Acid vs. Alkaline: the Electrochemical Shootout Richard Perez & Chris Greacen Ever wonder how different battery technologies stack up? Which is best? Which lives longest? How do different battery technologies compare in cost, weight or volume per amount of energy stored, temperature performance, self-discharge rate, and many other operating characteristics? Well, this is a comparison between the most used battery technologies. Included in these comparisons are three types of lead-acid cells and five types of nickel cells. All the info is in the table and the text here merely helps define how the info is categorized. Electrochemical Cells- the contenders In the table to the left, there is information about eight different types of cells. Three of these types are lead-acid cells: automotive, deep-cycle, and gel. Five of the types are nickel cells using alkaline electrolyte. Three of the nickel systems use cadmium as a cathode, while one uses a variety of metal hydrides, and the other iron. All of the cells mentioned are are secondary cells. Secondary cells can be recharged, while primary cells (like flashlight batteries) can only be discharged once and cannot be recharged. Lead Acid Cells All these cells use lead compounds as their anode and cathode material. The first type listed are automotive batteries. These are the standard car battery with plates constructed out of lead sponge. The car battery is designed to do one thing, start your car at the minimum price. The deep-cycle cells mentioned are heavy-duty types whose plates are made from scored sheet lead alloyed with antimony. For example, the Trojan L-16W is such a deep-cycle battery. The third lead-acid type is the gel cell. Gel cells have a jellied electrolyte and are sealed cells. All these lead-acid systems use a dilute solution of sulphuric acid as their electrolyte. Nickel Alkaline Cells Sintered plate nicads are small sealed cells, like AA, C or D sized cells. The pocket plate types are vented wet cells, and two types are mentioned- new and reconditioned. As an example of the pocket plate nicad consider the Edison ED-160. The nickel-hydride cells are a new type of sealed cell being made by Ovonics. They use a variety of metal hydrides as their cathode material. The last type of alkaline secondary cells is the venerable nickel-iron type. All of these cells use a dilute solution of potassium hydroxide as their electrolyte. The Comparisons Here are the criteria and standards used in these battery comparisons. Capacity This row details the range of available cell capacities, expressed in Ampere-hours, for each particular cell chemistry. The lead acid gel cells are becoming available in larger capacities, now up to over 100 Ampere-hours. The new nickel hydride types are not yet manufactured in sizes bigger than 3.8 Ampere-hours (a "C" sized cell). The other types are made in everything from tiny cells to ones you need a forklift to move. Cell Voltages Three types of voltages are covered for each cell at 78¡F. The first called "Cell Operating Voltage" is a nominal voltage value for the cell under moderate discharge. All the lead acid types produce about 2 Volts. The nickel alkaline technologies all have cell voltages around 1.2 to 1.25 VDC under moderate discharge rates. "Cell Full Charge Voltage" indicates the voltage of a cell that is full and still undergoing recharging at a C/10 rate. "Cell Discharge Cut-off Voltage" is the voltage at which the cell is considered to be fully discharged and still under a moderate (ÅC/10) discharge rate. Cost For the cost figures, expressed in dollars per kiloWatt-hour of power stored, we used the following collections of cells (batteries). The lead acid automotive battery was a standard type from the local Les Schwab tire shop. The lead acid deep cycle battery is a Trojan L-16 W. The lead acid gel cell is a Panasonic 6.5 Ah model. The sintered plate nicad is a Panasonic "D" sized cell. The pocket plate nicad (both new and reconditioned) is an Edison ED-160. The nickel hydride cell is an Ovonics "C" sized cell. The nickel iron cell is a Gould 35 Ah cell. What follows in the table is an analysis of the cell's cost for its cycle lifetime and calendar lifetime. Cycle life is the average number of discharges (to 20% State of Charge) that the cell will undergo before failure. Calendar lifetime is rated for cells in float service. Float service means that the cell is continually under charge and only rarely sees a shallow (<10%) discharge cycle. All these figures are averages and are placed at extremes of service, i.e. regular deep cycling and virtually no cycling at all. As such, these lifetime figures cover the spectrum of longevity that an average user may expect. Lifetimes on all electrochemical cells are greatly dependent on the cell's user. A careful user, one who follows the rules for that particular cell technology, will receive greater lifetimes than those on the chart. A slob will receive less. It's up to you to learn how to properly use your cells and then to do it. Energy Density We rated energy density in two fashions, one by the cell's weight in pounds, and the other by the cell's volume in cubic inches. One very notable feature here is the high powered Ovonics cells. Cycle Rate This row ranks the cells according to their ability to be rapidly cycled. It details the cell's ability to be discharged and recharged at fast rates (³C/5). Electrochemical cells that power an inverter must deliver high rates of current in relation to their capacity. Ease of Use This category summarizes the characteristics listed below it into an overall rating of low, medium or high. "Ease of Use" gives the prospective battery user a look how easy it will be to keep the battery up and running. Many things, like physical maintenance, equalizing charges, etc. are all part of life on some electrochemical cells. This category summarizes the characteristics listed below it into an overall rating of low, medium or high. Conclusions Not in a minute. The table contains the data, you can make up your own mind about what type of electrochemical battery best suits your application. We will, however, point out some interesting data on the table: 1) While lead acid automotive batteries are the cheapest to buy in $/kWh of storage, they are more expensive to operate than their lead acid cousins. Car batteries cost over three times more to use in either deep cycle or float service than do deep cycle types. 2) The self-discharge rate for all lead acid systems goes out of sight at the end of the cell's life. All nickel alkaline technologies maintain constant rates of self-discharge throughout their life. 3) Memory effect is limited to the sintered plate nicads and NOT shared by any other nickel alkaline types. 4) If you're running cells at low temperatures, then they had better be nickel alkaline types. Lead acid systems lose much of their capacity at low temperatures. 5) If you require maximum energy density, then the Ovonics nickel hydrides (NiH) are the only ones to consider. The energy density by weight of the Ovonics NiH is over 31% higher than any other type on the chart. The volumetric energy density is up over 50% from the nearest competitor. This, coupled with the NiH cell's sealed package, make it the best to use in portable service. Access Battery data and/or questions? No problem. Write or call: Richard, C/O Home Power, POB 130, Hornbrook, CA 96044 ¥ 916-475-3179.