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Sender: scott@cv.hp.com
Message-ID: <393E89F6.7AA8CAA5@agilent.com>
Date: Wed, 07 Jun 2000 10:44:22 -0700
From: Scott Linn <scott_linn@agilent.com>
Organization: Agilent Technologies
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Subject: Pilot: Memory effect urban legend
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Hi All,

I saw mention of the so-called "memory effect" wrt NiCad batteries in a
posting. I would like to clear something up:

Consumer users of rechargeable batteries WILL NOT be on the receiving
end of the so-called "memory effect".

Memory was something NASA saw in a satellite which got charged a very
precise amount every time the satellite was in sunlight, and discharged
a precise amount when it was in darkness (on the other side of the earth
wrt the sun).  It was this very controlled charge/discharge cycle which
caused a loss in capacity of the on-board NiCad cells.

In consumer applications, you will *never* be in such a well-controlled
charge/discharge cycle.  It just won't happen.

There are two problem areas in consumer NiCads which people have
attributed to "memory":

1) Overcharging of batteries and cooking the electrolyte.  This occurs
because the vendor who builds the charger you are using was *!&%! cheap
and didn't want to spend extra money on a decent charger.  Once the
cells reach full charge, the batteries get overheated and cook the
electrolyte, damaging them.  You can sometimes partially recover the
cells.  The overall effect is a voltage depression and loss of capacity,
which helps contribute to (2) below.

2) Reverse-charging a cell in a set.  If you over-discharge a set of
cells, one or more of the cells will reach 0 voltage before the others
(NiCad batteries have a very steep discharge curve at end-of-charge).
This causes the other batteries in the pack to reverse-charge the
battery which reached 0 first, which is *very* bad for the one battery. 
Not good.  I've seen people kill perfectly good battery packs by going
with the "memory effect" myth and trying to discharge their batteries
way down, thinking they are helping them.  If the batteries were fully
discharged *individually*, then you would be okay.  Never fully
discharge a NiCad battery pack.

In summary, "memory" is almost always improper charging.

My personal opinion is that it is easier for manufacturers selling
products using NiCads to propagate this "memory effect" myth than to
explain away their poor charging system.

I have some info from some battery mfgrs which explains this if people
still insist on believing in the memory myth.

Scott

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