I wrote:
> > ....However you charge your cells, charge just before use, not after use
> > (i.e.
> > store cells in an energy-depleted state)...
> >
Mike Lazzari wrote:
> Date: Fri, 02 Mar 2001 15:28:13 -0800
> From: Mike Lazzari <watershed@xxxxxxxxxxxxxxx>
> Subject: [OM] Chargers and max voltage
>
> Eric, I have to respectfully disagree with the last part of this
> statement. Since NiCads lose charge over time I agree that they should
> be charged before use. However you may damage the cells if they are
> stored in a depleted state. Remember that for most practical purposes we
> are not talking about individual cells but an array of cells in series.
> In an ideal world these cells would discharge at the same rate and reach
> zero potential at the same time but in reality this doesn't happen. So
> if one cell reaches zero potential before the others in the series it
> will be subjected to reverse "charge" with bad result. You end up with
> one dead cell and a worthless battery pack.
Mike,
My vague understanding is that self discharge is unlikely to lead to cell
reversal -- unless the cells are in a unit which actually draws current,
in which case this is not really self discharge (nor storage). If you
have more indication that this is not true (including personal experience)
please let me/us know further. I have a lawnmower at 24V with a sealed
battery pack which I have been leaving in a depleted state after each use.
I just charged it this weekend after its second winter in a depleted
state, so I guess I'll know soon!!!
The seemingly reputable source of http://www.cadex.com/b_16_0_storage.asp
recommends as follows:
"The NiCd battery can be stored unattended for up to five years. For best
results, a NiCd should be fully charged, then discharged to zero and
shorted at the terminals. If this procedure is impractical, a discharge
to one volt per cell and storage in a cool, dry place is acceptable. A
NiCd that is allowed to self-discharge is subject to formation of large
crystals (memory). Prolonged storage of NiCd (and NiMH) requires priming
the batteries before use by applying a slow charge followed by one or
several discharge/charge cycles." [Note this is not applicable to Li-ion
and SLA cells!]B
Mind you, most complaints about "memory" in NiCds are the results of
overcharging so this memory concern probably isn't much of a risk. For
myself, I care little about whether I get 400 or 800 cycles from a cell
since I seldom work with critical applications and when I do, I take
reasonably new cells anyway. (Well, I would like to avoid replacing that
lawnmower pack...) What has worked for me is to just put cells away after
use in whatever semi-depleted state that they happen to be in (i.e. I
don't further discharge them). The main practical advantage is that I am
forced to charge them before use (alkalines await if I forget) which means
that I am certain of a (nearly) full charge on use. On the other hand, I
don't store cells for longer than a couple months, since they cycle
through kid's toys, flashlights, and oh yes, camera flash equipment.
This is pretty far afield from OM topics now, so unless others request, perhaps
we should ajourn off-list...
Thanks for you comments,
Eric Pederson
PS: Please recycle those NiCd cells! They are too toxic for the waste stream!
=====
Eric Pederson
epederso@xxxxxxxxxxxxxxxxxxxx (no "n" in "epederso")
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