Experiences with NICAD Cells from Pacific West Supply George Patterson NICAD cells were picked up from Pacific West Supply in Amity, OR on Saturday, Dec. 8, 1989. They consisted of twenty-three ED-80's, nine ED-160's, four HIP-8's and one XWR7 dating back to 1933. The XWR7 is a pocket plate NICAD cell with a rated capacity of 35 amp-hour at the 8 hour rate. The Jungner Nickel-Cadmium Pocket-Type (NIFE) HIP-8's are high rate cells with a capacity of 80 amp-hr. and a cell weight of 6.4 kg. The Edison ED-160 cells are rated at 160 amp-hr. for the 5 hr. rate. The ED-80's at 80 amp-hour for the 5 hr. rate. Edison ED series cells are medium rate cells. The design of the ED-80 and ED-160 cells are the same except that the ED-80's are 12 1/4" tall while the ED-160's are 18 1/4" high. Two ED-80's are otherwise equivalent to one ED-160 cell electrically. Testing Testing this myriad of cells started with adjusting the electrolyte of each cell to a specific gravity of 1.190 gr./ml.. This was accomplished using a high quality hydrometer. Use only a brand new hydrometer that has NEVER been used to test lead-acid cells. All cells that were not yet reconditioned were filled with distilled water to the maximum level mark on the cell's case. The specific gravity was then adjusted either by adding distilled water or more highly concentrated electrolyte. The concentrated electrolyte is a solution of KOH in water with a specific gravity of between 1.19 and 1.22 gr./ml.. The cell was then charged and gassed for about 15 minutes in order to completely mix the solution. After another 10 minutes of charge, the specific gravity was measured with a hydrometer. It took several such episodes to achieve the desired value of 1.190 gr./ml., approximately 40 minutes per cell on average. Excess electrolyte was then removed from each cell to bring the level to the maximum mark. Titration for Potassium Carbonate Potassium carbonate concentrations in the cells' electrolyte were measured by titration and recorded. All cells were then charged prior to testing their ampere-hour capacity using a computer controlled system that produced the discharge curve of each cell with capacities to 1.1, 1.0, and 0.9 volts. Most of the Edison cells obtained were of the "Low Temperature" variety. They all had specific gravities for the electrolyte of approximately 1.220 gr./ml. after being filled with distilled water to the maximum level. Although the higher specific gravity has a low freezing point, <-36 degrees Centigrade, the higher density has a somewhat detrimental influence on the cycle life of the cells. The positive electrodes tend to lose capacity on cycling more rapidly than when the usual electrolyte concentration is employed. As cycle life is my most important consideration, the value of 1.190 gr./ml. was chosen. Foaming & Battery Oil During charging, three of the ED-160 cells foamed up and out of the vent caps. Upon inspection of all of the ED-160 cells, battery oil Chevron "Utility Oil 22" was added to bring the oil level on top of the electrolyte to 1/8"-3/16". This immediately reduced the foaming and the charging proceeded at the C/10 rate until >140% of rated capacity was reached. The cells were then allowed to rest for at least 24 hours, then discharged during the capacity test. Battery Cell Testing System The cell testing system consists of a computer with printer and digital voltmeter controlled over a IEEE-488 instrument control bus. The computer is a standard IBM-PC (IBM and PC AT are registered trademarks and PC XT is a trademark of IBM corp.) clone with software written in the Turbo Pascal (Turbo Pascal is a trademark of Borland International, Inc.) language. The software controlling the digital voltmeter functions over the IEEE-488 bus. All data is repeated every 30 seconds with the computer performing the necessary calculations and data storage. After all of the data is collected, a graph of the discharge curve is plotted on the color display and the printer provides a hardcopy of the discharge curve. Figure 1 shows a schematic of this computerized cell testing system. Insert Figure 1. Discharging Each cell under test is discharged through a resistor. This provides a constant current drain for the cell under test. Resistors and several feet of #14 copper wire were used since they were easy to fabricate. Almost any desired value can be made without special tools. To load the Edison cells at a C/5 discharge rate, reference the cell product literature. It was determined that for the ED-160 cells a current of 32 amps was desired and for the ED-80 cells a discharge current of 16 amps is correct. Copper wire has a temperature coefficient of +3900 ppm. For this reason we characterized the resistance of the wire over the range of cell voltages from 1.5v. to 0.8v. and programmed the computer with the resistor's characteristics. This allowed for reasonable measurement accuracy to be maintained. We characterized the local resistance while in use by measuring the voltage across the cell under test and the current through the load resistor with a DC clamp-on current meter. From ohms law E=IR, we calculated the resistance of the load while in use. Power dissipated in the resistor is proportional to the temperature (T1.5) of wire. On initially connecting the resistor, it heats up within a few minutes to its maximum value and then slowly cools until the end of the test at 0.9v. In practice the resistor value changes only by 1.7% during the test. The resistor for measurement of the ED-80 cells has a nominal value of 0.080½ initially with a final resistance of 0.0789½. The initial current was 15.6 amps and the end of the test current 12.7 amps. Monitoring Since the current was not constant during the test we needed to monitor it every 30 seconds. By integration the total amp-hour capacity of the cell was determined. The program, written in Turbo Pascal, requires the operator name, cell type and load resistance for input. The computer provides the time and date. After everything is under way, the computer commands the digital voltmeter to take a voltage reading every 30 seconds, 120 readings per hour. The voltage reading is divided by load resistance to calculate current. The current divided by 120 is used to convert it to amp-hours (coulombs). During the test the computer adds all of the amp-hour values until three voltages are found. The voltages are 1.1v., 1.0v., and 0.9v. The total amp-hour capacity is specified to a discharge of 1.0v. at the 5 hour rate. We are most interested in this value. The output from the computer produces a graph of voltage versus time (discharge characteristic) and a table of amp-hour capacities to the above three voltages. The test is terminated at 0.9v. Figure 2 shows shows a typical discharge curve obtained from testing an ED-80 cell, serial #088569192. This four-year old cell's tested capacity is 99 ampere-hours, while its rated capacity is 80 ampere-hours. INSERT FIGURE 2 Date Codes The Edison ED series batteries manufactured by SAB NIFE, Greenville, NC have date codes stamped into the top of the cells. Each cell has an individual date code/serial number consisting of nine numerals. If the cell has been reconditioned by Pacific West Supply, two additional characters follow the nine numerals. In the nine digit serial number date code, such as 088569159, the first two numerals represent the month and the next two numerals represent the year of manufacture. In this example, we see that the first two digits are 08, representing the month of August. The second two digits of this example date code are 85, representing the year 1985. The remainder of the date code is the cell serial number for the date of manufacture. If the two characters "RC" follow the nine digits, the cell has been reconditioned by Pacific West Supply. Capacity Test Results for ED-80 Cells Thirteen ED-80 cells were charged and then discharged through the capacity test system. The results of this test are shown in Figure 3. Average capacity of the cells (in use since 1985) was 98 amp-hrs. at a C/5 (16 ampere) discharge rate to a cell voltage of 1.0v. INSERT FIGURE 3 Testing a 57 Year Old Nicad Cell A Gould XWR7 nicad cell was tested. This cell was made in 1933 and was selected at random from a pile of cells at Pacific West Supply. Its rated capacity is 35 ampere-hours. The cell's exterior was physically cleaned (it was filthy) and the cell's electrolyte replaced. The cell was then charged and discharge tested for six complete cycles. The results of the testing are shown in Figure 4. Note the increase in capacity as the cell was cycled. This increase in capacity after a few cycles was demonstrated by many of the cells tested. By the third cycle, this 57 year old cell was testing at greater than its original rated capacity. INSERT FIGURE 4 Nickel Cadmium Cell Reconditioning The process for understanding the reconditioning of NICAD cells involves knowledge about the condition of the cell. Measure the K2CO3 (potassium carbonate) concentration by titration. If the potassium carbonate concentration is measured to be greater than 15%, it is time to recondition the cell. If it is impossible to determine K2CO3 concentration, assume a five year reconditioning period for aggressive use of NICAD cells. For service where the NICAD cells are not overcharged repeatedly and the oil levels are maintained properly consider ten years as the approximate renewal period for electrolyte. Reconditioning is defined as replacement of the electrolyte and oil followed by a complete charge/discharge cycle where the capacity is confirmed. Reconditioning Process The first step in cell reconditioning is to physically inspect the cell for damage. The case should be tight and without cracks. Clean the exterior of the cell with disposable towels. Titrate the electrolyte (see specific instructions below) to determine the degree of carbonate contamination of the electrolyte. If theK2CO3 concentration is less than 15%, then there is no advantage to electrolyte replacement. If the K2CO3 concentration is greater than 15%, then electrolyte replacement is required. Charge cell at C/10 rate for 16 hours. Remove electrolyte from cell by turning the cell upside down and pouring the electrolyte into a plastic bucket. Dispose of this caustic electrolyte in a responsible manner! Replace electrolyte with "NEW" electrolyte within five minutes. Damage will occur to the cell if it is dry for greater than five minutes. Use KOH electrolyte with a specific gravity of 1.190 gr./ml., consisting of KOH dissolved in H2O, and LiOH. The lithium hydroxide should be approximately 12 gr./liter of electrolyte. If you are mixing your own electrolyte using dry KOH & LiOH flakes, be sure to let the exothermic reaction cool before testing specific gravity. Add mineral oil to provide approximately 1/8" oil float on the surface of the electrolyte. Clean cell case and terminals of any dirt or electrolyte spillage. Test cell capacity. If the equipment is available certify the cell's capacity by cycle testing as described in this article. Place cell in use. Cell Electrolyte Levels Several episodes of electrolyte and oil foaming out of the cell caps were experienced. This was due to overfilling the cells with electrolyte and oil. Electrolyte levels were observed to change with the state of charge. If cells are filled when discharged and subsequently charged, they will be over full by as much as 3/8". Maintain the cell oil and electrolyte levels in the charged state to prevent this occurrence. If some oil foams out of the cell during charge, replace it as necessary with Chevron Utility Oil 22 (Product of Chevron USA). The proper amount of oil on top of the electrolyte will reduce foaming. If oil enters the plates due to a very low electrolyte levels excessive foaming can be the result. To prevent this, check electrolyte levels often to determine the rate of water usage for each cell. Replenish only with distilled water. Titration for K2CO3 in Alkaline Electrolyte This titration process was obtained via personal communications with David Dwyer at SAB NIFE. With this process I have measured carbonate concentrations between 0.74% and 19.27% accurately. Materials required Hydrochloric Acid, 1 N Buret, 25 ml. Buret stand, white porcelain base Pipette, 5 ml. Phenolphthalein pH indicator, 1% Methyl orange, 0.1% (w/v) Aqueous Erlenmeyer flask, 250 ml. Graduated cylinder, 100 ml. Distilled water Procedure 1. Using pipette remove 5 ml. electrolyte from cell and transfer to the 250 ml. Erlenmeyer flask. 2. Add 50 ml. distilled water to the Erlenmeyer flask. 3. Add 2 drops phenolphthalein pH indicator to the Erlenmeyer flask. Note the pink color to the liquid. 4. Using the buret, titrate with 1N HCL to clear. Record the number of milliliters 1N HCL to clear as A= ml. 5. Add 4 drops methyl orange to Erlenmeyer flask. Note the yellow-orange color. 6. Continue the titration until the yellow-orange color changes to pink-orange. Record this final value of the titration as B= ml. INSERT FORMULA For NICAD cells up to 15% is considered all right. If the K2CO3exceeds 15%, renew the electrolyte. Access George Patterson, 3674 Greenhill Rd., Santa Rosa, CA 95404