|
Chapter 10: Getting the Most from your Batteries
A common difficulty with portable equipment is the gradual decline
in battery performance after the first year of service. Although
fully charged, the battery eventually regresses to a point where
the available energy is less than half of its original capacity,
resulting in unexpected downtime.
 Downtime almost always occurs at critical moments. This
is especially true in the public safety sector where portable
equipment runs as part of a fleet operation and the battery
is charged in a pool setting, often with minimal care and
attention. Under normal conditions, the battery will hold
enough power to last the day. During heavy activities and
longer than expected duties, a marginal battery cannot provide
the extra power needed and the equipment fails.
Rechargeable batteries are known to cause more
concern, grief and frustration than any other part of a portable
device. Given its relatively short life span, the battery
is the most expensive and least reliable component of a portable
device.
In many ways, a rechargeable battery exhibits
human-like characteristics: it needs good nutrition, it prefers
moderate room temperature and, in the case of the nickel-based
system, requires regular exercise to prevent the phenomenon
called ‘memory’. Each battery seems to develop a unique personality
of its own.
Memory: myth or fact?
 The word ‘memory’ was originally derived from ‘cyclic
memory’, meaning that a NiCd battery can remember how much
discharge was required on previous discharges. Improvements
in battery technology have virtually eliminated this phenomenon.
Tests performed at a Black & Decker lab, for example,
showed that the effects of cyclic memory on the modern NiCd
were so small that they could only be detected with sensitive
instruments. After the same battery was discharged for different
lengths of time, the cyclic memory phenomenon could no longer
be noticed.
The problem with the nickel-based battery is
not the cyclic memory but the effects of crystalline formation.
There are other factors involved that cause degeneration of
a battery. For clarity and simplicity, we use the word ‘memory’
to address capacity loss on nickel-based batteries that are
reversible.
The active cadmium material of a NiCd battery
is present in finely divided crystals. In a good cell, these
crystals remain small, obtaining maximum surface area. When
the memory phenomenon occurs, the crystals grow and drastically
reduce the surface area. The result is a voltage depression,
which leads to a loss of capacity. In advanced stages, the
sharp edges of the crystals may grow through the separator,
causing high self-discharge or an electrical short.
Another form of memory that occurs on some NiCd
cells is the formation of an inter-metallic compound of nickel
and cadmium, which ties up some of the needed cadmium and
creates extra resistance in the cell. Reconditioning by deep
discharge helps to break up this compound and reverses the
capacity loss.
The memory phenomenon can be explained in layman’s
terms as expressed by Duracell: “The voltage drop occurs because
only a portion of the active materials in the cells is discharged
and recharged during shallow or partial discharging. The active
materials that have not been cycled change in physical characteristics
and increase in resistance. Subsequent full discharge/charge
cycling will restore the active materials to their original
state.”
When NiMH was first introduced there was much
publicity about its memory-free status. Today, it is known
that this chemistry also suffers from memory but to a lesser
extent than the NiCd. The positive nickel plate, a metal that
is shared by both chemistries, is responsible for the crystalline
formation.
 |
New NiCd cell.
The anode is in fresh condition
(capacity of 8.1Ah). Hexagonal cadmium hydroxide crystals
are about 1 micron in cross section, exposing large
surface area to the liquid electrolyte for maximum performance. |
 |
Cell with crystalline
formation.
Crystals have grown to an enormous
50 to 100 microns in cross section, concealing large portions
of the active material from the electrolyte (capacity
of 6.5Ah). Jagged edges and sharp corners may pierce the
separator, which can lead to increased self-discharge
or electrical short. |
 |
Restored cell.
After pulsed charge, the crystals
are reduced to 3 to 5 microns, an almost 100% restoration
(capacity of 8.0A). Exercise or recondition are needed
if the pulse charge alone is not effective. |
Figure 10-1: Crystalline formation
on NiCd cell.
Illustration courtesy of the US
Army Electronics Command in Fort Monmouth, NJ, USA.
In addition to the crystal-forming activity on
the positive plate, the NiCd also develops crystals on the
negative cadmium plate. Because both plates are affected by
crystalline formation, the NiCd requires more frequent discharge
cycles than the NiMH. This is a non-scientific explanation
of why the NiCd is more prone to memory than the NiMH.
The stages of crystalline formation of a NiCd
battery are illustrated in Figure 10-1. The enlargements
show the negative cadmium plate in normal crystal structure
of a new cell, crystalline formation after use (or abuse)
and restoration.
Lithium and lead-based batteries are not affected
by memory, but these chemistries have their own peculiarities.
Current inhibiting pacifier layers affect both batteries —
plate oxidation on the lithium and sulfation and corrosion
on the lead acid systems. These degenerative effects are non-correctible
on the lithium-based system and only partially reversible
on the lead acid.
|
