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Text File | 1993-03-01 | 25.4 KB | 442 lines

---------------------------------------------------------------- RECHARGEABLE BATTERIES AND LAPTOP COMPUTERS ---------------------------------------------------------------- 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 THREE BONUS DISKS which expand this software package with vital tools, updates and additional tutorial material for laptop users! Send $29.95 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.