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AMATEUR RADIO By GARRY CHATT, VK2YBX Getting the most out of nicads Proper charging techniques can extend the life of nicad batteries and 'repair' some common malfunctions. Here's how to get the most out of your nicads. High energy density and tolerance to abuse have made nickel cadmium [nicad) batteries popular for powering hand-held portable transceivers. They are arguably the most economical portable power supply available. While the nicad battery pack has the ability to stand up to all kinds of abuses, there are limits to the level of performance that the nicad can sustain. With some knowledge of these limits and knowing how to deal with some everyday difficulties, you can maintain peak performance levels and maximise operational life. The metal case is made from nickel plated steel, welded internally to the negative plate, and becomes the negative terminal. A sealing plate, located at the top of the cell, is welded to the positive plate and forms the positive terminal. A safety vent is also fitted to allow the escape of gas or electrolyte in the event of an abnormal increase in internal pressure. This vent is made from a special alkaline and oxidation resistant rubber which is self sealing and which maintains the normal internal operating pressure for the life of the battery. Construction Electrochemical processes A basic explanation of the operation of a typical nicad battery will show why maintenance and correct charging are so important. Fig.1 shows the internal details of a nicad cell. (A nicad battery pack is made up of a number of cells connected in series). Most nicads are cylindrical. The positive plate is normally made fFom porous sintered nickel which is filled with nickel hydroxide, while the negative plate is made from thin steel coated with a cadmium active material. The separator is made of polyamide fibre. These parts are all wound into a coil and inserted into a metal casing. An electrolyte is also included and this is a water-based alkaline solution which is totally absorbed into the plates and the separator. During the discharge process, oxy-nickel hydroxide combines with cadmium and water to form nickel hydroxide and cadmium hydroxide. The reverse occurs during the 84 SILICON CHIP Insulation gasket Current conector Nevattve plate Sepantor charging process. However, in the final stages of charging, oxygen is generated at the positive plate. This oxygen passes through the separator to the negative plate where it is absorbed to form hydroxide ions. This is why it is important not to overcharge nicads, as the oxygen liberated may not all be absorbed. If this is allowed to happen, the pressure inside the case increases and may even rupture the safety vent. When this occurs, electrolyte is lost and cell capacity is reduced. The information needed to correctly maintain a nicad battery should be clearly printed on its label. This information should include the nominal voltage rating, capacity and the recommended charge rate. Two charge rates should be indicated: a standard or slow rate, and a fast rate. The load voltage of a fully charged battery will vary between 1.2 and 1.3 volts per cell, depending on the cell design. A fully charged nicad battery will provide 1.2V per cell under load (see Fig.2). As the battery discharges its terminal voltage will be fairly constant until it is nearly depleted. A voltage of 1V or less per cell under load conditions indicates a fully discharged battery. Cell capacity The capacity (C) of a nicad battery is the amount of energy a cell or battery can provide. This is simply the time taken to discharge a cell to 1V multiplied by the current at Insulation plate which this discharge takes place. Nicad batteries are rated for Fig.1: internal details of a nicad cell. capacity based on a one hour The positive and negative plates and the separator are wound into a cylinder discharge rate at a temperature of 25°C. The unit of measure is the and inserted into a metal casing. Posttlve plate 1.3 1.2 110 - r-- §: ............... / l:i er ... "i\ \ 1.1 100 ► ~ w -' ./.,, 90 "' er = er \ ;. ~ 80 V / \ \ 0.9 70 -5 DISCHARGE TIME Temperature effects Nickel cadmium cells will operate over a wide temperature range although their performance will vary significantly when the operating temperature is far removed from room temperature (25°C). As temperature rises, useable capacity increases. This increase is due to the higher chemical activity at elevated temperatures but this is not considered when cells are rated for capacity. At 46°C, a cell will have approximately 106% of its room temperature capacity. Conversely, at - 6°C, the capacity will be 80% of room temperature capacity. Fig.3 shows the effect of temperature on nicad battery capacity. The standard charge rate for nicad batteries is the 10-hour rate or C/10. But higher charging rates are possible and practical for many modern cell designs. Five, three and one-hour chargers are common in communications equipment, and cells that can be fully charged in 15 minutes are available. 10 15 20 25 30 35 40 45 50 BATTERY TEMPERATURE (°C) Fig.2: the output voltage of a nicad cell is fairly constant at about 1.2V as the battery discharges. When the cell is depleted, its output voltage drops rapidly. milliampere-hour (mAh) or amperehour (Ah) for larger cells. For example, a cell that can provide a current of 450mA for one hour is rated at 450mAh or 0.45Ah. Capacity ratings at other than the one hour rate are not uncommon but the one hour rate is the most frequent reference. Some slight increase in capacity is available at the 10 hour rate but the improvement is usually not much greater than about 8%. 5 Fig.3: this graph shows how nicad battery capacity increases with temperature. Nicads should be allowed to reach room temperature (25°C) before recharging. Common nicad problems The most frequent complaint associated with nickel cadmium batteries relates to capacity loss and the consequent reduced operating time. There are a number of common causes for this: (1) incorrect battery for a given duty cycle; (2) effects of long term storage; (3) long term overcharge; (4) shallow discharge/full charge cycles; (5) cell depletion as a result of normal use; (6) insufficient charging time; and (7) charging at high temperature. Let's look at each of these common problems in turn. Duty cycle Poor discharge time or endurance may be due to excessive current drain under normal operation. For example, to estimate the energy required to operate a transceiver for a given period, we must know how much current is drawn from the battery pack during receive, transmit and standby modes. By then using a standard duty cycle (typically 5%, 5% and 90% for receive, transmit and standby respectively), the discharge time for the battery can be predicted. Fig.4 shows how this is done for a transceiver with the following current drains: receive, 45mA; transmit, 245mA; and standby, 1ZmA. The resulting figure of 236mAh represents the minimum capacity required for an 8-hour operating period. More active operations may require a 10%, 10%, 80% duty cycle or higher. A calculation of required capacity under more demanding service is shown in Fig.5 . A 450mAh battery, typical for many transceivers, will provide more than adequate service in both cases and still have reserve capacity for extended duty tours, or even heavier duty cycles. Long term storage Batteries that have been stored for long periods of time will not have full capacity when first placed in service. This is the result of two effects of long term storage. The first is called passivation. During storage, a crystal-like film grows on the positive plates (the anode) of nicad batteries. This passivation layer acts as an insulator and must be removed before the cells can provide full service. At the same time, the passivation layer prevents deterioration of the anode and, in that sense, is beneficial to the shelf life of the battery. The second effect of long term storage is pooling of the electrolyte. Electrolyte, as a result of gravity, will no longer be evenly distributed within the cell, leaving some portions of the cell dry while other areas are saturated. Both storage problems are easily corrected. After batteries are removed from storage, it will be necessary to "wake up" the cells with two or three charge/discharge cycles. This will "burn off" the passivation layer, and redistribute electrolyte evenly throughout the cells. Usually, about 40% of battery AUGUST 1988 85 STATUS STANDBY RECEIVE TRANSMIT CAPACITY CONSUMED IN 1 HOUR CURRENT X % 15 X .90 45 X .05 245 X .05 = 13.5 2 .25 12.75 STATUS STANDBY RECEIVE TRANSMIT CURRENT X % 15 X .80 45 X .1 0 245 X .10 X8 X8 328.0 mAh per 8 hr shift 236 .0 mAh per 8 hr shift Fig.4: this energy requirement calculation is for a portable transceiver with a 5-5-90% duty cycle. A typical 450mAh battery will provide capacity to spare. Long term overcharge Modern nickel cadmium cells have been designed to withstand the deteriorating effects of long term overcharge at the to-hour rate. Gassing, venting and leakage are rare, even when a cell has been left on charge for days or weeks at a time. The capacity of such a cell or battery will often appear to diminish after extended overcharge but this is not a permanent fault. Even batteries that appear to have lost as much as 35 % of total capacity can be resurrected by a single charge/discharge cycle. After this treatment, such batteries will typically exhibit 85-90% or more of their original capacity (Fig.6). Shallow discharge/full recharge of nicad cells is perhaps the most well-known effect, yet is probably the least frequent of nicad problems. Often called memory effect, it is the most misidentified problem associated with nicads. To explain, early nicad cells, 86 SILICON CHIP 12.00 4.50 24 .50 41.0 mAh per hour 29.5 mAh per hour capacity will be available after the first charging cycle, 70-80% after two cycles and more than 95 % after the third cycle. So it pays to cycle the batteries through several charge/discharge cycles before putting them into service. Apart from this, charged or discharged cells may be stored for indefinite periods of time with no significant degradation in performance. Where possible though, batteries should be charged before storage. Note that batteries which are stored in the charged condition will loose about 1 % of capacity per day due to self discharge. CAPACITY CONSUMED IN 1 HOUR Fig.5: the energy requirement calculation for a more demanding 10-10-80% duty cycle. Note that the energy requirement is still within the capacity of a 450mAh battery. when discharged to only a small portion of the total available capacity, would "memorise" that level of discharge. Such cells would then provide only the "memorised" capacity level and no more. Today's modern cell design has all but eliminated the memory effect. Special plate processing techniques have reduced the problem to the point where only repeated and identical discharges will cause a battery to exhibit memory. Even in cases where identical shallow discharge/full recharge cycles do produce a real memory effect, the condition may be corrected by several deep charge/discharge cycles (Fig.7). Cell depletion The electrochemical processes that occur when cells are charged and discharged are, in theory, fully reversible. In practice, the reformation of the chemical agents within the cell limits the life of the cell to a finite number of cycles. As time passes, less and less capacity is available and at some point, when available capacity is less than necessary for a given duty cycle, the battery should be replaced. Standard charge nicad batteries can take as many as 1000 full charge/discharge cycles before their capacity falls below 80%. Fast charge batteries should provide 600-700 cycles. Insufficient charge time Nickel cadmium batteries and cells are normally charged from a constant current source at some convenient rate. This rate is frequently chosen to provide fully charged batteries within a given time period. As stated previously, the "standard" rate is the 10 hour rate or C/10. If charge/discharge efficiencies were 100% perfect, then a cell charged at the 10 hour rate would be recharged in 10 hours. Unfortunately, this is not the case because charge efficiency is less than perfect. To recharge a fully depleted battery, it is necessary to provide 140% of the energy that the charged battery can deliver. This means that, when charging at the 10 hour rate, charge time must be increased by 40% to 14 hours for full recovery. This requirement also applies to " fast" chargers. These often use the temperature of a battery to trip a charge indication lamp. This usually means that the charge rate has been changed from fast to standard. The switch is often set to trip when the battery temperature reaches 45°C. It does not indicate end of charge. At this point the battery may be charged to only about 75-85% of full charge, so additional time should be spent in the charger to "top up" the charge. Charging at high temperatures Charging a battery when ambient temperature is high may reduce full charge capacity. When a battery or its environment is warmer than 25°C, full recharging will not occur and the battery will appear to have lost capacity (Fig.BJ. What ' s more , high ambient temperatures may cause premature tripping of fast chargers which are controlled by thermal sensors. 1.3 1·3 r--,-::TE::::ST::-:S::':'HA::-:-L:-:LO:::,W-r--r---r-r---.--r--,-...---,----, DISCHARGE I 1NmAL /DISCHARGE ....... 1.2 " - r-- r-. -..... N ~['..., r-,...._ ........ ARST DISCHAR~ AFTER EXTENDED OVERCHARGE I" '\ 1' 1---1---1-.--+--+--+----l--l---l---l---l--l---" \ \\ I'. ' J SECOND DISCHARr& AFTER EXTENDED OVERCHARGE 0.9 ,_ \ \ DISCHARGE TIME DISCHARGE TIME Fig.6: an extended overcharge does not render nicad batteries unusable. Recycling can typically restore capacity to 85% or more of new battery specification. 90 ~ z et I... l;l! ..,~ ls 4.21·c v- 100 80 J 70 'I' I 50 40 / ... i..---- /" J 60 / -- 45•c s1·c v I V Fig.7: the 'memory effect' is rarely a problem with modern nicads. Even if repeated identical charge/discharge cycles produce some memory effect, recycling restores full capacity. i..-- ..... r--..1, r--.... V e ao II 'I/ / 10 I r-.... -L WEAK,~ ~ t'-... I I '// 'I NORMAL DISCHARGE '\ e:.11y CELL SHORTE60R DEAD CELL ~ 20 ...... J ' "\ ~ "\ "' \ ~ '1 \ I DISCHARGE TIME CHARGE TIME Fig.8: a nicad battery accepts a reduced charge at high temperatures, lowering apparent capacity. In addition, thermal sensors may switch the charging rate to 'trickle' prematurely. Fig.9: weak or shorted cells result in abnormal discharge voltage curves. Recycling may restore weak cells but batteries with shorted cells should be replaced. Charging methods Testing batteries A simple, economical nicad charging circuit is shown in Fig.10. This circuit consists of a transformer, a bridge rectifier and a current limiting resistor in series with the cell to be charged. For best results, component values should be selected for a C/10 charge rate. Let's say that we want the circuit to charge a 6V nicad pack rated at 500mAh. Here's how to calculate the circuit values: • The transformer secondary voltage should be two or three times the battery terminal voltage; eg, 6V x 3 = 18V. • We must now calculate the value of the limiting resistor so that the battery pack charges at about the C/10 rate; ie, about 50mA or 0.05A. The equation is as follows: Rs = (18V - 6V}/0.05A = 2400. Before batteries are replaced it is a good idea to check the available capacity. This is quite easy to do and requires only a suitable load resistor and a voltmeter. The battery should be tested fully charged. All you have to do is to choose the load resistor so that the battery discharges at its "C" rate and then measure the time taken for it to discharge to 1V per cell. For example, a 15V 450mAh battery containing 12 cells would be checked using a load resistance of 320. At a discharge rate of 450mAh, the battery should last for 60 minutes or more before the voltage under load falls to 12V (ie, 1V per cell). A discharge time of only 48 minutes corresponds to 80% capacity while 36 minutes equates to 60% capacity, a level at which the battery should be replaced. To determine the test conditions for any battery, the terminal voltage of the battery is divided by the cell voltage (1.2, 1.25 or 1.3V the correct value depends on the battery manufacturer). The answer is the number of cells in the battery. The terminal voltage is divided by the rated capacity to find the test load resistance, ie: Load resistance = (number of cells x 1.2V) -;- capacity. Other common problems When a nicad battery discharges to less than 1V per cell under load, cell reversal may result. This happens because, when the weakest cell in a series string reaches its end point, the remaining cells may still have enough capacity to drive current through this cell. This effectively "charges" the cell in reverse. AUGUST 1988 87 AMATEUR RADIO - CTD FUSE + NICAD T BATTERY : PACK ..L. Fig.10: an economical nicad charging circuit. For best results, Rs should be selected for a C/10 charge rate. The text shows you how to calculate the correct Rs value. Dirty contacts on the charger or battery can cause a runaway charger. So can high AC mains voltage or current transients, surges and dips. Many batteries have fuses or thermal cutouts to protect them from damage due to high currents or temperatures. However, open fuses will result in a dead battery that must be replaced. Thermal cutouts, on the other hand, will reset themselves after cooling which means that the battery can be reused. Nicad do's and don'ts If this occurs, the cell volt-a.ge actually reverses; the positive terminal becomes negative and the negative terminal become positive. If the battery has not been badly reversed, it may be possible to correct this situation by subjecting it to a full charge cycle. In the long term, however, the effect can lead to excessive gassing within the cell and possible venting, resulting in electrolyte loss and premature failure. A battery with a weak cell in a series string of cells will exhibit a lower than normal terminal voltage under load after it has been discharged for a short time (see Fig.9). Batteries with this difficulty should be replaced if cycling does not restore the weak cell. Batteries with shorted cells exhibit lower than normal voltages under load, usually by multiples of 1.2V. Batteries that have shorted cells should also be replaced. Vented or leaking batteries It is most unusual to see leaking electrolyte as a result of can or seal failure in modern nicad batteries. The steel can construction and crimp seals used have virtually eliminated leakage as a source of concern. Cells which have vented are another matter. Cell venting, with consequential spillage of electrolyte, is always the result of some other. problem. Long term overcharge, forced discharge, runaway charging systems, cell reversal and cold battery charging are common reasons for electrolyte spillage. 88 SILICON CHIP Spillage leads to a loss of capacity and to cell failure. If it is suspected that venting has caused a battery to lose electrolyte, the loss may be verified by measuring the voltage of the battery after charging is complete. Batteries suffering from vented cells typically have an abnormally high terminal voltage at the end of the charge cycle. For example, a battery having a rated voltage of 15V could show a terminal voltage of 17 to 18V at the end of the charging cycle. Such batteries should be replaced if they fail to provide adequate service capacity. Cracked or broken cases Plastic battery case design takes into consideration the effects of rough treatment, accidental mishandling and just plain abuse. ABS, Lexan and other polycarbonate materials are used to reduce the probability of case breakage. Cases do break though. Batteries with cracks and splits can be used but with discretion. Any battery with pieces missing or with internal parts showing should be replaced, because the cells may accidentally short-circuit. A melted or swollen case can occur with fast charge batteries when the fast charge cycle has failed to terminate properly. If this happens, the battery should be removed from service (distortion of the case will likely prevent it from fitting in the transceiver), and the charger should be checked thoroughly for correct ~peration. In general, nicad cells should not be charged continuously at rates greater than 2C. Doing so can overheat internal cell components and cause premature failure. Short term high discharge rates are permissable, but caution should be observed whenever discharge rates exceed the 2C rate. Short circuiting nickel cadmium cells and batteries should be avoided. Because nicad cells have extremely low internal impedance very high currents can flow in a dead short, causing very rapid heating. Tools, jumper leads, wires and other shorting devices will get hot, leading to the possibility of burns or even a fire. Cold batteries can rupture if charging is begun before they are allowed to reach moderate temperature. Fast charge batteries should reach 15°C before charging while standard charge batteries should be at 10°C or warmer. Such temperatures can be achieved by allowing cold batteries to stand at room temperature for a few hours. Wet batteries should be allowed to dry thoroughly before being placed in a charger. Moisture can act as a conduction path that can lead to permanent charger or battery malfunction. Finally, nicads should not be discharged to less than 1V per cell, since this can easily lead to cell reversal. Footnote: The author wishes to thank Mr Walter Ullrich, President of Multiplier Industries, USA for his permission to use material from his publication How to Get the Most From a Nicad Battery. ~