Olympus-OM
[Top] [All Lists]

Re: [OM] Nickel Hydride AA cells

Subject: Re: [OM] Nickel Hydride AA cells
From: HI100@xxxxxxx
Date: Thu, 15 Jul 1999 05:23:32 EDT
Tom wrote a comparative review of different battery technologies and 
commented:

<<  The memory effect is a myth. NiCds die from overcharging. >>

If you talk to battery vendors they may or may not agree 
completely with this statement.

        The situation is a little more complex than revealed in the 
statement. 
The memory effect is often defined as a (reversible) loss of capacity caused 
by only 
partial discharge of the cells before recharging them. For this effect to 
occur the cells 
need to be partially cycled repetitively before the cell "remembers" it's 
capacity has
 been reduced to the level it was previously discharged to. Some vendors 
claim their 
cells do not suffer from this effect at all but their competitiors cells do! 
In reality it appears 
that at least some vendors cells suffer from a "voltage depression" effect 
that is 
interpretted by the user as memory.  This is most common in electronic 
equipment 
because the electronic cut -off voltage in the equipment is set too near the 
normal 
NiCd operating voltage. Thus when the voltage is depressed by repetitive 
partial 
cycling the equipment warns you the voltage is low much too soon and then 
shuts off even 
though you could have extracted a lot more energy if the equipment was 
properly 
designed to run at the sligtly lower "depressed voltage".   The solution 
commonly 
recommended  to always completely  discharge the battery to eliminate memory 
is often 
as bad as the problem. This is particulary true for consumers who buy single 
cells. 
In a properly made battery pack the cells are carefully matched in capacity 
by the manufacturer, this is generally not true for individual cells 
(especially from different lots). 
        As remarked by others on the list deep discharging a battery usually 
reverse charges
the lowest capacity cell in the pack. This is very bad for the cell and 
usually causes 
 low resistance shorts that cannot be cleared by recharging. (You can usually 
recover 
these cells by applying a 100A pulse from a lead acid D cell for a second or 
two but
 this is usually only a short term fix). The higher the volage battery pack 
the greater the chance of 
damaging the pack in deep discharge since the likelihood of mismatch in cell 
capacity is 
much greater.
        As a gross generlization high capacity NiCd cells (much greater than 
say 500mAH for 
an AA size cell)  are significantly less reliable than the more common low 
capacity cells. 
The high capacity cells have very thin seperators to maximize active plate 
material. 
In repetitive overcharge the heating mentioned in the above posting causes 
the seperators
 to be damaged through cyclical mechanical stress from expansion. Some 
vendors offer 
high temperature cells with thick seperators and hence lower capacity and 
these cells 
are much more reliable but are generally not available to consumers directly.
        A third common failure mode in NiCds is from being allowed to 
completely self discharge 
("left in cupboard failure"). This results from the fact that the cells are 
sealed with a plastic seal.
When charged the cells run at a pressure higher than atmospheric. This helps 
compress the seals and 
helps prevent electrolyte leakage. When completely discharged the seals are 
no longer held closed 
by pressure and leak more easily as they age. The cell then grows tell tale 
crystals around the 
edge of the positive connection (where the seal is). Eventually enough 
electrolyte is lost and the 
cell fails. When not in use float charging at a very low rate (a few mA ) 
helps stop this failure mode.
        To maximize cell life it is important to keep individual cells 
together as a group to keep their 
capacity matched when charging and discharging: this reduces the chance of 
reverse charging damage 
in deep discharge. Since most flashes don't have low voltage cutoff circuits 
you should not experience 
"memory/volage depression" problems. In fact to maximize battery cycle life 
avoid completely 
discharging them.

Sorry for this over long post.

Tim Hughes
Hi100@xxxxxxx

< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >


<Prev in Thread] Current Thread [Next in Thread>
Sponsored by Tako
Impressum | Datenschutz