THE CHARACTERIZATION OF NICKEL-CADMIUM BATTERIES FOR TELECOMMUNICATIONS APPLICATIONS PART 2

Transcription

1 THE CHARACTERIZATION OF NICKEL-CADMIUM BATTERIES FOR TELECOMMUNICATIONS APPLICATIONS PART 2 Anthony Green Saft Advanced And Industrial Battery Group 156, Avenue De Metz Romainville France anthony.green@saft.alcatel-alsthom.fr Abstract Traditional telephone systems have generally been based on central exchanges having large power back-up systems. This arrangement has meant that the battery back-up system has been housed in its own battery room or, at least, in a temperature controlled, well ventilated location. However, in recent time, there has been a move away from this type of system to more outdoor installations. The result is a change to a more hostile battery environment with regard to temperature variations and climatic conditions. This has proved difficult for the lead acid battery and various methods are being studied to overcome the problem by using either special air conditioned battery cabinets to protect the battery, alternatives battery couples, electro-mechanical devices such as flywheels or energy storage devices such as capacitors. The nickel-cadmium battery is an viable option in terms of alternative battery couples, as its structure and electrochemistry is such that it is able to withstand abusive conditions and is generally used where either high reliability and/or the ability to operate under difficult environmental conditions are required. In this paper the work relating to a number of different nickel-cadmium structures is described. These are the sintered positive, plastic bonded negative high performance plate stack, the double plate sintered positive / plastic bonded negative medium performance plate stack and the pocket plate stack. The technologies of the different structures are briefly described. In the paper given at Budapest (The Characterization of Nickel-Cadmium Batteries for Telecommunications Applications - Part 1) results were presented from recent relevant telecom application testing for high temperature operation particularly relating to the nickel-cadmium sintered positive, plastic bonded negative technology. This work is being carried as part of a development of new products designed expressly to meet the requirement of telecom applications. This paper describes experiments which have been carried out to establish the performance of these various types of nickel cadmium plate technologies at elevated and standard temperatures to allow a cross comparison to be produced. Amongst the results given are the charge acceptance of the different nickel cadmium technologies at normal and elevated temperatures and maintenance periods at normal and elevated temporaries based on the change in floating current with temperature and with water consumption at room temperature. This is compared to testing on the effect of evaporation and the actual water consumption at elevated temperatures. In addition information is given illustrating the long lifetime of nickel cadmium batteries in sensitive applications and field trials being set up in telecommunication applications in high temperature environments are described. 1. The Nickel Cadmium Technologies The major nickel cadmium plate technologies available in the world market are the traditional pocket plate technology, which was developed at the beginning of the century, and the sintered positive technology, which was developed originally for aircraft batteries during the 1940 s. The pocket plate technology cell is shown in the Appendix. The active materials, nickel hydroxide for the positive and cadmium hydroxide for the negative, are retained in perforated nickel plated steel strips or «pockets» which are welded to a current carrying busbar. Positive and negative plates are organized alternately and separated by a molded polypropylene grid of vertical bars. This gives the correct spacing between the plates, gives the maximum open area for electron transfer and allows free movement of electrolyte within the stack.

2 This technology allows plates of various thickness to be produced and so cells can be produced to suit high, medium and low discharge rates. The sintered positive / plastic bonded negative cell is also shown in the Appendix. It has a positive plate of the sintered type which is obtained by chemical impregnation of nickel hydroxide onto a porous nickel structure. The porous nickel structure is obtained by sintering nickel powder onto a thin, perforated, nickel plated strip. The plastic bonded cadmium negative electrode is produced by a continuous process. This involves mixing the active material, binder and additives together, continuously spreading this onto a perforated nickel plated steel substrate, drying and, finally, passing the coated band through rollers for dimensioning. Positive and negative plates are organized alternately and separated by a sandwich of sintered micro-porous polymer and non-woven felt. This gives a precise spacing between plates and allows free electrolyte movement within the stack. The sintered positive is a thin high performance plate. However, the plastic bonded negative plate can be produced in a range of thickness. The sintered positive, plastic bonded negative cells are produced for performances from very high to medium rates. The electrolyte used for all nickel cadmium technologies is an aqueous solution of potassium hydroxide (KOH) and lithium hydroxide (LiOH). During the electrochemical reaction, the electrolyte is only used for ion transfer and it not chemically changed or degraded during the charge/discharge cycle. Figure 1 gives a comparison of typical energy densities available from the two technologies. Due to the construction, the pocket plate has, for the same capacity, thicker plates than the sintered positive energy density. It is also interesting to note that, for both technologies, as the capacity increases then the energy density increases. This is because this comparison is based on individual cells, whereas, if the highest energy density is required, the smaller capacities would generally use an optimised multi-cell construction. The M type pocket plate uses a higher capacity, thicker plate than the H type and so, within the same container, it is possible to put a higher capacity. Thus a higher energy density is achieved. In the case of the sintered positive, this is a high performance plate and it is not possible to produce a thicker plate. However, this is possible with the plastic bonded negative. Thus, the medium performance product is produced using a double sintered positive and a thick plastic negative. Thus in this way a higher energy density can be achieved. An important advantage with the sintered positive products when searching for increased energy density is the range of separators which can be used. Using thin aircraft type separators it is possible to increase significantly the energy density and this is shown for the sintered positive plastic bonded negative compact cell. All the examples shown in Figure 1 are for open cells i.e. they are not recombination sealed cells. Thus they have a reserve of electrolyte of at least 3 cm 3 per Ah of cell capacity. The development of a recombination sealed cell would give a further increase to these figures. High Density VRLA 'compact' 'monobloc' Ah Ah 300 Ah Pocket 'H' Ah Ah 300Ah 'sealed' Pocket 'M' 'H' 'M' 'compact' Fig Energy density - Wh / dm3 Comparison of different NiCd Options or plastic bonded negative plates. Thus the effect is that, for the same performance type, the sintered positive/plastic bonded negative cell will have a higher Energy density - Wh / dm3 Fig 2 Comparison of different technology options Figure 2 shows what can be achieved if the energy density is optimized. The VRLA battery shown is the highest energy density battery available. When compared to the existing nickel cadmium compact flooded cell they are similar at the higher capacities but the nickel cadmium battery has a lower energy density for the smaller capacities. Progressing to a more volumic efficient construction, epitomized by the use of multi-cells instead of single cells improves the situation, and, removing the electrolyte reserve by passing to a

3 recombination cell allows similar energy densities to be achieved. 2. Water Consumption In the paper given at Budapest the following graph (Fig 3) was presented for the sintered positive / plastic bonded negative product. 0 C 10 C 20 C 30 C 40 C 50 C 1.41 V/cell 1.37 V/cell Topping-up interval in years per cm3 of reserve per Ah current by a bout 15%. Thus, to maintain an equal floating current at 40 C to that at 20 C by modifying the voltage would require very large voltage modifications. From the current passing into a battery on floating it is possible to calculate the theoretical water loss. The quantity of water used can be found by the Faradic equation which states that each ampere hour of overcharge breaks down cm 3 of water. In Table 1 the theoretical values of water loss are calculated from the floating current measured and compared to the actual water loss found and the water loss calculated from the specified topping up intervals given in Fig 3. Table 1 - Summary of Water Loss Volts per cell Temperature 20 C 40 C 40 C Current (ma) Fig 3 Water consumption at different temperatures Calculated water loss (g) These are calculated values based on the water consumption at room temperature and the floating current at different temperatures. In order to verify this data a floating test was performed over a 3 month period. Ah cells were used and these were floated at 1.41 volts per cell at 20 C and 1.37 and 1.41 volts per cell at 40 C. The floating current and voltage for the battery was measured during the test and the water consumption over the three month period was also measured Floating current (ma) Fig 4 Number of days 1.41 vpc at 40 C 1.37 vpc at 40 C 1.41 vpc at 20 C Floating current over a 3 month period. Five cells were used for each test and figure 4 gives the evolution of the average current in the battery of 5 cells over the 92 days of the test. As would be expected the current in the battery increased with temperature and with floating voltage. The effect of temperature was very marked. At a floating voltage of 1.41 volts per cell the average current in the battery at 40 C was more than double the value found at room temperature (20 C - 25 C). Reducing the volts per cell at 40 C from 1.41 vpc to 1.37 vpc had an effect of only reducing the Actual water loss (g) Ratio calculated/actual % «recombination» 83% 73% 64% Water loss - Fig 3 (g) Looking at the actual water loss and the calculated water loss at 20 C it can be seen that the actual water loss is only one sixth of the calculated value. This is because some of the floating current is used to compensate for the natural self discharge of cells. However, this ratio drops significantly when the temperature increases due to increase in evaporation at the higher temperatures Fig 5 mm of Hg Vapor pressure of water below C. The increase in evaporation can be best illustrated by looking at the change in vapor pressure of water related to temperature (Fig 5).

4 The evaporation of water is related to the vapor pressure and this is directly related to the temperature and the air pressure. Referring to Fig 5, it can be seen that a temperature change from 20 C to 40 C results in an increase in the vapor pressure from 17.5 to 55.3 mm of Hg i.e. the vapor pressure increases by a factor of more than 3. Thus the level of evaporation will increase by a similar amount. This should be considered when looking at the actual water loss at 40 C. As would be predicted, the current at 1.37 volts per cell is lower than that at 1.41 volts per cell by about 15% and so it would be expected that the water loss would be lower by a similar amount. However, the actual water loss appears to be a few percent higher, although within the limits of such a measurement we can say that there is no difference between the two values. If the ratio of the calculated value and the actual water loss is examined, it can be seen that it is significantly lower at the higher temperatures than at normal room temperature. This is because evaporation has a significant affect on the water loss at these high temperatures. Thus, regardless of the charge control which is used there will always be a significant water loss at higher temperatures. This shows clearly the need of flooded cells, which can be refilled with water, in these environments. It should not be a surprise that starved electrolyte cells have significant difficulties in this environment. The gas recombination level of a cell is often quoted, and is even used in standards. In Table 1 such values, calculated from the actual water loss and the current flowing, are given. Clearly there are at least two other factors involved, the evaporation, which effects the water loss, and self discharge, which effects the amount of current which can be considered to be used for electrolysis. Thus, in our view, the use of gas recombination values as a measure of the quantity of water being used and gas given off has no practical basis. If the water consumption actually found is compared to the published table (Fig 3), there is a good safety margin at 20 C. However, at 40 C there is no safety margin as the figures were based on current and do not take into account the effect of evaporation which is dependant on external conditions. However, these values are only intended as a guide and, as these are flooded cells, in practice the water level will be monitored and the cells refilled as necessary. 3. Recharge at High Temperatures The capacity charged can be expressed in more than one way. It can be expressed as the percentage of the rated capacity of the cell which is available after a certain time or, it can be expressed as the percentage of the actual capacity of the cell. In general, it is the former which is used in publications as this is the «practical» value which will be seen by the user. However, in practice, the actual cell capacity is 10% to 20% higher than the rated capacity and so, for a technical comparison it is more correct to use the actual capacity of the cell. Thus for these comparisons the actual capacities of the cells tested are used and these will give lower values than those generally published, which will be at least 10% higher. Recharge tests were carried out for the high performance and medium performance sintered/plastic bonded products at room temperature (20 C-25 C), 40 C and 50 C. The tests were carried out at the standard recommended charge voltages for these products of 1.41 volts per cell and 1.45 volts per cell. Fig 6 gives the 12 hours from a fully charged state for the H type simple plate cell. At room temperature the 1.45 volts per cell charged the cell to 93% of its actual capacity which, in terms of its published nominal capacity would make it fully charged. At 1.41 volts per cell, which is the recommended voltage for a single level charging system, the cells were charged to 89% of their actual capacity volts per cell 1.41 volts per cell hours charge from a fully discharged state Fig 6 H type sinter/plastic H type sinter/plastic - capacity recharged. As the temperature was increased then the level of charge achieved in the 12 hours was reduced. At 40 C the capacity achieved in this time was reduced by about 10% and, increasing the temperature by another 10 C had the effect of reducing the level of charge achieved in this time by a further 10%. Thus increasing the temperature reduces the charging efficiency and it will require a longer time to achieve the same level of charge. In Fig 7, a similar curve to Fig 6 gives the capacity restored after 24 hours from a fully charged state for the M type double positive sinter / plastic bonded type cell. A time period of 24 hours is chosen as the charge acceptance of a M type cell would be expected to be less than an H type cell. In this case, over the 24 hour charge period at room temperature the 1.45 volts per cell charged the cell to % of its actual capacity which, again, in terms of its

5 published nominal capacity would effectively be fully charged. At 1.41 volts per cell, which is the recommended voltage for a single level charging system, the cells were charged to 85% of their actual capacity. repeated in all the tests and so can be considered to be a valid experimental result. It is probably due to combination of the higher current at the higher temperature and the relatively low levels of charge achieved volts per cell 1.41 volts per cell M type sinter/plastic The H type plate begins to have a good level of recharge (89% in 15 hours) beyond 1.45 volts per cell. It falls short of 12 hours at 1.41 volts per cell in which the sintered positive / plastic bonded negative high performance cell can achieve the same capacity. In general, the pocket plate cell requires a higher charge voltage than an equivalent sinter / plastic bonded cell hours charge from a fully discharged state Fig 7 M type sinter/plastic - capacity recharged. In a similar way to the H type cell, as the temperature was increased then the level of charge achieved was reduced. At 40 C the capacity achieved in 24 hours was reduced and, increasing the temperature by another 10 C had the effect of further reducing the level of charge. However, the overall change in charge level seeemed to be less affected by temperature than the H type cell C 40 C H type 15 hours charge from pocket plate a fully discharged state Fig 8 Charge voltage per cell H type pocket plate - capacity recharged. A similar range of tests were conducted for low, medium and high performance pocket plate cells. The cells were around Ah and two temperatures, room temperature (20 C-25 C) and 40 C were evaluated. In this case a range of charge voltages from 1.40 volts per cell to 1.65 volts per cell were tested. As the medium rate cells showed a behavior in between that of the low and high cells it is not described here. Fig 8 shows the results for the H type plate after 15 hours of charge. It can be seen that, at the lowest voltage, there appears to be a better recharge at 40 C than at 20 C. This was As the charge voltage is increased then the capacity restored is increased and, at 1.65 volts per cell virtually all the capacity of the cell can be restored in 15 hours. The effect of increasing the temperature from 20 C to 40 C at charge voltages above 1.45 volts per cell is to reduce the capacity charged by between 8% and 15%. The situation with regard to the low discharge rate type cell is given in Figure 9. In this case, the same data is presented as for the high performance plate type cell, i.e. 15 hours charge C 40 C L type 15 hours charge from pocket plate 65 a fully discharged state Fig 9 Charge voltage per cell L type pocket plate - capacity recharged The same phenomena is found with the 1.40 volts per cell charge voltage. The higher temperature gives a higher result. Above 1.50 volts per cell at least % of the cell capacity is restored after 15 hours of charge. The effect of temperature is less marked than with the high performance plate, mirroring the behavior seen with the sintered / plastic bonded product. Thus, the effect of temperature for both types of plate technology, is to reduce the capacity available over a specified charge period. However, the level of charge achieved is still at a high level and an extended recharge time will allow a higher capacity to be achieved. 4. Applications and Test Batteries

6 Nickel cadmium batteries are known for their long life and resistance to abusive conditions. This is why they are now being considered for telecommunication outdoor cabinets. They are used, for example, for offshore navigation aids. In a paper presented by Northern Lighthouse Board at the International Association of Lighthouse Authorities meeting in April of this year, systems using nickel cadmium batteries which had been in service for nearly 20 years were described and it was expected that a «maximum life in excess of 20 years would be achieved from the batteries». At this conference, a paper presented by Saft Nife BV (Netherlands), shows the results of a long term use of nickel cadmium batteries in an outdoor application. In terms of long term reliability of nickel cadmium batteries it is possible to give many examples. Here in Australia, the Sydney Opera House were so satisfied by the 15 years service obtained from their original nickel cadmium batteries that they replaced them in 1992 with the same technology. The Norwegian Lighthouse Service, after 6 years of testing without any failures, moved exclusively to nickel cadmium for their photovoltaic systems for lights and lighthouses. Rome Airport replaced the VRLA batteries in their UPS systems directly with nickel cadmium batteries to meet their reliability needs. The Eurostar and Le Shuttle trains passing through the Channel Tunnel use exclusively nickel cadmium batteries in steel containers. These have been in service since 1994 and are based on French Railway s 20 years experience with the product. To demonstrate the advantages of the nickel cadmium technology to potential telecomms users we have embarked on a series of user trials. At this moment in time we have test batteries in the USA and the UK. In the USA we have installed a 48 volt Ah battery, in conjunction with a telephone company, in an outdoor cabinet. There is no change to the system other than the change of battery technology. The standard VRLA battery charger is used and the nickel cadmium battery is accommodated within the same footprint as the existing battery. This installation is situated in the Southern States where the temperature outside can reach 40 C in the summer and there is a high humidity. In this application the existing VRLA batteries do not survive one summer. The battery is connected to a data logger which is interrogated through the telephone system and temperatures, voltages and currents are monitored. This is planned to be a 1 to 2 year trial. charger is used. The battery is installed in a pit next to the curbside cabinet and will be given a discharge cycle every month. The trial is planned to be for 1 year. 5. Conclusions Part 1 of this paper, presented at Telescon in Budapest, Hungary, in April 1997, compared the behavior of different nickel cadmium technologies at room temperature. In this paper the work has been extended to cover high temperature operation. Over a three month test period, floating at 40 C, it has been shown that, at the same floating voltage, the water consumption increased fourfold. Reducing the floating current by decreasing the floating voltage does not have a significant affect on this value. This indicates that, for high temperature operation, a flooded cell which will allow water replenishment will give the highest reliability to the system. The testing has shown that at high temperatures the charge efficiency of the nickel cadmium cells is reduced, but an acceptable level of charge is still achieved. The figures confirm that the sintered positive / plastic bonded negative technology can be charged at a lower voltage than the pocket plate technology. The ability of the sintered positive / plastic bonded negative technology to operate in a smaller voltage window than the pocket plate technology, its higher energy density when compared to that technology and its long life, makes it a potential obvious choice for telecommunication outdoor applications. Trials have now begun using the sintered positive / plastic bonded negative technology in outdoor telecommunication systems in the USA and the UK and more are planned. These will be reported at a later stage. In the USA we also have plans to install a number of other test batteries with other operators to prove the technology. In the UK, a test battery has been installed with a cable company. As in the USA, the standard VRLA battery