From: elmore@rap.ucar.edu (Kim Elmore)
Subject: NiCd Batteries, Part 3/3
Organization: NCAR, Research Application Program
Date: Fri, 18 Dec 1992 17:04:38 GMT

*Chapter 3*

                NICAD CELL CHARACTERISTICS
                            by
                          W6WTU


The operating characteristics of a Nicad cell are very basic in
nature.  During charge, only current and cell temperature are
critical.  Discharge may occur at any current rate until the cell
terminal voltage falls below 1.1 volts, but the cell temperature rise
must not cause the cell electrolyte to boil (gas).  When connected in
a pack, the pack must be unloaded when the individual cell voltage
falls to 1.1 volts to prevent reverse charging of the lowest voltage
cell in the pack.

Few applications discharge cells at an excessive high rate where
gassing becomes a problem, with the exception of toy electric cars
(these are "toys" for men).  Cells designed for this application
usually have larger than average plate area and large connecting tabs
to support the higher peak current.  Cell temperature is kept below
the critical value by selecting the cell capacity such that the cell
will dissipate its charge before gassing occurs.  In essence, the toy
car runs continuously with a nearly zero series load resistance
allowing the cell temperature to rise near critical just as cell
discharge is reached.

The following brief notes will further amplify an understanding of
Nicad cell characteristics.

1.  Nicads are very easily damaged due to gas pressure caused by
over-charging at high charge rates.  Unfortunately, Nicads do not have
a positive indicator for indicating the state of charge.  It is up to
the user to estimate the relative charge state.  Typically, the charge
rate is calculated at C/10 where C equals cell capacity and the 10
represents hours for charging.  At C/10, a nominal or normal charge
would occur.  A fast charge can be accomplished, cell design
permitting, by changing the ratio, i.e., 5C/2.

2.  Cell temperature does rise at the completion of the charge cycle,
particularily at high charge current values, and can be used as a
charge completion indicator.  At the moment of temperature rise, the
charge current must be reduced immediately to prevent excessive gas
pressure build-up.  Some chargers monitor the cell temperature and
reduce the charge current to a small sustaining value.  For 500ma
cells, the sustaining value is typically between 10-25ma, or just
enough current to compensate for the chemical change ( internal
current leakage).

3.  Pulse charging of Nicads is most preferred as it is more efficient
than a steady-state current value.  Steady state current creates gas
bubble formation on the plates of the cell, while pulse current tends
to create fewer gas bubbles allowing the charge to be more uniform
across the plate area.  The charge uniformity is enabled by gas
bubbles being moved around due to current pulsations.  Bubble movement
allows more plate area to be exposed during charge.  Pulse charging
also creates less of a temperature rise.  The charge rate still has to
be calculated as a function of C/10 during each charge pulse to
determine total charge time (The accumulated charge is equal to the
sum of the charge area under the pulse curve).

4.  Older Nicad cells would not accept high charge rates above C/10
and would develop electrolyte seal leakage at attempted higher charge
rates.  A leaky seal, in any case, means the eventual loss of the
cell.  Newer cells, as used in most handheld radio applications seem
to tolerate high charge rates without developing cell leakage as long
as the temperature is controlled (kept low).

5.  In the event corrosion around the seal of a cell develops, the
corrosion can be removed.  Wipe the corroded area with a vinegar
soaked cotton swab.  Use an ample amount of liquid and dry the cell
with a tissue.  Keeping the cell clean allows it to be used until it
fails completely.  Take note that weeping cells can cause corrosion
problems if they are not inspected and cleaned regularly.

6.  Discharge of a Nicad is critical only if the cell is under a load.
When loaded, the cell voltage should not be allowed to drop below 1.1
volts which is just over the edge of the discharge end of the curve.
Allowing the cell to drop to 1.0 volt or less while under load may
cause permanent plate/electrolyte damage.  The result would be a loss
of cell capacity.  Of course, a loss also takes place naturally with
age which limits the number of charge-discharge cycles the cell is
capable of supporting before it is exhausted.

7.  Unloaded cells may be stored almost indefinately and allowed to
discharge to whatever cell terminal voltage the cell can maintain on
its own.  When putting a previously stored cell into use, charge the
cell, at least the first cycle, at C/10 for 12-14 hours.  Shelf life
of a cell is usually a function of how fast nickel whiskers develop
internally that can short the cell.  Two methods have been proposed as
a way of extending cell life when whisker formation is of concern.  a)
Cycle the use of the cell/pack periodically.  It is believed that the
rate of whisker formation is reduced when cells are charged and
discharged periodically.  b) Allow the cell to sit on the shelf as
long as needed and if a whisker short should occur, blast it away with
a high current pulse from a capacitor or other source.  It has been
observed that once whisker growth/shorting begins, it can not be
permanently stopped.

8.  It has been observed that the terminal voltage across a charged
Nicad cell will increase as the cell ages.  Using this observation,
the life expectancy of a cell can be evaluated and monitored.  As an
example, a new/fresh cell will exhibit a terminal voltage of 1.25
volts while an old cell will exhibit, say, 1.5 volts.  It is believed
that the cell dehydrates with age and the rising voltage is a function
of the increase in cell impedance.

Many hints and kinks have been published regarding the nature and care
of Nicads and all point to the fact that Nicads require user TLC to
obtain long cell life.  The bottom line is that the user must pay
attention to how the cell is being treated.