Question:

=A0"Calendar life degradation" =3D loss of capacity with time, regardless
of amount of use, with capacity not being restored by subsequent
charge-discharge cycles - is rare in NimH batteries. Has anyone seen
this occur to a significant extent in any NimH cells?


        Russell


I may be seeing what appears to be calendar life capacity degradation
in NimH cells.
Unused cells may have a significantly reduced capacity than when new.
Capacity may be below 75% and may continue to fall with time, even if
the calls are unused.

I am used to initial low capacity in NimH cell, which normally
approach full capacity after one or maybe two initial charging cycles.
This is different - it's an apparently permanent loss of capacity that
seems to relate only to battery age.

You could imagine that some part of cell reactants are becoming
unavailable. Could perhaps be caused by venting and dryout, some sort
of non reversible reaction etc.

______________________

Most manufacturers claim that their NimH cells do NOT have this effect.

Duracell Months. Charge before reuse.

Varta:   Recharge yearly

Sanyo:  Cycle several times to restore capacity. 2 to 3 cycles to fully res=
tore.

Panasonic:  Recharge yearly


        =A0=A0 =A0 =A0 =A0 =A0=A0http://www.powerstream.com/Storage.htm

BUT
1997
Abstract only.
Lots of change in last 14 years.
Various things can happen:

   http://www.mrs.org/s_mrs/sec_subscribe.asp?CID=3D12302&DID=3D236622&acti=
on=3Ddetail

Mechanisms Causing Capacity Loss on Long-Term Storage in NiMH System
printer-friendly
Author(s):
D. Singh, T. Wu, M. Wendling, P. Bendale, J. Ware, D. Ritter, L. Zhang

Capacity recovery after long term storage and loaded storage is a
critical issue with the NiMH system since its inception. A measurable
loss in capacity is observed when cells are stored for long periods of
time or discharged deeply to zero volts. The different mechanisms that
are known to cause self discharge and capacity loss after storage and
loaded storage will be the focus of this paper. Capacity loss after
long term storage involves two main events. One is self discharge
which causes the open circuit voltage(OCV) of the cell to drop. Self
discharge is caused by decomposition of NiOOH, migration of metal ions
and possible degradation of separator. Selfdischarge can be prevented
by using separators which are stable at high temperatures and pH and
have good ion trapping capability. Various separator types and
treatments can play an important role in inhibiting metal ions from
migrating thus reducing self discharge. Selfdischarge during storage
causes a severe suppression in the voltage of the foam positive
electrode. This drop in voltage causes a breakdown ofthe cobalt
conductive network in the nickel positive electrode. Reduction of high
valence cobalt(III) which forms the electrode's conductive network
takes place at these low voltages. A permanent breakdown in the
conductive network results in low efficiency of the cell on
consecutive charge and discharge cycles. In addition, the cobalt in
its lower valence states can migrate away from the electrode into the
separator causing shorts. These events effect the charge and discharge
efficiency of these cells thereby resulting in capacity loss. Various
mechanisms causing self discharge which affect capacity recovery after
long term storage and loaded storage are discussed in this paper.

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