IC 8938 Bureau of Mines Information Circular/1983 New Developments in Personal Lighting Systems for Miners By William H. Lewis and Elio Ferreira UNITED STATES DEPARTMENT OF THE INTERIOR Information Circular 8938 New Developments in Personal Lighting Systems for Miners By William) H. Lewis and Elio Ferreira UNITED STATES DEPARTMENT OF THE INTERIOR James G. Watt, Secretary BUREAU OF MINES Robert C. Horton, Director This publication has been cataloged as follows: Lewis, W. H. (William H.) New developments in personal lighting systems for miners. (Information circular / United States Department of the Interior, Bu- reau of Mines ; 8938) Supt. of Docs, no.: I 28.27:8938. 1. Safety-lamp. 2. Nickel-cadmium batteries. 3. Mine lighting- Equipment and supplies. I. Ferreira, Elio. II. United States. Bureau of Mines. III. Title. IV. Series: Information circular (United States. Bureau of Mines) ; 8938. IE205tIM^' [TN307] 622s [622'. 473] 83-600085 c ^! ^ CONTENTS Page Abstract 1 :<^ Introduction 2 ^Electrochemical design , 3 ,,^ Roll-bonded electrode process 4 >v Positive electrode fabrication 4 ^ Negative electrode fabrication 4 -^ Electrolyte 4 Cell design 5 Separator 5 Battery construction 6 Cycle testing 9 Conclusions , 10 ILLUSTRATIONS 1. Conventional lead-acid battery and prototype nickel-cadmium battery 2 2. Roll-bonded nickel-cadmium cell with separators 6 3 . Complete cell assembly 6 4. Molded battery case 7 5. Fill port-level indicator 7 6. Fill port-level indicator installed in case 7 7. Terminal cover attached to battery case 8 8. Outer battery cover 8 9. Lamp cord wiring arrangement and intercell connections 9 10. Typical discharge curve of nickel-cadmium caplamp battery 10 11. Battery-end cutoff voltage versus time 10 TABLE 1 . Comparative battery specifications 3 do ^ '^ ^ UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT A ampere min minute A»h ampere hour ym micrometer g gram pet percent h hour s second in inch V volt in3 cubic inch W'h watt hour in'lb inch pound yr year lb pound NEW DEVELOPMENTS IN PERSONAL LIGHTING SYSTEMS FOR MINERS By William H. Lewis ^ and Elio Ferreira^ ABSTRACT Energy Research Corp., under contract to the Bureau of Mines, has developed a new miners' caplamp battery. The new battery is based on nickel-cadmium technology and offers significant improvements in per- formance with reduced size and weight when compared to the conventional lead-acid battery presently in use. This report describes the design and fabrication of the nickel-cadmium battery, which utilizes a roll- bonded electrode structure. The final battery design has a 15-A*h capacity in a 2-1/2-lb package, which is over 2 lb lighter than the present lead-acid caplamp battery. 'Electrical engineer, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. ^Project engineer. Energy Research Corp., Danbury, CT. INTRODUCTION The present-day miner's caplamp and battery system has been a highly reliable and indispensable aid to miners for many years and has undergone few changes since its general acceptance in the 1930' s. Recent developments in ogy, however, have made ments possible. battery technol- several improve- Over the past 3 yr, Energy Research Corp., under contract to the Bureau of Mines , has been developing a new caplamp and battery system. The new battery is based on nickel-cadmium technology and features significant improvements in per- formance when compared to the conven- tional lead-acid battery presently in use (fig. 1). Among the more significant improvements are a 48-pct reduction in battery weight and a 15-pct reduction in battery volume. The over-2-lb weight reduction should be an attractive feature to miners , who must carry the battery while performing their work. Battery capacity has been in- creased from 12 to 15 A*h and will pro- vide a greater margin of safety and cap- lamp burning time that may be required in emergency situations. Charging cycle life has been also increased, and al- though testing has not been completed, it is expected to be in the 1,000-cycle range. This will extend the usable life of the battery to approximately 2-1/2 times that of the present lead-acid bat- tery, providing 3 to 4 yr of normal use. The extended battery life should more than offset the higher cost of the nickel-cadmium system. Table 1 compares the specifications of the old and new batteries. FIGURE 1. - Conventional lead-acid battery (left) and prototype nickeUcadmium battery (right). TABLE 1. - Comparative battery specifications Specification Nickel-cadmium (prototype) Lead-acid (typical) Weight lb. . Voltage (average) V. . Capacity A*h. . Ene rgy W • h . . Energy density W*h/lb.. Size in. . Volume in^ . . Energy density W*h/in^ . . Charging cycle life cycles.. Cost 1.64 X 4.90 X 4.66 3.7 12 44 9.5 6.65 53.4 0.82 <400 $30 ^Cost is a function of production volume. The projected is based on a production volume of 20,000 units per year. ELECTROCHEMICAL DESIGN cost figure Nickel-cadmium batteries were first developed in Europe and have been in use for more than 60 yr. The basic nickel- cadmium cell is a rechargeable system which consists of a combination of active materials that can be electrolytically oxidized and reduced repeatedly. The overall chemical reaction of the system can be considered as follows: Cd + 2NiOOH + oHoO (CHARGED) 2"2^ ^=^ Cd(0H)2 + 2Ni(0H)2. KOH (DISCHARGED) In a charged condition, the system con- sists of a positive electrode (nickelic hydroxide) and a negative electrode (me- tallic cadmium). Potassium hydroxide is used as an electrolyte. In an uncharged condition, the positive electrode be- comes reduced to nickelous hydroxide and the negative electrode oxidized to cad- mium hydroxide. The oxidation of the negative electrode occurring simulta- neously with the reduction of the posi- tive electrode generates electric power. In a rechargeable battery, such as the nickel-cadmium system, both electrode re- actions are reversible and by supplying an electric current from an external source, the reactions can be driven backwards and in effect recharge the electrodes. Over the years many techniques and pro- cesses have been developed for economi- cal manufacture of the nickel-cadmium electrode structures, and basically two types have evolved: sintered plate elec- trodes, and nonsintered electrodes. The following discussion will focus on the development of the roll-bonded (non- sintered type) nickel-cadmium electrodes that are used in the design of the new caplamp battery. The term "roll-bonded electrode" refers to electrodes fabri- cated by forming a conductive mix of Ni(0H)2 (nickel hydroxide) or CdO (cad- mium oxide) and Teflon^ into a bonded structure by rolling in a calendaring mill. The steps comprising the manufac- ture of roll-bonded electrodes are all semicontinuous or are automated to a greater degree than those for sintered electrodes. Moreover, the nonsintered, or roll-bonded, process offers the fol- lowing advantages: • Lower nickel requirements • Lower material cost ^Reference to specific products does not imply endorsement by the Bureau of Mines. • Lower labor cost • Lower capital equipment cost • Lighter weight • Lower pollution stream • Lower process energy requirements ROLL-BONDED ELECTRODE PROCESS The processing steps in the manufacture of the nickel-cadmium electrodes consist of similar operations. The discussion begins with the process for fabricating nickel electrodes. Positive Electrode Fabrication Nickel hydroxide is prepared by precip- itation from NiSO^ (nickel sulfate) solu- tion containing a small percentage of CoSO^ (cobalt sulfate); Co(0H)2 (cobalt hydroxide) is coprecipitated with Ni(0H)2 (nickel hydroxide). The Co(0H)2 is com- bined with the active material to give good charge efficiency and capacity re- tention to the electrode. The precipi- tate is washed to remove K2S04 (potassium sulfate) and KOH (potassium hydroxide) and dried. It has been found that the particle size of the Ni(0H)2 powder is of primary importance in determining the utilization and voltage characteristics of the finished electrodes. In the next step, Ni(0H)2/Co(0H)2 is blended with graphite, Teflon, and an or- ganic lubricant. The mixture of nickel hydroxide, graphite, and Teflon is kneaded to fibrillate the Teflon, which acts as a binder or matrix for containing the active materials. Specially selected graphites are used as the conductive dil- uent to impart electrical conductivity to the Ni(0H)2, which is otherwise an insu- lator. The organic lubricant is added to act as a rolling and extrusion aid to help "work" the mixture to the proper consistency. At this point, the doughlike material is sent through a rolling mill which forms it into sheets and calendars it to the proper thickness. The electrode sheets are then dried to remove the solvent and cut to size with a shear. The electrode strip at this point is self-supporting, flexible, and mechani- cally rugged, and can be easily handled for subsequent manufacturing. The nature of the roll-bonded process allows manu- facture of electrodes with variations in thickness from 0.005 in to more than 0.25 in. This permits a greater latitude in cell design than is attainable with the sintered electiode process. The final steps in the process are the fabrication of the current collector and its lamination to the positive active material. The current collector consists of a nickel foil, which is cut to size and perforated. Negative Electrode Fabrication The roll-bonded cadmium electrode con- sists of a mixture of CdO (cadmium ox- ide) , carbonyl nickel powder, and Tef- lon, which is processed as previously described and laminated to a perforated nickel foil current collector. To obtain maximum utilization of active material, the particle size of cadmium oxide must be between 2 and 6 urn. For- tunately, commercially available CdO pow- der satisfies this requirement. Carbonyl nickel powder is added to improve the wettability and conductivity. This provides good capacity, especially during early formation cycling, and prevents re- crystallization of the active material. ELECTROLYTE The electrolyte used is a solution con- taining 35 pet KOH (potassium hydroxide) , 1 pet LiOH (lithium hydroxide) , and other additives. The use of 35 pet KOH is based on the optimum low-temperature characteristics of the electrolyte con- centration; also its conductivity is optimum at this percentage. Lithium hydroxide is added to improve cycle capacity stability. CELL DESIGN The Code of Federal Regulations, Min- erals Resources, CFR 30, Part 19.9 (a) specifies that "Permissible electric cap- lamps shall burn for at least 10 con- secutive hours on one charge of the bat- tery and shall give during that period a mean candlepower of light beam of not less than 1." Based on the above specifications and targeted physical dimensions, a cell de- sign was developed with a nominal capac- ity of 15 A*h and an output voltage of 3.6 V. The calculations used in designing the cell are based on previous experimental tests on similar cells. Faradaic cal- culations show that it takes 3.65 g of nickel hydrate [Ni(0H)2 • 1/3 H2O] to produce 1 A'h of capacity. Experimentally, it has been found that, using a 10-pct overcharge, the conserva- tive utilization of nickel hydrate is 80 pet and that of CdO is 60 pet of theo- retical. Slight variations in these fig- ures can be obtained as the number of electrodes, amount of graphite, and dis- charge current density are varied. Based on the amount of active material (nickel hydroxide) on the positive electrode of 11.3 g, the nominal capacity of the cell can be calculated as follows; Nominal capacity = The basic cell consists of six positive electrodes, five full-thickness negative plates, and two half-thickness negative plates. The positive mix formula has been optimized for the current drain of the bulb. In this design the low dis- charge current density allows the graph- ite content to be reduced, thereby allow- ing more hydrate to be used. The optimum particle size of the graphite was deter- mined experimentally and is a compromise among several factors. First, if the particle size is too large, the graphite will not blend homogeneously with the nickel hydrate and Teflon and will yield a poor physical mix that will affect the performance of the electrode. Second, if the particle size is too fine, the bulk density decreases, making it difficult to obtain the proper capacity density (A-h/in3). The quantity of graphite is based on several factors: Owing to the low cur- rent density of 1.3 A, this being the rate of discharge with a 3.6-V caplamp bulb, it is not necessary to increase the 11.3 g X 6 plates/cell x 0.80 utilization 3.65 g/A-h = 15 A-h. amount of graphite, where very high con- ductivity would otherwise be required. It has been shown experimentally that when the graphite content is below the design value, conductivity is much poor- er, impairing the performance of the electrode and reducing its capacity. SEPARATOR The separator system is composed of one 5-mil Pellon (nonwoven polyamide) bag heat-sealed around the positive elec- trode, followed by a layer of U-wrapped Celgard K 306 microporous polypropylene film. The separators are important in vented nickel-cadmium cells in preventing cadmium shorts from penetrating to the positive during extended cycling. The Celgard K 306 film has shown good long- terra oxidation resistance during acceler- ated chemical testing and can be commer- cially produced in quantities for this type of battery. Figure 2 shows the as- sembled nickel-cadmium cell with sepa- rators in place. FIGURE 2. = RolUbonded nickel-cadmium cell with separators. FIGURE 3. = Complete cell assembly. BATTERY CONSTRUCTION Based on capacity requirements (15 A*h) , the final battery size was estab- lished at 1.77 In deep by 3.92 In wide by 6.65 In high. The battery consists of three cells connected in series. Each cell Is composed of six positive elec- trodes, five full-thickness negative plates, and two half -thickness negative plates assembled to a terminal cover as shown in figure 3. Each cell is termi- nated with two threaded studs, attached to the terminal cover with nuts , and torqued to 15 in -lb. 0-rlng seals are used between the terminals and cover to prevent leakage of the electrolyte. The battery case is molded of Polysul- fone and is compartmented for each cell assembly (fig. 4). Polysulfone was chos- en as the molding material because of its high impact strength and good chemical resistance. Chemically, it is stable in KOH solutions and highly resistant to aqueous mineral acids and salt solutions; its resistance to detergents and hydro- carbon oil is good even at elevated temperatures and under moderate levels of stress. Another important property of Polysulfone is that it can be easily solvent-bonded to itself by using methyl- ene chloride. FIGURE 4. - Molded battery case. FIGURE 5. - Fill port-level indicator. Based on the MSHA (Mine Safety and Health Administration) drop test require- ments and application of the battery, the case and cover were designed with rein- forced corners and a wall thickness of at least 1/8 in. The battery case is de- signed with three viewports which are fitted with Polysulfone fill ports-level indicators. These devices facilitate maintenance of the battery by permitting the user to visually check and adjust the electrolyte level (fig. 5). ' 1 ^ ^ ft The fill port-level indicator is de- signed with a baffle arrangement to pre- vent electrolyte spillage, even though the battery is positioned upside down or sideways. Sufficient headroom is also provided in the cell to allow operation in any position without spilling the electrolyte (fig. 6). The primary seal between the battery case and terminal cover is attained by solvent-bonding with methylene chloride, followed by a secondary epoxy seal which is poured around the well provided at the top edge of the terminal cover (fig. 7). The battery is provided with an outer cover constructed of 0.025-in-thick stainless steel. The cover is attached to the battery with two L-shaped brackets and two No. 6-32 screws (fig. 8). Battery charging is accomplished through the lamp cord, which fits into the outer cover by means of a rubber boot j 1 L J, FIGURE 6. - Fill port-level indicator in- stalled in case. FIGURE 7. - Terminal cover attached to battery case. FIGURE 8. - Outer battery cover. FIGURE 9. - Lamp cord wiring arrangement and intercell connections. and bushing. The connections between the cord and terminal studs are made through lugs and secured with nuts. Six termi- nals are connected in series through an intercell connector and a thermoswitch (fig. 9). The thermoswitch serves not only as an intercell connector but also as a safety switch. Several types of thermoswitches were tested for their trip and reset temperature and time. The thermoswitch selected for this design has a trip tem- perature of approximately 120° C at a trip current of 8.1 A. Its reset temper- ature is approximately 78° C. Trip and reset times are 2 and 3 s, respectively. CYCLE TESTING Several cell des factured utilizing ments developed dur of these improveme least 360 cycles, discharge period a charge period of a typical plot as a function of conditions. igns have been manu- the design improve- ing this study. Some nts have produced at each cycle being a of 10 h followed by 14 h. Figure 10 shows of battery voltage time during discharge When studing the cycle capacity of the battery, two major factors were con- sidered: (1) the number of times that the battery can be cycled (discharged and charged) and (2) the variation of battery-end cutoff voltage (voltage of the 10th h of discharge) with cycling. The battery-end cutoff voltage is impor- tant, because it determines the amount of light output from the caplamp. Federal 10 4.00 FIGURE 10. - Typical discharge curve of nickel-cadmium capiomp battery. >3.6 100 150 200 250 NUMBER OF CYCLES, lO-h discharge FIGURE 11. - Battery-end cutoff voltage versus time. 300 350 regulations require the battery to have sufficient capacity to maintain a beam candle power of not less than 1 during 10 h of continuous use. It has been de- termined experimentally that the voltage to the caplamp must be at least 3 V to meet these requirements. Since the battery-end cutoff voltage gradually de- creases as the battery is cycled, eventu- ally the voltage will fall below the re- quired limits. Figure 11 is a plot of the battery-end cutoff voltage as a func- tion of the number of cycles. It can be seen from the plot that the voltage after 360 cycles has fallen to approximately 3.4 V, which is more than adequate to meet the candlepower requirements. Based on previous experience with similar types of batteries and data on the present de- sign, it is expected that the life will be in the 1,000-cycle range. CONCLUSIONS The nickel-cadmium battery developed during this study offers great promise to the mining industry as a long-life, lightweight battery for caplamp appli- cations. The reduction of over 2 lb of weight when compared to the present lead-acid battery and increased perform- ance should be attractive features to mining personnel. The projected 250-pct increase in service life should more than offset the battery's higher initial cost. INT.-BU.OF MINES, PGH., PA. 26925 1 1 CTP 1 K^Mh^'-ti H32 84 < <« ^&^Sj^^4;^&it^^^%v^yfr^^^j.'tWJ;'^--w..:::.^^i^^ .. 'bv^ ^""^t. ^ %.** .•:^'-- \-/ --'Mk'. %/ A •07 ^>>•. o <^' VT^''/ \/^-\/ %'*^*/ ^-^ \/ V^*/ \/^^\/ %^^-/ ^,;^^\/ -- ■'^- '•-^5 DEC 83 ftl«VOC5TI% IHCMm 46962