad in series with
a 2.0 amp-hour alkaline cell, the end-of-life scenario is
that the 1.2 amp-hour NiCad drops to zero voltage first.  Then
the other cell (and the load) put current through anyhow, and
the voltage drops BELOW zero.
	Reverse-biasing the cell is a guaranteed path to early
and possibly catastrophic failure.
	This is why many battery packs are factory-connected cells;
the matching of individual NiCad cells makes a battery that will
last years (and the swapping of one cell from such a pack would
not significantly prolong the life of the remaining cells; the
mismatch of the new and old cells would kill the old ones early).

	John Whitmore