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RECHARGEABLE BATTERIES AND LAPTOP COMPUTERS

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No other topic seems to inspire more opinion and comment than

     the proper care and handling of rechargeable laptop and notebook 
     computer batteries. Should you slow or fast charge them? What is 
     the true life of a rechargeable battery after which it must be 
     disposed? Do rechargeable batteries have a "memory" effect? Can 
     nickel-cadmium batteries explode when charging or discharging? 
     Although this tutorial may seem technical in places, try to read 
     ALL of it since battery power may be your only source of laptop 
     power on many occasions. 
     For many portable computers a variety of rechargeable battery 
     options exist today. But frequently it comes down to the old 
     standby: nickel-cadmium batteries. Ubiquitous in consumer 
     electronic items such as shavers, flashlights, toothbrushes and 
     radios, nickel-cadmiums or "nicads" are a reasonable balance of 
     power, cost and weight and are used by many computer 
     manufacturers as the portable power source of choice. Let's 
     scratch the surface on the topic since there is QUITE A BIT the 
     manufacturer doesn't tell you about nicads.... 
     
     Glance at the following chemical equation which is at the heart 
     of the nickel-cadmium cell reaction. Don't get overly anxious 
     because high school chemistry was not your favorite subject. 
     We'll take things slowly.... 
     
                         <-----
     Cd + 2 NiOOH + 2 KOH -----> Cd(OH) + 2NiO + 2 KOH 
                                      2 
      
     In this highly simplified reaction sequence, electricity is 
     generated when the reaction proceeds in the direction of the 
     right pointing arrow, the discharge cycle. If the reaction 
     proceeds in the left direction the cell is charging. 
     
     In simplest terms, a nicad cell (a battery is constructed of 
     several cells hooked together) has a positively charged plate of 
     nickelic hydroxide and a negative plate of metallic cadmium. The 
     liquid between the positive and negatives plates which 
     facilitates this chemical reaction is usually a dilute solution 
     of potassium hydroxide - similar to lye or the Draino (tm) 
     solution your pour down the sink to clean your plumbing. When 
     discharging and thus producing electricity, the nickelic 
     hydroxide is reduced to nickelous hydroxide as hydroxyl ions 
     from the potassium hydroxide electrolyte combine with the 
     cadmium metal of the negative plate of the cell to form cadmium 
     hydroxide. Cadmium is oxidized when this happens and electrons 
     are provided into the external circuit, such as your laptop 
     computer. 
     When charging, the process reverses and hydroxyl ions combine 
     with the nickel which accepts electrons from the external 
     charging circuit. Notice that the electrolyte, potassium 
     hydroxide is unchanged with two atoms or units produced on both 
     sides of the chemical equation whether charging or discharging. 
     This is why you do not need to add more water to a nicad battery 
     which operates as a sealed reaction container. It regenerates 
     its electrolyte in both the charge and discharge cycles. 
     
     All of this is an ideal nicad cell. The real world of computers 
     and rechargeable batteries is not quite that simple. The first 
     SERIOUS item to consider is that all nicad cells and batteries 
     generate gas during both the charging, and to a lesser extent, 
     discharging cycle. 
     
     During recharging, oxygen gas is generated at the positive 
     electrode while hydrogen gas is produced at the negative 
     electrode. In other types of rechargeable cells, a standard lead 
     acid car battery for example, these gasses are usually released 
     into the atmosphere. The nicad cell does not have this luxury 
     since it must operate cleanly and with minimum release of gasses 
     or liquids. To minimize hydrogen gas release, nicad cells 
     usually have an oversized negative electrode which tends to 
     reabsorb hydrogen gas. In addition oxygen is recycled by 
     combining with metallic cadmium to produce cadmium oxide. So 
     called "fast-charging" nicad cells prevent gas buildup and 
     dissipate some of the heat generated during the quick charge 
     cycle by further enlarging the electrodes. Heat and gas buildup 
     is thus controlled and kept to tolerable limits in quick charge 
     nicads. 
     The first of several lessons which can be derived from this 
     technical discussion is that the buildup of hydrogen and oxygen 
     gas during the charging cycle is normally dissipated unless HIGH 
     recharging rates are attempted or unusually high temperatures 
     are produced. If the nicad cell is charged at abnormally high 
     rates the oxygen gas cannot dissipate and will EXPLOSIVELY 
     rupture the cell. 
     
     A safety system of sorts exists within the design structure of 
     most nicad cells via a pressure venting system - a plastic 
     diaphragm membrane at the top of the cell and small external 
     hole or "exhaust vent." In theory the system safely vents excess 
     pressure and then reseals. In practice the resealing is never 
     complete and the cell may continue to ooze caustic electrolyte 
     or worse the vent may not open soon enough and the cell may 
     simply explode. The vent is really designed for SEVERE charging 
     or discharging rates. In normal use it should NEVER activate; if 
     it does, the battery should be discarded. In cases of massive 
     overcharge or discharge the safety vent is usually too little 
     too late and a dangerous battery explosion takes place anyway. 
     
     During rapid discharge - short circuiting the nicad cell or 
     battery with a piece of wire, for example - gas buildup and heat 
     can be generated and a violent explosion can occur. Another 
     reason why nicads can explosively burst when short circuited and 
     forced to discharge quickly is that they have relatively low 
     "internal resistance" which allows them to dump their electrical 
     capacity quickly and with explosive force. 
     
     Common zinc carbon batteries have a much higher internal 
     resistance and when shorted may produce serious burns to your 
     fingers from melting wire but usually will not explode due to 
     sudden gas buildup. On the point of sudden nicad discharge by 
     short circuit you might be tempted to say that it would be highly 
     unlikely with a portable computer battery. Not so. Tales are 
     told of laptop computer batteries which have exploded when a 
     careless owner shoved several fully charged nicad batteries in a 
     travel case with a set of spare keys. If the keys accidentally 
     contact both the positive and negative poles of the nicad 
     simultaneously, a violent explosion reaction can occur!   
     
     Clearly nicads have some unusual features to be respected and 
     understood. Be careful with charged nicads and treat them as the 
     small "hand grenades" which they can become. Heat, sudden short 
     circuits and high rates of charging are the problem in this 
     area. 
     
     The correct operating temperature for discharging and recharging 
     nicads is from 65F to 85F, according to most manufacturers. High 
     and low ranges of from +32F to 115F are possible as upper and 
     lower limits if nicads MUST be used in extreme environments 
     although discharge and recharge efficiency may be adversely 
     affected - it may require more power to fully charge the battery, 
     charge may not be held for as long on the shelf after charging 
     and finally discharge may not produce a full three or four hour 
     computing session at these severe temperature ranges.
     Electrically, individual nicad cells - the units which are 
     hooked together to produce the final battery - have a charged 
     voltage of 1.25 volts. Nominally this drops to 1.2 volts under 
     actual discharge use or "load" in the electrical device. 
     Individual cells are strung together in "series" with the 
     positive terminal of one cell touching the negative terminal of 
     the next cell in sequence to raise the voltage to that suitable 
     for the electrical device. Thus two cells hooked in "series" as 
     a battery produce 2 X 1.2 volts = 2.4 volts. Likewise, three 
     cells connected as a battery produce 3.6 volts. By the way, 
     ordinary flashlight batteries of the carbon zinc type have a 
     nominal voltage of 1.5 volts compared to the 1.2 volts of the 
     nicad cell.
     Nicad batteries have an unusual and highly characteristic 
     discharge behavior which is best described as "a stable 
     discharge plateau then sudden voltage drop." Essentially a fully 
     charged nicad battery provides constant voltage and current 
     until near its exhaustion at which point the voltage SUDDENLY 
     DROPS and the cell is, for practical purposes, completely 
     discharged. 
     
     Compare this to standard carbon zinc and alkaline batteries 
     which gradually drop in voltage and amperage through the 
     discharge cycle of the battery. In use nicads tend to be stable, 
     then die suddenly at the end while conventional non-rechargeable 
     batteries slowly decay in voltage as their power is consumed. 
     One conclusion you might draw from this is that when your 
     portable computer beeps that the nicad battery voltage is 
     nearing exhaustion you literally have only moments of use left! 
     The good news is that nicads produce dependable power through 
     their discharge cycle which is highly desirable with digital 
     data and computer memory devices. 
     The "memory effect" of nicads is perhaps the most discussed and 
     misunderstood phenomenon associated with nicad cells and 
     batteries. An undesirable and somewhat unique characteristic of 
     nicad batteries that they can develop a "memory" which can 
     decrease either the capacity or voltage of the battery. 
     
     The first type of memory problem in nicads - voltage memory - is 
     caused by sustained charging over many days or months. This 
     memory effect can be accelerated by high ambient temperature 
     extreme duration of charge and high rate of charge. In effect 
     the battery is charged for such a long period of time or at such 
     a high rate or high temperature that the efficiency of the 
     chemical reaction is impaired and proper terminal voltage 
     readings are not achieved. 
     
     In the second, more common "memory capacity" problem, the nicad 
     loses the capability to deliver its full power capacity. One 
     cause of this peculiar memory problem is the FREQUENT PARTIAL 
     DISCHARGE of the battery - use for perhaps 30 minutes - and then 
     full recharge again. In effect the nicad battery "learns" that 
     only part of its capacity is used and over several cycles of 
     "partial depletion and then full recharge" that less then full 
     capacity is needed. It will then be unable to deliver a full 
     two or three hour standard discharge in normal use. Fortunately 
     memory effects are usually temporary and can be reversed.
     
     The chemical basis for these two memory effects is not fully
     understood, but may have to do with obscure oxidation reactions 
     which temporarily coat the internal electrodes of the battery 
     with thin layers of complex non-reactive chemical compounds 
     which can be removed by more fully "exercising" a nicad through a 
     complete charge/discharge cycle. 
     It is claimed by many manufacturers that this odd memory effect 
     of nicads has been largely eliminated due to modern 
     manufacturing methods. However to some degree this may in fact 
     be a result of newer charging systems and the relatively 
     complete discharge of nicad power by modern laptops. In effect 
     the batteries are charged and discharged in a more appropriate 
     manner by most laptop users so memory effects "appear" to be no 
     longer a problem.
     Both memory problems - voltage memory and capacity memory - are 
     usually temporary and can be corrected by discharging the 
     battery to or very near its exhaustion point (optimum drawdown 
     voltage is about 1.0 to .9 volts for a standard 1.2 volt nicad) 
     and then recharging it to full capacity. Repeat this discharge-
     recharge cycle from 2 to five times and frequently the nicad 
     will lose its memory for the "partial capacity" and again 
     provide a full 3 or 4 hours of use in most laptops. Actually, 
     frequent FULL discharge and recharge prolongs the life of a 
     nicad. The more you use them the longer they last! 
     Most folks who want to completely discharge laptop nicads simply 
     leave the computer on until it runs down. A much faster method is 
     to use the following batch file which continuously reads the 
     directory of a disk and writes the contents to a disk file. 
     The continuous disk access drains nicad power much faster. If 
     you are not familiar with batch files, read the batch file 
     tutorial elsewhere in this program. Here's the three line batch 
     file. To stop the batch file at any time press the control and 
     break keys simultaneously. When finished you may wish to erase 
     both the batch file and the small file named "test" which it 
     creates.
     :start
     dir>test
     goto start
     As an aside, the newer nickel-hydride batteries used in some 
     laptop and notebook computers do not seem to suffer from memory 
     effects. But these batteries are more expensive and not in 
     common use by most laptop manufacturers.
     Nicads do eventually fail. And for various reasons. Temporary or 
     partial failure due to memory effects was discussed in the 
     previous paragraphs. 
     
     Permanent failure - usually between 3 to 5 years into the life 
     of a typical nicad can happen due to the growth of 
     characteristic "whiskers" of conducting chemical compounds which 
     effectively bridge the internal gap between the positive and 
     negative electrodes inside the battery. Effectively these small 
     contamination deposits gradually short circuit the battery 
     internally which leads to inability to charge or discharge. Some 
     clever electronic hobbyists build high current "surge" power 
     supplies which can burn open these internal deposits and reopen 
     the gap between positive and negative electrodes. A risky 
     practice at best - given the explosive reputation of nicads - but 
     "zapping" nicads in this manner has been documented as one way 
     to add life to an otherwise dying battery. A risky an usually 
     ill-advised attempt to salvage an otherwise dying battery.
     A different permanent failure can result from premature loss of the 
     liquid electrolyte from the battery. High temperature and/or 
     high charging rates are usually the cause here. Quick-charge 
     batteries frequently fail due to this problem if their charging 
     circuits are not properly designed. If the top edge of the cell 
     which contains the fail safe pressure release valve has a 
     buildup of white corrosion powder this is probably the residue 
     ot the expelled electrolyte and the cell may be on its way to 
     failure and should be replaced. Note that you can only see this 
     corrosion buildup on the top of the SINGLE nicad cells which are 
     usually encased within a surrounding plastic battery housing. 
     The plastic housing may show little problem externally. 
     Generally, however, the average computer user should not attempt 
     to open the protective plastic case of the battery to examine 
     each cell. If the manufacturer seals several individual nicad 
     cells in a plastic battery container it is for GOOD reason and 
     your own personal safety. As a rule quick charge nicads do not 
     last as long a regular nicads due to heat build up during the 
     charging cycle.
     So how long will a nicad battery last before complete failure 
     occurs? Manufacturers estimate LOW figures between 500 and 1,000 
     full charge and discharge cycles or about 3 to 5 five years, as 
     noted above. Some nicads have been known to approach 5,000 to 
     10,000 charge and discharge cycles before permanent failure. 
     Excessive quick charging, heat buildup, infrequent use and lack 
     of full charge all contribute to shortened nicad lifespan. 
     Charging and discharging mathematics... 
     Charging nicads is generally done automatically by a charging 
     circuit. Two practical pieces of advice: 1) if the battery 
     becomes VERY hot something could be wrong 2) if the manufacturer 
     tells you that the battery will be fully charged after a certain 
     length of time although it can be left charging longer you will 
     probably do the nicad a favor by removing it after full charge 
     is reached. Some clever nicad users simply attach an inexpensive 
     electrical timer - similar to those used to turn lights on and 
     off in the evening - directly to the nicad charger to prevent 
     overcharging. 
     
     Generally nicads have a proper charging rate which depends on 
     each manufacturers recommendation. For standard nicads which are 
     NOT quick charge types the proper slow or "trickle" charge rate 
     is determined by dividing the ampere hour capacity of the 
     battery by 10. For example if a nicad has a total capacity of 1 
     ampere hour, dividing this by ten (1/10) produces a correct 
     trickle charging rate of .1 amps or 100 milliamps. Quick-charge 
     nicads can accept a charge rapidly and the suggested charging 
     rate is determined by dividing the ampere hour capacity of the 
     battery by 3 rather than by 10. These figures represent the 
     trickle charge rate which theoretically means the nicad "could" 
     be safely left charging indefinitely without harm. 
     Higher efficiency chargers are designed not to simply trickle 
     charge nicads but start a discharged battery at a HIGH rate of 
     charge and then taper the charging current back quickly to the 
     safer "trickle" charge rate once full charge is reached. Usually 
     for regular nicads this "initial surge charge" can be as high as 
     the ampere hour capacity divided by 3. For quick charge nicads 
     this "initial surge charge" can be as high as the ampere hour 
     capacity divided by 1. Obviously these are very high charge 
     rates and are provided to discharged batteries and then 
     quickly discontinued once full charge is approached. Clearly a 
     charging circuit of this sophistication is expensive and may 
     even contain its own microprocessor to sense the discharge level 
     of the nicad and calculate the optimum charge rate, time and 
     trickle charge transition. Since we have previously discussed 
     the adverse affect of heat on nicads it is essential to note 
     that NICADS SHOULD BE CHARGED IN A COOL OR ROOM TEMPERATURE 
     location since they normally generate heat when charged. If you 
     minimize heat buildup - especially during the charging cycle - 
     you will prolong the useful life of your nicad battery. 
     
     Discharging a nicad - especially if you are trying to remove a 
     "memory" problem such as that discussed earlier does NOT mean 
     discharging a cell to zero volts. Usually the correct discharge 
     voltage is about 1.0 volts. This may seem odd when you consider 
     that the fully charged cell has a 1.2 volt reading, but in fact 
     at 1.0 volts a typical nicad cell has released about 90% to 95% 
     of its energy - another eccentric, but predictable behavior of 
     nicads given the rapid "voltage drop off" as they near the end 
     of their three or four hour life in a laptop computer. 
     
     Shelf life. While carbon zinc and alkaline batteries can hold 
     their charge for years, nicads lose their charge relatively 
     quickly. Although it varies, one quick rule of thumb is that a 
     typical fully charged nicad will lose roughly 25% to 35% of full 
     charge in one month. Then another 25% to 35% of THE CHARGE 
     REMAINING in the next month. And so on and so on. Thus if you 
     have several nicad batteries you want to charge for a trip you 
     will be taking in a month, it is probably better to charge ALL 
     OF THEM the final week just before the trip rather than the 
     month before. For want of a better phrase, this might be called 
     "shelf discharge" and is normal with all nicads and has to do 
     with slight electrical leakage and chemical compound decay 
     internally within a charged nicad which sits on a shelf. Cooling 
     or refrigerating the nicad (but NOT freezing) will slow this 
     "shelf discharge" since you are cooling and slowing the 
     breakdown reaction. In fact ALL batteries will last longer when 
     refrigerated until they are used. Simply store them in 
     individual sealed plastic bags (to minimize moisture 
     condensation) and place them in the refrigerator. 
     And so we conclude with a little summary....
     
     1) Do exactly what the manufacturer suggests for both 
     discharging and recharging a nicad.
     
     2) Keep temperatures - especially during charging - cool or at 
     normal room temperature.
     
     3) Never short circuit a nicad intentionally or accidentally.
     
     4) Try cycling a nicad through several COMPLETE discharge and 
     recharge cycles if it "appears" to be faulty an incapable of 
     operating your equipment for a normal three or four hour 
     operating period. 
     
     5) Remove nicads from charging circuits or discontinue charging 
     when full charge has been reached. 
     
     6) Watch for white flaky corrosion deposits on the upper edge of 
     the cell near the pressure vent this can mean impending cell 
     failure and electrolyte loss.
     
     7) Dispose of permanently defective nicads properly - contact 
     the manufacturer for instructions since cadmium is a dangerous 
     toxic metal and has been banned from many dump sites. Try 
     calling your local city hall and ask who can answer a question 
     about cadmium metal waste disposal. 
     
     8) When the nicad battery power begins to drop near the end of a 
     discharge cycle it will drop VERY QUICKLY due to the rapid 
     characteristic dropoff of nicads. Prepare for laptop shutdown 
     quickly. 
     
     9) Cycle your nicads through a FULL DEEP discharge and FULL 
     COMPLETE recharge frequently - they will last LONGER before you 
     must dispose of them and deliver MORE power when used. 
     
     10) Infrequently used nicads should be charged and discharged at 
     least once or twice every two or three months to prolong their 
     usable lifetime before permanent failure.
     
     11) If your nicads are stated by the manufacturer to be quick 
     charge type, you can probably prolong their life by slow or 
     trickle charging them (if your charger provides that option) 
     since you will minimize heat and gas buildup within the cell. 
     Just because they can be quick charged does not mean they MUST 
     be quick charged. Nicads last longer and deliver more power when 
     not driven to extremes of temperature or overcharging. 
     
     Tutorial finished. Be sure to order your FOUR BONUS DISKS which 
     expand this software package with vital tools, updates and 
     additional tutorial material for laptop users! Send $20.00 to 
     Seattle Scientific Photography, Department LAP, PO Box 1506, 
     Mercer Island, WA 98040. Bonus disks shipped promptly! Some 
     portions of this software package use sections from the larger 
     PC-Learn tutorial system which you will also receive with your 
     order. Modifications, custom program versions, site and LAN 
     licenses of this package for business or corporate use are 
     possible, contact the author. This software is shareware - an 
     honor system which means TRY BEFORE YOU BUY. Press escape key to 
     return to menu. 
                                         
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