Path: utzoo!attcan!uunet!motcid!pasik
From: pasik@motcid.UUCP (John Pasik)
Newsgroups: sci.electronics
Subject: Re: NiCad "Deep Discharger"
Keywords: NiCad Discharge Memory
Message-ID: <3702@sienna9.UUCP>
Date: 25 Jun 90 21:02:25 GMT
References: <1990Jun15.154259.2666@ux1.cso.uiuc.edu> <9634@pt.cs.cmu.edu> <1990Jun17.033046.18236@ux1.cso.uiuc.edu> <3653@sienna9.UUCP> <1727@ns-mx.uiowa.edu>
Organization: Motorola Inc., Cellular Infrastructure Div., Arlington Heights, IL
Lines: 40

emcguire@cadfx.ccad.uiowa.edu (Ed McGuire) writes:

>In article <3653@sienna9.UUCP> pasik@motcid.UUCP (John Pasik) writes:
>> The typical maximum discharge voltage reccomended for Ni-Cad battery packs
>> is 1v per cell.  The 1v/cell value is chosen to minimize the chance of
>> electrically reversing the polarity of any cell in the pack (very bad)!
>                                                               ^^^^^^^^
>How bad?  Are these the battery packs typically used for cordless
>drills and charging flashlights?  I've got a flashlight which came with
>instructions to discharge completely to avoid a memory effect, which
>sounds like the NiCd effect being discussed.  The instructions had no
>warning against complete discharge.  They also did not avoid constant
>charging -- the trickle charger is obviously intended to store the
>flashlight whenever it's not being used.

>The battery would not take a charge after the flashlight was
>accidentally stored "on" for several months.  If this is due to cell
>reversal, is there a way to restore the battery?  Is it dangerous to
>charge a reversed battery?
>---
>peace.  -- Ed
>"He looked again.  No, these cows were strangers."  (Milo and Otis)

Trickle charging is OK as long as the current level doesn`t cause cells
to overheat or outgas.  You can discharge a single cell completely without
fear of "reversal", many small cordless devices only use one or two cells.
In such cases it is harder to cause a cell reversal.  If a cell is reversed,
it typically begins to short-out.  It is possible to remove cell shorts
with brief high-current pulses (>10C, were C is the amp/hr rating of the
battery).  The typical batteries used in small flashlights and power tools
are lead acid.  Sealed lead acid batteries are "starved electrolyte" systems.
This means the plate capacity is greater than the electrolytes ion capacity.
If such batteries are deep discharged, they will lock all the free ions up
in the plate material.  The result is basically water as the electrolyte.
Water is not a good conductor so the battery will not take a charge easily.
It is sometimes possible to recharge such a battery by charging it for an
extended interval (10 to 100 times normal).  If the battery can be connected
to a higher than normal voltage charging source, the process can be speeded
up.  Monitor the charging current and reduce the voltage as the current begins
to ramp up.