Nickel-Cadmium Batteries Richard Perez The lead-acid batteries are the weakest component in home power systems. Large nickel-cadmium cells offer us a better way to store the electricity we make. Nickel- cadmium cells are more efficient, longer lived, and much more rugged than the lead-acid cells. This is the first in a series of articles about using large pocket plate nickel- cadmium batteries in home power systems. It is an introduction to the nickel-cadmium cell and how it works. The next article will contain actual test and performance data of a working PV/nicad system. Meet the nickel-cadmium cell While it may seem that nickel-cadmium (or nicad) cells are a recent development, they have been in use since the early 1900s. The nicad stores electricity in a reversible chemical reaction, just like a lead acid cell. Electrochemical cells convert chemical energy into electrical energy. Rechargeable, or secondary, cells are happily able to reverse the process and convert electrical energy back into chemical energy. Electrochemical cells consist of three basic elements- 1) an anode, 2) a cathode, and 3) an electrolyte. The anode and cathode are made from two materials (usually metallic compounds) which form an "electrochemical couple". This means that the two metals making up the couple release free electrons (electricity) as they chemically react. Since the anode and cathode materials are not in actual physical contact, a medium for electron exchange between the anode and cathode is necessary. This medium is called the electrolyte and is usually an electrically conductive liquid. In the lead-acid battery the electrolyte is a dilute (Å25%) solution of sulphuric acid in water. However, the nicad uses an alkaline (caustic) chemical reaction rather than the ascetic (acid) reaction the the lead-acid cell uses. The electrolyte of the nicad cell is a dilute solution (Å21%) of potassium hydroxide (KOH) in water. The differences between lead-acid and nickel-cadmium reactions doesn't stop here. In the lead-acid reaction, the sulphuric acid electrolyte actually participates in the cell's chemical reaction. The amount of sulphuric acid in the electrolyte solution decreases as the cell is discharged. In the nickel-cadmium cell, the potassium hydroxide electrolyte acts only as an electron transfer medium and does not chemically change as the cell discharges. For this reason, it is impossible to determine the state of charge of a nicad cell using a hydrometer. Nickel-cadmium cell electrochemistry The anode (or positive pole) of a nicad cell is chemically nickel oxide hydroxide (NiOOH) when fully charged, and nickel hydroxide [Ni(OH)2] when fully discharged. The cathode (or negative pole) of the nicad cell is chemically composed of cadmium (Cd) when fully charged, and cadmium hydroxide [Cd(OH)2] when fully discharged. The potassium hydroxide electrolyte has a density of 1.17 to 1.30 irrespective of the cell's state of charge. The electrolyte also contains a small amount of lithium hydroxide (LiOH). The discharge and charge chemical reaction is shown below: INSERTCHEMEQA The nicad cell's particular electrochemistry yields a working voltage of about 1.2 VDC. A wet, pocket plate, nicad cell should be considered fully discharged at between 1.00 and 1.15 VDC. Under charge, the nicad cell's voltage will vary from 1.35 to 1.65 VDC depending on state of charge, amount of recharging current in relation to the cell's capacity and temperature. These types of cycle characteristics mean that a battery pack for a 12 VDC system would use 10 nicad cells in series. A 24 VDC system would use 20 nicad cells in series. Nickel-cadmium cell construction The active materials in the nicad cell are impregnated into pockets in the cell's plates. The actual plates are made of nickel plated steel and do not participate in the cell's chemical reaction. INSERTILLUSTRATION This is a very different type of cell construction from that used in lead-acid batteries. In the lead-acid cell, active materials ARE the plate and everything undergoes chemical change. This means that the plates are continually being chemically broken down and rebuilt. In the nicad cell, the reactive ingredients are held in pockets in an inert grid of nickel plated steel. The net result of "pocket plate" construction is that the reactive compounds stay where they belong and the cell lasts much longer. In the particular nicad cells we are testing, the active materials are formed into long strips which are encased in perforated pockets in the nickel plated steel plates. The plates are intermeshed with separators between them to make up the working cell. The illustration below shows a cross section of a typical pocket plate design. INSERTCROSSSECTION Nickel-cadmium types & sizes Wet, pocket plate, nicad cells are available in several types. These types are designated by their discharge current rating in relation to the cell's capacity. High current nicads are designed to deliver large amounts of current within a very short interval, i.e. total discharge of the cell in a few minutes. Medium current nicads cells are designed to have their total electrical capacity withdrawn in a 7 to 48 hour period. Low current nicads are designed to be emptied slowly over longer period, up to several weeks. While the chemical reaction is the same for all types of nicads, their physical construction differs slightly. The major difference is the number and thickness of the plates within a equal volume cell case. The high discharge rate cells have a greater number of thin plates, the medium rate cells have fewer and thicker plates, and the low rate cells have very few, very thick, plates. Considering the requirements of home power systems, the best types for us to use are the Medium rate nicads. They have a combination of relatively thick plates (for longevity) and high enough discharge current ratings to handle the surge demands of a large inverter. Wet, pocket plate, nicad cells are available in capacities of between 80 and 1,200 Ampere-hours. Over 400 Amp- hrs.(Å180 pounds), the cells get so heavy that you need a forklift to move them. To give you an idea of their size and weight, let's look at the Edison ED-160 cells we are currently testing. The ED-160 nicad cell is a medium rate cell with a capacity of 160 Ampere-hours at a discharge rate of 32 Amperes (its C/5 rate). Each cell is 6.37" wide, 18.25" tall, 3.37" long, and weighs 21 pounds. Nickel-Cadmium performance Voltage vs. state of charge (SOC) The nicad has vastly superior voltage to SOC performance over the lead-acid cell. The voltage of the nicad remains fairly constant during cell discharge, while the lead acid cell's voltage decreases more rapidly as the cell is discharged. The chart below shows our actual test data on an ED-160 nicad cell. Note that the voltage remains fairly stable throughout the discharge cycle. INSERTCHART The chart shows voltage for a single cell. To extrapolate the performance of a 12 VDC battery pack (10 series cells) multiply the voltage figure by 10. What this translates to in actual service is vastly improved operation of 12 VDC gear because they are being fed a higher average voltage. For example, one evening we put the entire house on the ED-160 test pack. The pack had been recently recharged and its voltage was running about 13.4 VDC under a small load of two car tail lights (Å4 Amps). The lights were much brighter than normal due to the higher voltage. Our 2m Ham radio put out more power. My 12 VDC Weller soldering iron got hot faster. And so on. Current The nicad cell can deliver more current faster, with less voltage loss, than the lead-acid cell. The reason for this is the internal resistance of the cell. Lead-acid cells have an internal resistance that is about twice that of the nicad cells. The nicads lower internal resistance makes them more able to deliver very high current in relation to their electrical capacity. For example, the ED-160 cells we are testing can be discharged at rates over 600 Amperes (and they are Medium rate cells!). The current handling capabilities of the nicad cell make it possible to reduce the ampere-hour capacity of a battery pack and still deliver the high surge currents needed by equipment like inverters. Temperature It is in low temperature performance that the nicads really shine. You can even freeze these cells without damaging them. For example, at 50¡F. (10¡C.) the lead-acid cell has 90% of its capacity available, while the nicad has 97% of its energy available. At 32¡F. (0¡C.) the lead-acid cell has 75% capacity available, while the nicad has 92%. At 14¡F. (- 10¡C.) the lead acid's capacity has dropped to 53%, while the nicad still has 85% of its rated capacity available. Self-discharge rate Here the lead-acid cell starts out even with the nicad. They will both loose about 10% of their stored energy in a 1 month period. The nicad cell's self-discharge rate remains constant over its entire lifetime. The self-discharge rate of the lead- acid cell increases as the cell ages. For example, a 6 to 8 year old deep cycle lead acid cell will loose about 30% of its stored energy monthly to internal self-discharge. Nickel-cadmium maintenance All that's necessary is adding distilled water to the electrolyte to maintain it at the proper level. As with all batteries, keep them clean and all their connections tight and bright. Nickel-Cadmium cells & Abuse It's very easy to abuse a lead-acid battery. For example, just leave it discharged for several months and it will permanently loose most of its capacity. This is not the case with pocket plate nicads. They can be totally discharged and stored for a year. When they are recharged, they will still have all their rated electrical capacity. The nicads are more resistant to overdischarge and overcharge damage than are the lead-acid cells. It makes no difference to a nicad if it is operated extensively without being totally refilled. Operation of lead-acid cells without periodically totally refilling them will result in diminished capacity. Longevity The maximum lifetime for a properly maintained nicad cell can be as long as fifty (50) years. Average nicad lifetime is around 20 to 25 years. Lead-acid cells will last, if properly maintained, for about ten (10) years. In terms of how many cycles the cell will deliver, the nicad is well ahead of the lead-acid cell. While a lead-acid cell will deliver about 1,000 cycles, the nicads will deliver more than 2,000 cycles. The actual limiting factor of the nicad's lifetime is how they are used and maintained. If the nicad pack is properly sized, recharged and if the pack has its water level maintained, then the nicads may last much longer than described above. Price & cost-effectiveness Currently a brand new nicad battery pack will cost between 6 and 10 times the amount of a similar capacity lead-acid pack. This appears to be a major wrinkle. Sure the nicads work better, but at that price there is no way that they'll pay for themselves. True Enuff. However, if the nicads are purchased used and reconditioned, then the cost is about twice that of a lead-acid system and the nicads do in fact pay for themselves by lasting longer. Will the reconditioned nicad cells last? Are they worth what they cost? Will they work in home power systems? Stay tuned. Home Power #13 will feature our complete test report on a reconditioned nicad pack in actual PV system service. We're dong the testing now, but it will be six weeks before the data is complete.