From: myers@fc.hp.com (Bob Myers)
Newsgroups: sci.electronics.basics,sci.electronics.design
Subject: Re: NiCds: 0V and - X Volts ok. Was: NiCad tester needed.
Date: 6 Jun 1996 21:08:25 GMT
Organization: Hewlett-Packard Fort Collins Site
Message-ID: <4p7hc9$ot4@fcnews.fc.hp.com>
NNTP-Posting-Host: hpfcrlm.fc.hp.com

Richard Steven Walz (rstevew@armory.com) wrote:
> You are certainly wrong
> about "the memory effect", and I am awaiting a replay from Bob Myers,
> Senior Engineer for HP in Colorado, Fort Collins, to reply.

OK, Steve, you got it:

You like the Varta technical data, SO...

Form the Varta "Sealed NiCd Batteries Product Range and Technical Handbook"
(my copy's from 1987, but I don't think that chemistry has changed all that
much in 9 years) -

"Sealed NiCd batteries may be stored for years regardless of the charge
state they are in."

1.0 V/cell is set as the end-of-discharge voltage in capacity curves
based on a nominal discharge rate of 0.2C, but this does NOT mean that it
is *necessarily* dangerous to the cell to allow its voltage to go below this
level.  (In a constant-current discharge at 0.2C, 1.0V might be a good
point to stop, just because AT THAT RATE, you're going to pass through the
voltages between 1.0 and 0.0 pretty quickly.)  In fact, many of the Varta
specs actually list permissable discharge rates to an end voltage of 0.7 or
0.8V.  There's nothing especially magic about ANY of the points between
1.0 and 0.0 V, other than the fact that you ARE on a very steep point on
the curve and better be watching the discharge carefully.

Switching sources for the moment, to a G.E. technical bulletin on the
supposed "memory" phenomenon:

"Realistically, however, 'memory' cannot exist if any one of the following
 conditions holds:

	A. Batteries achieve full overcharge*.
	B. Discharge is not the same each cycle (+/- 2-3%)
	C. Discharge is to LESS THAN 1.0 VOLT PER CELL. [emphasis mine]"

* - I wanted to point out that "full overcharge" in the context of G.E.'s
not is NOT in conflict with the notion that overcharging is responsible for
voltage depression.  You would need to read the full note to understand
what they're meaning by "full overcharge" here.

So here's at least one source which not only isn't concerned about 1.0V/cell
being a minimum, but actually is recommending discharge BELOW that point!

Again, the 1.0V/cell point isn't magic.  The few times that I HAVE seen
recommendations for an absolute, thou-shalt-not-drop-below-this-point sort
of end-of-discharge voltage spec, it's been more in the 0.5-0.6V range.
                                  rstevew: emphasis --> ^^^^^^^^
1.0 V/cell is simply a convenient point at which to stop drawing the
capacity curves, but this should NOT be taken as implying that something
bad happens as soon as you get below it.

> I'll
> wait for Bob Myers to officiate this one, and do it my way. I have
> many of mine at an extreme age now performing wonderfully! And they
> don't and have never needed to be "deep discharged". That's a MYTH!

100% agreed.  Intentional deep discharge is just a waste of time and
energy, even if we could ignore the risk of reverse charging in the case
of NiCd BATTERIES.


Bob Myers  KC0EW  Hewlett-Packard Co.      |Opinions expressed here are not
                  Workstations Systems Div.|those of my employer or any other
myers@fc.hp.com   Fort Collins, Colorado   |sentient life-form on this planet.