The
Care and Feeding of NiCad Batteries
By
Jerry Smith
(copyright
@ commonlaw)
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A
Quick Summary
When using devices employing NiCad
batteries that use a trickle charger (wall plug-in unit), be sure to follow
the manufacturer's recommendations for recharging. The batteries
should be allowed to discharge to a "battery low" condition before recharging
to prevent a memory condition which, in effect, reduces the capacity of
the battery. Don't leave the charger plugged in and forget about
it. Continued abuse by overcharging will result in poor performance
in a relatively short period of time and you'll be looking for a new battery
pack before you know it.
That's it!
Now, if you're interested in more of the technical stuff, read on!
The study of NiCad batteries and
their characteristics could evolve into volumes of information. However,
The goal here is to provide the end user with enough information to decide
which charging technique is most suitable and cost effective for a particular
application and to extract the longest possible life from NiCad batteries.
It is important to understand
the effects of over-charging NiCad batteries before proceeding to the various
types of charging techniques. Most, if not all NiCad battery manufacturers,
suggest a specific charging period in terms of hours and generally the
rate of current in milliamperes. This specification assumes that
the battery has been completely discharged to a certain level before recharging
takes place.
During an “over-charging” condition,
excessive heat and gases can build up from the chemical reaction which
takes place in the battery during the recharge cycle. Most NiCads
found in electronic equipment today utilize a sealed cell arrangement.
Although these cells are sealed,
they usually have a mechanism for releasing excessive pressure so that
an explosion can be avoided when the cells are over-charged. One
of the mechanisms is a "reseal" type valve which means it will open when
the pressure reaches a certain level (150-300 psi) and close when the pressure
goes below that level.
The other type is a "one-time"
relief valve which opens and remains open even after the pressure and temperature
has been reduced. The problem with this type of cell is that once
the valve is opened, the chemistry of the battery will have a tendency
to dry out and the life of the battery is shortened dramatically.
This should provide the user
with an appreciation for why the manufacturer provides specific instructions
for recharging their NiCad batteries. By following the manufacturer's
recommendations, many hundreds of recharges are possible, providing capacity
close to the original design specification of the battery.
Are you sure
you want to continue with all this technical jargon?
When a battery is
exposed to over-charging, its projected life can be shortened considerably.
This usually shows up in terms of capacity and endurance of the battery,
that is, a 500 ma (milliampere) hour battery should provide the equivalent
of 50 ma of current for a period of 10 hours (50 ma * 10 hours = 500 ma/hours).
Another example is 100 ma of current for 5 hours. Milliampere is
a unit of measure of the amount of current (electrons) flowing in a circuit
or in this case, to and from the battery.
When a battery is
over-charged excessively, the length of time it can provide a specified
current draw, such as 50 ma for ten hours, is slowly reduced to the point
where it may only provide the same required current draw for a few hours
or less. This is due to progressive degradation of the battery chemistry.
Obviously there are many types
of electronic devices available today which employ rechargeable batteries
as their source of power. Most of these devices do not draw a fixed
amount of current for a fixed period of time as shown above.
An example would be a portable
or handheld radio transceiver. This device was selected to show the
extreme changes in duty cycle which can occur. Duty cycle is the
amount of time the unit is ON or OFF between recharges and the varying
amounts of current being drawn in the ON state as well as the OFF state.
Most microprocessor based radio
transceivers will draw current from the battery even when they are turned
off. These units usually have a clock that keeps running and memory
that requires power to maintain information about each channel. When the
unit is turned ON, it remembers the frequency previously programed for
each channel. In addition, a nicad not connected to anything, will
lose from 1 to 3 percent of its charge per day at room temperature.
When the transceiver is turned
on, additional current is required for the display and the receiver circuitry.
When a transmission is received, the squelch is opened and additional current
is required to power the speaker or headset. Most of the current
required for any of these functions is minimal. However, when a transmission
is made, up to several hundred milliamperes of current can be drawn for
the length of time the transmitter is keyed. This describes the duty cycle
of the device as mentioned above.
So what's the
point?
The point in all this
is, that when recharging the battery to the manufacturer's recommendation,
it is virtually impossible to know the amount of discharge which has occurred
and how long to leave it on the charger for proper recharge without damaging
or shortening the life of the battery. Keep in mind that when the
battery is depleted to its "discharged state" and then recharged
for the length of time specified by the manufacturer, then proper recharging
will occur.
This applies to almost any
device employing rechargeable batteries. The difficult part in the
proper "Care and feeding" of NiCad batteries is, knowing "how much" the
battery has been depleted and "how long" to leave the charger plugged in
to produce a proper recharge. A brief review of several charger techniques
should lead you to a solution which will eliminate the problem of "How
Long?" and "How Much?"
Yes, there's
more!
A TRICKLE charger
is perhaps the most common type of charger available today. It is
very low in cost to produce because of minimal parts count. Usually
seven parts including the label. It is the obvious choice of most
manufacturers of electronic devices for that reason. TRICKLE chargers
typically supply a CONTINUOUS current to the battery in the range of 45
to 65 milliamperes. This current level is required as a minimum to
produce the chemical reaction required to recharge the battery. When
the battery reaches its fully charged state, the TRICKLE charger should
be removed. If not, the chemical reaction will continue to take place
and any or all of the symptoms alluded to earlier could occur.
Another type of charger utilizes
an automatic TIMER. The TIMER based charger typically will recharge
the battery for a preset time starting at the point the charger is turned
on and turning itself off after the specified time has past. Again
this works well when the battery is discharged to an appropriate point.
The problem arises when the
battery is not completely discharged and then recharged again. Although
the battery may only require a 30 minute recharge, it receives a full recharge
cycle of three to four hours, possibly more, depending on the preset time
for normal recharging. Obviously, damage may occur because of the
over-charged condition. TIMER based chargers typically deliver 100
to 250 ma per hour which is considerably more current per hour than the
TRICKLE charger. This higher level of current provides an opportunity
for faster deterioration of the battery with improper use of the charger.
Still another type of charger
is the TEMPERATURE based charger which is very similar to the TIMER based
unit in that the charger senses the temperature of the cell. When
it reaches a preset temperature limit, the charger turns itself off.
The same problem exists with the TEMPERATURE based unit as does the
TIMER based unit.
The possibility of over-charging
exists with a partially discharged battery because the charger is looking
for a temperature limit to be reached before turning off. Suppose
a fully charged battery at room temperature is placed in the charger, it
doesn't know that the battery is fully charged until the temperature rises
to the cut-off point. To do this, the charger will over-charge to
the point where the battery temperature rises to the cut-off point.
Once again, the battery is being exposed to unnecessary deterioration through
an overcharging condition.
With todays
advanced technology, there must be a solution!
Part of the problem
with properly recharging NiCad batteries is that most manufacturers are
concerned with the end-user connecting the wrong charger to their device
or product. Worst case is a charger with the polarity reversed.
When this happens a very rapid discharge takes place causing the cells
to heat up and usually results in damage or destruction of the battery
and/or the device containing the battery.
To provide protection from
this situation, manufacturers usually insert a "diode" and a current limiting
resistor in series with the battery and the charger connector. A
diode allows current to flow only in one direction so that if an incorrect
or reversed polarity charger is used accidently, damage will not occur.
The ideal way to avoid these
problems is to monitor the voltage so that when the battery reaches a predetermine
voltage, the current flowing into the battery can be reduced to just maintain
the battery at it's peak. However, the "Diode" which is in series
with the battery, prevents the battery voltage from being "read" through
the external connector. This is one of the reasons why the three
types of chargers mentioned earlier are used instead of a voltage monitoring
type charger. These three types work on a time or temperature basis
and rely heavily on the user for proper use.
Here comes
the commercial!
SmartCharge
was developed to resolve the problems associated with time and temperature
type chargers. It uses a unique circuit which in effect is able to
know what the battery voltage is in spite of the presence of the "Diode"
and then control the amount of current being sent to the battery.
This is accomplished without
any modification to the device. However, it does require that SmartCharge
be designed to work with each individual product or device. The major
benefit is that once connected to the device it can be left on continuously
without damaging or shortening the battery life. The end-user no
longer has to be concerned with how much or how long to recharge his NiCad
batteries. In fact, most devices such as portable computers and radios
can be left on and operated with SmartCharge plugged in. It
acts like an Uninterruptable Power Supply (UPS).
Another area of concern about
NiCad batteries is "Memory Effect". This condition is caused by not
discharging the battery to an appropriate level each time the batteries
are used prior to recharge. The result of this condition is shortened use
between each recharge. Unlike the damage which can occur from over-charging,
"Memory Effect" is a temporary condition which can be corrected by cycling
the battery 2-3 times between full-charge and the normal discharge state.
When using the SmartCharge
technology for maintaining batteries, it’s recommend that every two to
three months, the batteries be recycled between discharge and charge at
least 2-3 times. Discharge means, leaving the device on until it
indicates "BATT-LOW". Most devices which SmartCharge is manufactured
for have some type of low battery indicator. It is NOT recommended
that a battery be discharged to "0" voltage!
One of the greatest benefits
in using the SmartCharge technology is that the user need only remember
to plug the charger into the device and forget about it. With the
three types of chargers mentioned earlier, each time an over-charged condition
occurs, the life of the battery is shortened by some small fraction and
eventually, these small fractions add up to a useless battery long before
the anticipated life span of the battery is reached. To gain the
most benefit from this article, do not leave nickel cadmium batteries on
charge longer than the manufacturer recommends. Always let the battery
discharge until you see a 'battery low' indicator or the device turns itself
off. This will prevent the memory problem from developing in the
battery.
One last thought....
Many of the newer
laptop computers are using 'nickel metal hydride', 'lithium ion' and other
chemistries in their rechargeable batteries. These types of batteries
are designed to have quick charge and discharge rates and are generally
not prone to memory problems. They typically require very accurate
charging technologies to prolong the batteries useful life. In addition,
they provide substantial performance over the nickel cadmium rechargeable
battery. When your considering the purchase of a new, high performance
laptop computer, look for the Lithium-Ion type battery.
SmartCharge products are available
for EPSON HX-20 Portable Computers and the new 'lithium ion' battery packs
(Sony 520 series) used in the Epson EHT Series and Fujitsu's Stylistic
500 Series, Pen-Based Computers and selected Sony Camcorders. A four
battery gang charger for the Sony batteries is presented on our web page.
Custom design units to fit your OEM applications are also available.
In addition, some aviation
hand held tranceivers such as the STS-7600, 800 Series Narco, and Terra
radios can be used with SmartCharge. A new Charger for the
ICOM A22 is in the works.
Contact GS Technologies
for
SmartCharge
applications and product information:
Email at info@gtechno.com
Go To
WebSite at GS
Technologies
WebSite at Culpeper
Aero Squadron
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