Memory: myth or fact?
Isidor Buchmann
Cadex Electronics Inc.
isidor.buchmann@cadex.com
www.buchmann.ca
April 2001
The word ‘memory’ was originally derived from ‘cyclic memory’,
meaning that a Nickel Cadmium (NiCd) battery can remember
how much discharge was required on previous discharges. Improvements
in battery technology have virtually eliminated this phenomenon.
The problem with NiCd is not the cyclic memory but the effects
of crystalline formation. The active cadmium material 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 Nickel Metal Hydride (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.
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 1. The enlargements show the negative
cadmium plate in normal crystal structure of a new cell, crystalline
formation after use (or abuse) and restoration.
 |
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 1: Crystalline formation on
NiCd cell.
Illustration courtesy of the US
Army Electronics Command in Fort Monmouth, NJ, USA.
How to restore and prolong nickel-based batteries
The effects of crystalline formation are most pronounced
if a nickel-based battery is left in the charger for days,
or if repeatedly recharged without a periodic full discharge.
Since most applications do not use up all energy before recharge,
a periodic discharge to 1V/cell (known as exercise) is essential
to prevent the buildup of crystalline formation on the cell
plates. This maintenance is most critical for the NiCd battery.
All NiCd batteries in regular use and on standby mode (sitting
in a charger for operational readiness) should be exercised
once per month. Between these monthly exercise cycles, no
further service is needed. The battery can be used with any
desired user pattern without the concern of memory.
The NiMH battery is affected by memory to a lesser degree.
No scientific research is available that compares NiMH with
NiCd in terms of memory degradation. Neither is information
on hand that suggests the optimal amount of maintenance required
to obtain maximum battery life. Applying a full discharge
once every three months appears right. Because of the NiMH
battery’s shorter cycle life, over-exercising is not recommended.
Exercise and Recondition — Research has shown that
if no exercise is applied to a NiCd for three months or more,
the crystals ingrain themselves, making them more difficult
to break up. In such a case, exercise may no longer be effective
in restoring a battery and reconditioning is required.
Recondition is a slow, deep discharge that removes the remaining
battery energy by draining the cells to a voltage threshold
below 1V/cell. Tests performed by the US Army have shown that
a NiCd cell needs to be discharged to at least 0.6V to effectively
break up the more resistant crystalline formation. During
recondition, the current must be kept low to prevent cell
reversal. Figure 2 illustrates the battery voltage during
a discharge to 1V/cell, followed by the secondary discharge,
know as recondition.
Figure 2: Exercising and reconditioning
batteries on a Cadex battery analyzer.
If no exercise is applied to
a NiCd for three months or more, exercise may no longer be
effective in restoring a battery and reconditioning is required.
Recondition is a slow, deep discharge to 0.4V/cell.
Figure 3 illustrates the effects of exercise and recondition.
Four batteries afflicted with various degrees of memory are
serviced. The batteries are first fully charged, then discharged
to 1V/cell. The resulting capacities are plotted on a scale
of 0 to 120 percent in the first column. Additional discharge/charge
cycles are applied and the battery capacities are plotted
in the subsequent columns. The solid black line represents
exercise, and the dotted line recondition. On this test, the
exercise and recondition cycles are applied manually at the
discretion of the research technician.
Figure 3: Effects of exercise and
recondition.
Four batteries afflicted with various
degrees of memory are serviced. Battery ‘A’ improved capacity
on exercise alone; batteries ‘B’ and ‘C’ required recondition.
The new battery improved further with recondition.
Battery ‘A’ responded well to exercise alone and no recondition
was required. This result is typical of a battery that has
been in service for only a few months or has received periodic
exercise cycles. Batteries ‘B’ and ‘C’, on the other hand,
required recondition (dotted line) to restore their performance.
Without the recondition function, these two batteries would
need to be replaced.
After service, the restored batteries were returned to full
use. When examined after six months of field service, no noticeable
degradation in the restored performance was visible. The regained
capacity was permanent with no evidence of falling back to
the previous state. Obviously, the batteries would need to
be serviced on a regular basis to maintain the performance.
Applying the recondition cycle on a new battery (top line
on chart) resulted in a slight capacity increase. This capacity
gain is not fully understood, other than to assume that the
battery improved by additional formatting. Another explanation
is the presence of early memory. Since new batteries are stored
with some charge, the self-discharge that occurs during storage
contributes to a certain amount of crystalline formation.
Exercising and reconditioning reverse this effect.
Case studies
A certain organization continually experienced NiCd battery
failure after a relatively short service time. Although the
batteries performed at 100 percent when new, their capacity
dropped to 20 percent and below within one year. We discovered
that their two-way radios were under-utilized; yet the batteries
received a full recharge after each short field use.
After replacing the batteries, we advised the organization
to exercise the new batteries once per month by discharging
them to one-volt-per cell with a subsequent recharge. The
first exercise took place after the batteries had been in
service for four months. At that stage, we were anxious to
find out how much the batteries had deteriorated. Here is
what we found:
On half of the batteries tested, the capacity loss was between
25 to 30 percent; on the other half, the losses were around
10 to 20 percent. With exercise — and some needed recondition
cycles — all batteries were fully restored. Had maintenance
been omitted for much longer, the probability of a full recovery
would have been jeopardized.
The importance of exercising and reconditioning NiCd batteries
is emphasized by a study carried out by GTE Government Systems
in Virginia, USA, for the US Navy. To determine the percentage
of batteries needing replacement within the first year of
use, one group of batteries received charge only, another
group was exercised and a third group received recondition.
The batteries were used for two-way radios on the aircraft
carriers USS Eisenhower with 1500 batteries and USS George
Washington with 600 batteries, and the destroyer USS
Ponce with 500 batteries.
With charge only (charge-and-use), the annual percentage
of battery failure on the USS Eisenhower was 45 percent (see
Figure 4). When applying exercise, the failure rate was reduced
to 15 percent. By far the best results were achieved with
recondition. The failure rate dropped to 5 percent. Identical
results were attained from the USS George Washington and the
USS Ponce.
|
|
| Maintenance
Method |
Annual Percentage
of Batteries
Requiring Replacement |
|
|
| Charge only (charge-and-use) |
45% |
| Exercise
only (discharge to 1V/cell) |
15% |
| Reconditioning (secondary
deep discharge) |
5% |
|
|
Figure 4: Replacement rates of
NiCd batteries.
The annual percentage of NiCd
batteries requiring replacement when used without any maintenance
decreases with exercise and recondition. These statistics
were drawn from batteries used by the US Navy on the USS Eisenhower,
USS George Washington and USS Ponce.
The GTE Government System report concluded that a battery
analyzer featuring exercise and recondition functions costing
$2,500US would pay for itself in less than one month on battery
savings alone. The report did not address the benefits of
increased system reliability, an issue that is of equal if
not greater importance, especially when the safety of human
lives is at stake.
Another study concerning NiCd batteries for defense applications
was performed by the Dutch Army. This involved battery packs
that had been in service for 2 to 3 years during
the Balkan War. The Dutch Army was aware that the batteries
were utilized under the worst possible conditions. Rather
than a good daily workout, the packs were used for patrol
duties lasting 2 to 3 hours per day. The rest of the time
the batteries remained in the chargers for operational readiness.
After the war, the batteries were sent to the Dutch Military
Headquarters and were tested with Cadex 7000 Series
battery analyzers. The test technician found that the capacity
of some packs had dropped to as low as 30 percent. With the
recondition function, 90 percent of the batteries restored
themselves to full field use. The Dutch Army set the target
capacity threshold for field acceptability to 80 percent.
This setting is the pass/fail acceptance level for their batteries.
Based on the successful reconditioning results, the Dutch
Army now assigns the battery maintenance duty to individual
battalions. The program calls for a service once every two
months. Under this regime, the Army reports reduced battery
failure and prolonged service life. The performance of each
battery is known at any time and any under-performing battery
is removed before it causes a problem.
Summary
Each battery system has a few shortcomings: The lithium family
ages and has limited load current; the NiMH has a relative
shore service life; the lead acid family is bulky and requires
long charge times; and the NiCd has memory. But the
NiCd makes up by being the most enduring battery. In addition,
it has the lowest cost-to-energy-ratio of all commercial rechargeable
batteries.
- To achieve long life, some simple guidelines must be observed.
They are:
- Do not leave a nickel-based battery in a charger for
more than a day after full charge is reached.
- Apply a monthly full discharge cycle. Running the battery
down in the equipment may do this also.
- Do not discharge the battery before each recharge. This
would put undue stress on the battery.
- Avoid elevated temperature. A charger should only raise
the battery temperature for a short time at full charge,
and then the battery should cool off.
- Use quality chargers
This article contains excerpts from the second edition book
entitled Batteries in a Portable World — A Handbook on Rechargeable
Batteries for Non-Engineers. In the book, Mr. Buchmann evaluates
the battery in everyday use and explains their strengths and
weaknesses in laymen’s terms. The 300-page book is available
from Cadex Electronics Inc. through book@cadex.com,
tel. 604-231-7777 or most bookstores. For additional information
on battery technology visit www.buchmann.ca.
About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics
Inc., in Richmond (Vancouver) British Columbia, Canada. Mr.
Buchmann has a background in radio communications and has
studied the behavior of rechargeable batteries in practical,
everyday applications for two decades. The author of many
articles and books on battery maintenance technology, Mr.
Buchmann is a well-known speaker who has delivered technical
papers and presentations at seminars and conferences around
the world.
About the Company
Cadex Electronics Inc. is a world leader in the design and
manufacture of advanced battery analyzers and chargers. Their
award-winning products are used to prolong battery life in
wireless communications, emergency services, mobile computing,
avionics, biomedical, broadcasting and defense. Cadex products
are sold in over 100 countries
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