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New from Smart Fastchargers, this nicad and NiMH charg­er caters for a wide range of battery voltages and capacities and uses the patented Reflex charging method. It has eight buttons to set the rate of charge, a rotary switch to select the battery voltage and a LED bargraph to indicate the cell voltage. An audible beep, at one second intervals, gives an indication that the main charge is still in progress. Recharging nicad batteries for long life Nickel cadmium and nickel metal hydride batteries are widely used in all sorts of portable equipment but they often don’t last long before they must be replaced. One solution is to use “burp charging” which is claimed to provide many thousands of charge/discharge cycles. By HORST REUTER* Battery powered equipment is undeniably practical – lightweight, portable and small, with no cables to drag around. But there is a price to pay for that convenience. Rechargeable bat­ teries are costly to buy and often don’t last long. The problem can usually be traced back to the type of charger used. Unfortunately, most of the nicad chargers supplied with ap­pliances at 10  Silicon Chip present require manual termination; ie, the user has to switch off the charger and disconnect the battery. This makes it practically impossible to avoid overcharging the batteries, thereby reducing their life expectancy. The key to long life lies in the charging method. In this article, a battery is defined as consisting of one cell or several cells connected in series. Internal cell im­ pedance is defined as the sum of the resistance of the internal connections and plates (both constant) and the degree of diffi­culty the ions encounter passing through the separators and electrolyte (variable). Nicad or NiMH? From an environmental point of view it would be an advan­ tage to change to NiMH batteries. Nickel Metal Hydride (NiMH) batteries are made without cadmium and are therefore less damag­ing to biological systems. At present, they have about 20% higher energy density than nicad cells (AA) and produce no memory effect (more about memory effect later). However, typical NiMH cells have a higher internal im­ pedance than nicad cells; 50mΩ instead of 10mΩ for 1200mAh cells. As a consequence, NiMH batteries have lower maximum discharge currents. This means they are only suitable for low current ap­ pliances like handheld radios. The maximum discharge current is 3C for NiMH AA cells and 2C for NiMH button cells, whereupon the cell voltage drops to approximately 1.1V. “C” is defined as the current that equals the rated battery capacity. For example, charging a 1.2Ah battery with a 4.8A current is a 4C charge. The same 4.8A current applied to a 4.8Ah battery is a 1C charge. In practice, the useful discharge currents for NiMH batteries are limited to less than 1C (the cell voltage remains above 1.2V). For currents above 1C, nicad batteries are superior. Fig.1 is a comparison of the load characteristics of one 1200mAh AA size NiMH cell, one 600mAh AA size nicad and one 1200mAh sub-C size nicad cell. The load was only applied for 5 milliseconds. The tests showed that a fully charg­ ed 12V 1200mAh nicad battery as used in power drills delivers a maximum of 11.9V with a 10A load. Even a 12V 600mAh nicad battery delivers a maximum of 10.9V with a 10A load. However, a 1200mAh NiMH battery with the same load delivers only 7.65V. NiMH batteries also differ from nicad cells in that the chemical reaction during charge is exothermic; ie, the charging process produces heat. The chemical reaction in nicad cells is endothermic; the reaction absorbs heat. However both battery types produce some heat during the main charge cycle because of internal impedance and both produce heat when overcharged. Over­ charging creates heat and gas but does not produce any further energy storage in the cells. The heat produced during the main charge in nicad cells due to cell impedance is absorbed in the endo­ thermic reaction. In NiMH cells, the cell heating due to internal impedance is added to the heat of the exothermic reaction. When NiMH batteries reach the overcharge region, they are therefore hotter than nicads. All available NiMH cells I have tested vented at around 43-45°C case temperature – much lower than for nicads. This means that charge termination at high charge rates is criti­cal and cannot safely be achieved with delta V termination charg­ers. The case temperature should not exceed 40°C Fig.1: a comparison of the load characteristics of a 1200mAh AA size NiMH cell, a 600mAh AA size nicad and a 1200mAh sub-C size nicad cell. The load was only applied for 5 milliseconds. This clearly demonstrates that the higher internal impedance of NiMH batteries limits their usefulness in delivering high currents. for NiMH cells and 45°C for nicads. Delta V termination utilises the voltage drop at the begin­ning of the overcharge region of the cell voltage curve (see Fig.2). The magnitude of this voltage drop is generally not as well defined in NiMH cells as it is in nicad cells. It depends on factors like charge current, ambient temperature, cell impedance, cell capacity, etc. The situation can be worse in battery packs. Several unmatched cells may cause the battery voltage to reach only a very shallow peak if some cells reach their individual peaks while others are still charging. Even if the Fig.2: delta V termination utilises the voltage drop at the beginning of the overcharge region of the cell voltage curve. The magnitude of this voltage drop is generally not as well defined in NiMH cells as it is in nicad cells. It depends on factors like charge current, ambient temperature, cell impedance, cell capaci­ty, and so on. January 1996  11 the nickel hydroxide to nickel oxyhydroxide. This process is reversed during discharge. If each cell in the battery pack is discharged completely and then charged, the individual crystal sizes on the cell plates remain unaltered. However, if the cadmium is not completely converted back into cadmium hydroxide during partial discharge, on the following charge the cadmium hydroxide crystals will clump together, forming larger crystal structures. Although it is not yet fully understood how Fig.3: the patented Reflex or “burp” charge this happens, scanning method consists of a positive charge pulse followed by a high current, short duration electron micrographs discharge pulse. This is quite different from of batteries with and other chargers which have an essentially without memory effect pulsed output but no discharge pulses. clearly show the difference in crystal sizes. The net result is that we charger circuit is able to detect a very are left with a smaller, less reactive small voltage drop (<10mV) at the very surface area and therefore reduced start of the overcharge region, the fact capacity. remains that we are already operating The clumping of crystals is mostly in the overcharge region. a slow process but is cumulative. Overcharging is not acceptable if we However, as we will see later, it is want to achieve maxi­mum battery life reversible. for nicad cells. For NiMH cells it can End point voltage be dan­gerous if used in combination with high charge currents. It can lead The usual strategy to prevent memto venting and consequent loss of ory effect is to discharge each cell to capacity and in extreme cases to cell 1.1V or 1.0V, a level where very little explosion, due to a build up of gas useful energy is left. This is only partly pressure. effective with single cells and with If NiMH cells are charged with new and well matched cells in battery delta V termination char­ gers, then this has to be done at the rate the manufacturer recommends, typically C/10 (120mA) for 1200mAh cells. At this rate, any heat produced during charging and overcharging will be safely dissipated. packs. Not all cells in a battery pack will age equally or charge and discharge equally at different operating temperatures. In the end, some cells will only be partly discharged when others are deep dis­charged. At the final stage of the battery discharge, a sudden sub­stantial voltage drop occurs. This can lead to reverse charging of the weakest cell in a battery pack of more than 12 cells and will still cause a clumping of crystals in all cells (except the weakest cell) during the next charge. The magnitude of memory effect in each cell depends on the depth of discharge. Unlike some other types of cells, nicads can be totally discharged and then even shorted to avoid the memory effect but not without reducing life expectancy. The life expectancy of all types of batteries, including nicads, is partly dependent on the depth of discharge. Hence, a total discharge will reduce life expectancy (up to a factor of 10 in cases of frequent total discharge). Totally discharging a battery to 0V – unlike discharging a single cell – is a sure recipe for extremely short battery life due to cell voltage reversal. Shallow discharge, less than 25% of total capacity, makes for long battery life but creates the conditions for memory effect. A 1.1V or 1.0V discharge voltage is only a compromise, not a magic value. Freezing cells Another strategy to combat memory effect, the practice of freezing batteries to break up the clumping of the crystals, creates mechanical stresses in the cells. This can also lead to Memory effect Let’s look at the major problem of nicad cells: memory effect. This is caused by charging a partially discharged battery and enhanced by slow charging and high operating temperatures. During charging, the negative plate loses oxygen and converts cadmi­um hydroxide to metallic cadmium, while the positive plate goes to a higher state of oxidation, changing 12  Silicon Chip Fig.4: the essential characteristic of the Reflex charging method is a high current charge pulse, followed by a short rest period and then an even higher discharge pulse for 5ms. The battery voltage is then measured before the next charge pulse. This is the view inside the prototype from Smart Fastchargers. It uses a total of three PC boards and can charge batteries at a rate of up to 9A. reduced life expectancy since a high degree of mechanical preci­sion goes into the production of today’s high capacity cells. It is also a time consuming method, since all cells in the battery have to be slowly warmed to above 10°C after freezing for efficient fast charging. All these are makeshift solutions. The problem should be tackled at the roots, by using a charge method that will reduce crystal size in batteries where crystal clumping has occurred and avoids crystal clumping during the charging of partially dis­charged batteries. Another area that needs improvement is the small number of recharge cycles suggested for most nicad batteries. In the case of some hand-held radios, the batteries are supposed to have only 300 recharge cycles. Batteries for other appliances are rated for 500 and 1000 cycles. Theoretically, 5000 charge/discharge cycles are possible over a minimum life span of 10 years. One power hand tool manu­ facturer advertises 3000 cycles and 10 minutes charging time. This is achieved by using advanced charger technology and fast charge batteries. 3000 cycles represent approximately 6.5 cents per cycle as compared to 55 cents per cycle for the hand-held radio batteries (at presently quoted prices). Another problem is the excessive time required to charge nicad and NiMH batteries with delta V termination chargers: generally between one hour for fast charge nicad batteries and 15 hours for standard nicad batteries and NiMH batteries. Only in exceptional cases, as with some chargers for battery powered tools, is it possible to achieve charge rates of less than one hour for nicad batteries. Burp charging One overseas company has designed a fast charger that achieves an amount of recharge cycles close to the theoretical limit. This patented charger, well proven in industrial and military applications, is used to charge aircraft batteries, emergency standby batteries for hospitals, etc and operates fully automatically. It automatically detects the type of battery (nicad, NiMH, lead-acid, etc), battery capacity and voltage and adjusts itself accordingly. These complex chargers use the patented Reflex or BURP charge method. This consists of a positive charge pulse followed by a high current, short duration discharge pulse. This should not be confused with pulse or switchmode chargers which switch the charge current on and off but do not apply a discharge cur­rent – see Fig.3. By using a charger circuit with the patented Reflex method incorporated in a licensed integrated circuit, it is possible to obtain a dramatic increase in the charge/discharge cycles of nicad batteries, to at least 3000 cycles if reasonable care is exercised. There is no need to run appliances until the batteries are flat to avoid the mem­ory effect. It is now possible to recharge the batteries after each use. Partial dis­­ charge, as opposed to full discharge, will significantly increase the life of the batteries. A microprocessor calculates and accurately terminates the applied charge by evaluating the inflection points on the charge voltage curve. The termination point varies according to the charging characteristic of the battery; it occurs just prior to the transition into overcharge (see Fig.2). The circuit provides a fast charge, preceded by a series of soft start charge pulses. Then, if the battery is left in January 1996  13 Fig.5: the timing for soft start, fast, topping and maintenance charges. The charge/discharge pulse combination for the topping and maintenance modes remain the same as for the fast charge cycle; only the rest time is changed. the charger, the fast charge will be followed by a topping charge and a non-destructive indefinite maintenance charge. All of the above can be done by one charger with an adjust­able output current sufficient for batteries of 7000mAh capacity at the 1C (1 hour) charge rate or for 1900mAh capacity batteries at the 4C (15 minute) charge rate, taking the charge efficiency into account. To fully charge a battery, approximately 20% more charge than has been withdrawn has to be put back into the bat­tery if charged at or above C/10 at 20°C. The charge efficiency of batteries depends on charge cur­rent and ambient temperature. High or very low ambient temper­ atures and/or low charge currents decrease the charge efficiency; in extreme cases to a point where the battery cannot be fully charged. Soft start Batteries can exhibit a high impedance during the initial stages of charging. The resulting voltage peak can be interpreted by the processor as a fully charged battery. However, with the soft start cycle, at first only short duration current pulses are applied to the battery. Starting at 200ms, the pulse width is gradually increased to approximately one second in duration. This gradual increase in pulse width takes place over a period of two minutes to avoid voltage peaks. Fast charge During the main charge cycle, each positive current pulse is followed by a discharge pulse, as shown in Fig.4. The dis­charge pulse is 2.5 times the amplitude of the charge pulse. After the main charge, if the battery is left on the charger, it will be fed a topping 14  Silicon Chip charge. This charge is at a current low enough to prevent cell heating but high enough to convert all active material in the cells to the charged state. Due to higher temperatures and gas bubbles (see explanation further on), 100% charge cannot be achieved with fast chargers. Standard constant current chargers create heat and gas bubbles on the cell plates during charging. This results in less than 90% efficiency. This version of the Reflex charger is approximately 95% efficient, since the termination method largely avoids cell heating and the charge/discharge pulse sequence removes most of the gas bubbles from the cell plates. The 2-hour C/10 charge tops up the battery if the time is available or 100% capacity is required. Maintenance charge After the full charge and topping charge, the C/40 charge compensates for the internal self-discharge of the battery, at the same time preventing dendrite formation and maintaining the crystal structure. The battery can remain on the charger until used – there is no time limit. This charge cycle can be useful in standby applications, as in security installations. Fig.5 shows the timing for soft start, fast, topping and maintenance charges. The charge/discharge pulse combination for the topping and maintenance modes remain the same as for the fast charge cycle; only the rest time is changed. The removal of gas bubbles from the cell plates during charge keeps the cell impedance low, reduces operating tempera­ture and allows higher charge currents for nicad and NiMH batter­ies. The following charge times can be achieved: fast-charge nicad batteries in less than 15 minutes at the 4C rate, standard nicad and NiMH batteries in less than one hour. As well, memory effect in batteries can be eliminated. This works even when the battery no longer holds any charge. It re­quires a minimum of three complete charge/discharge cycles. A typical case in practice involved a 4.8V 600mAh cellular phone battery pack. This had only 20% of its stated capacity, after it had been used over a period of six months with the supplied charger. After five charge/discharge cycles, it had recovered to approximately 95% of capacity. The possibility to rejuvenate shorted nicad batteries is also a feature. Whenever a nicad battery has been stored charged and has then slowly self-discharged over a very long period of time at an elevated temperature, or has been charged at a low current over a long period, as in constant current trickle charging in standby applications, crystals on the cell plates can form crystalline fingers, or dendrites, which can propagate through the plate separators and across the cell plates. In extreme cases, these crystalline dendrites can partially or completely short-circuit a cell. Such cells can be rejuvenated by this charger. Charger circuit Fig.6 shows the block diagram of a charger using the pat­ented Reflex charging method. The charger covers a battery vol­tage range from 1.2V to 13.2V at charge currents from 0.1A to 9.0A. The central part of the battery charger is basically a reduced instruction set microprocessor (RISC) to handle the complex calculations for the charge termination point. The microprocessor uses an analog-to-digital converter (ADC) with 300µV resolution to convert the battery voltage, normalised to one cell by the input attenuator VR1. The ADC is followed by a filter to limit the effects caused by battery voltage jumps and ADC noise and to eliminate Fig.6: the block diagram of a charger using a RISC microprocessor programmed with the patented Reflex charging method. The charger covers a battery voltage range from 1.2V to 13.2V at charge currents from 0.1A to 9.0A. any large aberra­tions in the battery voltage curve. The microprocessor controls the charge, topping and main­ tenance modes. One input of the microprocessor controls the charge rates (1C or 4C) and is linked to the bank of push- buttons for selection of charge current. One input resets the microprocessor to repeat a charge cycle or to charge shorted cells. In this case, the reset button has to be activated until the LED “cell voltage” display indi­cates acceptance of the charge current. A battery voltage guard circuit avoids au­tomatic charging of shorted batteries. This is necessary since the current required to kick start a shorted battery varies from case to case and should be controlled manually. Another detect circuit avoids the automatic charging of batteries with a voltage or more than 2V per cell. This condition is due to high internal impedance, as found in new batteries that have not been cycled and in some batteries which have been stored for several months. Charging these batteries would cause exces­sive heating. The DC input to the charger can range from 11.5V to 28V, depending on the number of cells in the battery to be charged. Essentially, this is a minimum of 2V per cell plus an additional 2V. Hence a 6V battery (5 cells) requires a minimum of 12VDC to the charger while a 12V battery (10 cells) requires a minimum of 22VDC. Safety cut-off In case the voltage sensing for end of charge does not work there is a timeout circuit which is set for 72 minutes at the 1C rate and 18 minutes for the 4C rate. In addition, there is a heatsink temperature sensor to interrupt the charge as a safety measure in extreme hot weather conditions. The microprocessor controls three output circuits and two LED indicators. The charge circuit is a switch­ mode current source, adjustable from 0.1A to 9A with VR2 (a bank of pushbutton switches). The discharge circuit is a pulsed constant current sink adjusted to between 0.25A and 22.5A (2.5 times the charge current). During the main charge cycle, a small piezo speaker emits a brief tone once a second, synchronised to the discharge pulses. This is a convenient audio cue to tell the user the battery is still in the main charge sequence. The tone control on the front panel actually adjusts the volume, so that the tone is not obtru­sive. Other details of the operation can be gleaned from the block diagram. By this time this issue goes on sale, the charger will have been released for sale. For information concerning availability and price, contact Smart Fastchargers, R.S.D. 540, Devonport, Tas 7310. Phone/fax (004) 921 368. *Horst Reuter is Technical Manager of Smart Fastchargers. January 1996  15