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.

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That's it!  Now, if you're interested in more of the technical stuff, read on!

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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.

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.

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?"

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.

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.

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.

Many manufacturers of the newer laptop computers are using 'nickel metal hydride', 'lithium ion' and other types of chemistry 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 you're 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. In addition, some aviation hand held transmitters such as the STS and 800 series Narco radios can be used with SmartCharge.  A four battery gang charger, the Model L4-A for the Sony batteries, is presented on our web page.  Custom design units to fit your OEM applications are also available.

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