Virtually every boat carries a variety of what are commonly but mistakenly called flashlight batteries. Over the course of a year, a typical boat owner may purchase dozens of such batteries, often spending $50 or more.
Of the various types of batteries purchased in the U.S. each year, the alkaline type alone accounts for an estimated 3 billion units. Understanding the attributes and limitations of various batteries can help one to use the best battery for the job.
A few definitions are useful in discussing batteriesactually, these devices are properly called primary cells, not batteries. A battery is a group of interconnected cells. When two or more cells are inserted into a flashlight they form a battery. Cells designed for one-time use are defined as primary cells, those that can be recharged are secondary cells. Some primary cells can accept a limited number of recharge cycles. Of considerable interest to mariners are newly developed primary cells that can provide more than 20 discharge/recharge cycles.
Primary cells started with Alessandro Volta (for whom the unit of electromotive force, the volt, is named). He first produced wet primary cells in 1796. These cells consisted of tin or zinc and copper or silver separated by a piece of pasteboard or animal hide which was soaked with a vinegar or salt solution. Improvements in cell design and chemistry continued throughout the 1800s. In 1866, George Leclanché developed a practical sealed cell in which the electrolyte, ammonium chloride, was immobilized in an inert powder. This cell, in cylindrical form, was the forerunner of the now common carbon zinc cell. (In France, the common dry cell is often referred to as a pile Leclanché.) It came to be called a dry cell, even though the electrolyte was still wet.
This basic type of cell has seen many improvements over the decades. Shelf life was quite short until the late 1930s, but has been greatly improved. Until leak-resistant housings became available in 1939, damage to equipment using these cells was a common problem. The zinc anode of both the carbon zinc cell and the zinc-chloride heavy duty cell also serves as the container for the electrolyte. When these cells reached the end of their life, it was common for the zinc can to have deteriorated to the point where it leaked electrolyte. The sealed carbon zinc and heavy duty cells sold today still use a zinc anode as the container for the electrolyte, but this is further surrounded with a steel shell. (Interestingly enough, due to the industry’s track record of battery leakage, the Federal Trade Commission prohibits the use of the word “leakproof” on any primary cell.)
Alkaline cells, on the other hand, use a different type of construction. The outer container of the alkaline cell, a steel can, is the cathode or positive terminal of the cell, the anode is located in the center of the cell and is typically a brass pin or tube. Careful comparison of typical D-size carbon zinc or heavy duty cells with an alkaline cell will show that the positive terminal of the former cells is a cap connected to the center anode, while the negative terminal is the case of the cell (although the steel outer jacket, which is part of the leak-proofing of the cell, is electrically insulated from the remainder of the cell). For the alkaline cell, the positive terminal is attached to the steel can, while the negative terminal disc is connected to the center anode. The outer surface of the alkaline cell is most often a plastic cover/label, which is necessary to prevent shorting of cells when they are inserted in a metallic housing, such as a flashlight.
The energy density, watt-hours per pound, and per cubic inch of primary cells has significantly improved over the years. Today, the common carbon-zinc cell (more properly a zinc-manganese dioxide cell) offers very good value for many low-current applications. These cells are low in cost, have good shelf life and possess reasonable energy density. At low current drain, these cells can provide almost 45 watt-hours per pound of weight. By comparison, a typical lead-acid deep-cycle battery provides approximately 20 watt-hours per pound.
The chemistry of primary cells has been improved by tailoring the energy storage/delivery characteristics of the cell to the characteristics of the load. Primary cells suitable for general purpose applications include the zinc-zinc-chloride-manganese dioxide cell, often called a “heavy duty” cell, and the zinc-alkaline-manganese dioxide cell, called an alkaline cell. The heavy-duty cell chemistry was first patented in 1899. The alkaline cell first entered the market in the 1950s.
Since all primary cell types offer the same open-circuit voltage of 1.5 volts, they can be used interchangeably in most applications. There are some additional factors that enter the choice of which battery to use. One of these is performance at low temperatures. Basic carbon zinc cells perform poorly here (this type of cell can freeze, usually at about 0° F), heavy duty and alkaline cells perform better at low temperatures. At very low temperatures, below approximately -20° F, only a few types of cells, including some lithium cells, will function. When current drains are low the carbon zinc cell offers good value per dollar.
Name brand cells usually have very good shelf life. This applies for all three types of cells, with the alkaline offering the longest life. My personal experience with low-cost, private-label cells, such as those offered by drug stores, indicates that shelf life and total performance is likely to be less than for the name brand products.
Cell leakage can have a disastrous effect on expensive devices such as radios. Here again, personal experience indicates that the name brand products are more leak-resistant. The zinc chloride cells (heavy duty) appear to offer greater leak resistance, especially near the end of their useful life, than the less expensive carbon zinc cells. This effect is the result of the water contained in the cell combining with the active ingredients as the cell is used. In fact, this type of cell can be almost completely dry internally when fully discharged. Heavy duty cells also offer somewhat better low-temperature performance when compared with the carbon zinc cell. Regardless of the brand of cell, it is a good idea to remove cells from an electrical device when it is not in use. Promptly removing a cell when it reaches the end of its service life is absolutely necessary.
Primary cells don’t have fixed service lives. The energy delivery capability of a cell varies with current drain, operating schedule, cutoff voltage, operating temperature, and storage conditions prior to use. The General Services Administration specifications for cell performance in different use scenarios is illustrative of the variations in cell performance.
General purpose flashlight
Cell is capable of supplying power for a total of 400 minutes when a load of 2 1/4 ohms is applied for one five-minute period each 24 hours until closed circuit voltage drops to 0.65 volts.
Light industrial flashlight
Cell is capable of supplying 950 minutes of power when a four-ohm load is applied for one four minute period each hour for eight consecutive hours per day until closed circuit voltage drops to 0.9 volts.
Heavy-duty industrial flashlight Cell is capable of supplying 800 minutes of power when a four-ohm load is applied for four-minute periods at 15-minute intervals for eight consecutive hours per day until closed circuit voltage drops to 0.9 volts.
Note that the cut-off voltage specified for the general purpose flashlight test is only 0.65 volts. When that voltage is reached the light output of the flashlight will be quite low. The higher cut-off voltage of 0.9 volts specified for the light and heavy duty industrial flashlight test is more realistic since using a really dim flashlight is rarely satisfactory. In all cases, the test load is applied for the specified period of time, after which the cell is allowed to rest. For flashlights, the life of the cell would be far less if the power drain were continuous. The test conditions are somewhat analogous to the various load ratings given lead-acid storage batteries.
The amount of energy output available from a cell is a function of the temperature during discharge. Generally, the higher the temperature, the greater the energy output available. Most cells are inoperative when the temperature reaches -20° F. Fortunately, little pleasure boating activity requires battery use at temperatures much below 32° F.
While output increases with temperature, shelf life decreases with increasing temperature. Shelf life is defined as the period of time a cell can be stored before it drops to 90% of its fresh capacity when tested at a temperature of 70° F and 50% relative humidity. High storage temperatures degrade shelf life; for example, storage at 90° F decreases the shelf life of a cell by about 1/3 when compared with storage at 70° F. Low temperature storage can increase shelf life. Storage at 39° F can double or even triple shelf life.
When stored at low temperatures, the cells must be packaged to prevent condensation and subsequent corrosion of the terminals. Storage in a freezer appears to further increase shelf life. An alkaline cell will retain up to 85% of its capacity after five years in storage. A heavy-duty cell provides four years of shelf life, and a general-purpose cell (zinc carbon) generally has two years of shelf life. The shelf life of very small cells, such as AA and AAA and of miniature nine-volt batteries that are made with flat cells may be somewhat less than for conventional C and D size cells. The shelf lives mentioned assume storage at temperatures about 70° F. Shelf-life-dated cells are available, since these cells are required for some equipment on inspected vessels and aircraft. Cell dating was common practice in the past, when shelf life was shorter and leak-proofing was less effective than it is today. Dating is common on the package of cells, usually as a “best used before” date. Individual cells are date coded by the manufacturers; however, this date code is encrypted and not available to the public.
If a voyage of great length is contemplated, it may be worthwhile to purchase cells from a large supplier, thereby obtaining the freshest possible products. Storing the cells in a cool, dry location is obviously best. It is unlikely that sufficient space will be available in the freezer of most voyaging sailboats for storage of a large number of cells. By placing the cells in an airtight plastic box (of the type readily available for food storage), the area beneath the floor boards can be used for spare cell storage.
Cells in flashlights or radios packed in deck-mounted life raft containers will be subject to very high storage temperatures in most parts of the world. These cells should be replaced at the annual inspection of the raft. A leaky cell in the raft container can not only damage the device in which it is installed, it might also damage the raft.
There are numerous other types of primary cells in common use, including lithium, zinc mercuric oxide, Zinc silver oxide, and Zinc air (oxygen). Some of these electrochemical systems use quite expensive ingredients. When cells are small, as they must be for applications such as watch and calculator cells, the cost of fabrication of the cell becomes large compared with the cost of the active materials, thus use of expensive active materials is practical.
Some types of cells, such as mercury and magnesium, which were previously favored for use in emergency strobe lights and some EPIRBs, due to their long shelf life, are now being replaced by various types of lithium cells. Virtually all of the new 406 MHz EPIRBs are equipped with lithium batteries. Fixed lights in life rafts typically use water-activated cells, which employ magnesium negative electrodes and either silver chloride or cuprous chloride positive electrodes. When immersed, sea water acts as the electrolyte. These types of cells will also operate, although less efficiently, in fresh water. They exhibit a time delay, which can reach 20 to 40 minutes when supplying relatively large currents, before achieving full output. The locator lights on life jackets typically are of the water-activated type.
Rechargeable cells make sense both to reduce the cumulative cost of battery power and on long voyages where sources of fresh, high-quality cells at reasonable prices may be rare. Charging equipment that will operate from the vessel’s 12-volt bus is available for most types of rechargeable cells. Rechargeable cells will provide full performance only in devices that are specifically designed for their use. The most common type of rechargeable cell in use on boats is the sealed NiCad cell. It is important to recognize that the output voltage from a typical fully-charged NiCad cell is only 1.25 to 1.33 volts. The initial voltage from a fresh carbon zinc, heavy-duty, or alkaline cell is 1.5 volts or slightly more. A flashlight designed to operate with primary cells will not supply the same light output with NiCads as it does with normal cells. While NiCads are a fine power source in many applications, the energy density of NiCads is only 12 to 17 watt-hours per pound, typically 1/3 that of alkaline cells and 1/2 that of heavy-duty cells.
NiCads exhibit some characteristics that must be taken into account to insure satisfactory service results. During discharge, the output voltage of a NiCad remains close to the initial voltage when fully charged and then drops sharply when the cell reaches a discharged state. This characteristic is useful in insuring full performance from the device being powered, however, it does make advance notification of discharge more difficult than with cells whose output voltage declines gradually. NiCad cells self-discharge at rates approaching 1% per day and are, therefore, completely discharged in 60 to 90 days. Remaining in a discharged state does not diminish the life of these cells, but they should be fully recharged before further use.
Some NiCads exhibit a “memory” effect if they are not fully discharged in some of the use cycles. To a degree, small, sealed NiCad cells share this characteristic with the large open-cell NiCad batteries used in aircraft applications. These liquid electrolyte (potassium hydroxide) cells require extensive servicing, including total discharge and recharge, normally at 100-use-hour intervals. The internal construction of these cells is very different from that used in the sealed cells. A major attribute of these NiCad batteries is their ability to deliver a current of 1,000 amps or more for a short time for engine starting and then accept a charge current of 400 amps of more in preparation for the start of the next engine. They have found occasional use on boats. However, cost constraints (a 32-amp-hour 28-volt battery can cost $3,000) and the need for special servicing have prevented their widespread use.
A new type of cell has recently become available, with major application in portable computers. The nickel metal hydride (NiMH) cell offers about 50% more energy capacity than the typical NiCad cell. Their primary advantage for a portable computer is the possibility of a trade-off of weight or operating-time increase.
Although primary cells are not generally considered to be rechargeable, they can, in fact, be recharged to a very limited degree if the recharging is done before the cell is too deeply discharged and with careful control of the recharging process. The very limited ability of most primary cells to accept recharging has precluded the general development and sale of charging devices. Some devices are sold for recharging normal primary cells, however, the results are rarely satisfactory.
Recent work by Rayovac Corporation has resulted in the introduction of a rechargeable version of the alkaline cell. Rayovac calls their product the Renewal Reusable Alkaline and currently produces the cells in size D, C, AA, and AAA. Recharging a standard alkaline cell might provide a few discharge cycles (perhaps up to six), provided the depth of discharge before recharging was carefully limited (if the cell is discharged below 0.9 volts it cannot be recharged). Even after recharge, standard alkaline cells never reach full energy level, with output voltages being less than 1.5 volts. The chemistry and mechanical construction of the standard alkaline cell does not permit recharge cycles.
According to Rayovac, however, rechargeable alkaline manganese (RAM) cells have existed in the laboratory for more than 40 years. The problems with the rechargeable cells have included: separation of the cathode from the outermost steel jacket (causing an open circuit within the cell); and generation of hydrogen gas (causing over-pressure within the cell and the growth of metallic hair-like structures, zinc dendrites, which cause internal short circuits). The research and development work accomplished by Rayovac has resulted in their ability to market a rechargeable alkaline cell that can be expected to deliver 25 or more discharge/recharge cycles. Unlike the NiCad cell, the Renewal cell is fully charged when removed from its package and does not require and should not be charged before first use. The developments which resulted in the Renewal technology are covered by numerous patents. There are 11 patents on the cell alone!
Rayovac data shows that for continuous flashlight use, to a cutoff voltage of 0.9 volts, the Renewal cell will provide 540 minutes of operation during the 5th use/recharge cycle, 490 minutes during the 10th cycle, and 310 minutes service during the 25th cycle. The 25th use cycle data equals 53% of the 588 minutes use available during the initial cycle. When the cells are used to power equipment whose performance is unacceptably degraded when operated to a cutoff voltage of 0.9 volts, it will be necessary to remove the cells from service and recharge them more often. The size-AA Renewal cells, used in a miniature flashlight of the type most popular among yachtsmen, will initially provide five hours of service to a cutoff voltage of 0.9 volts.
The Renewal cells must be recharged in a special charger available from Rayovac. The charging process requires periodic monitoring of battery open-circuit voltage during the charge period. Utilizing a custom-designed integrated circuit to control the charge process, the charger terminates the charge when open-circuit voltage of a cell reaches a full charge indication of 1.65 volts.
At present, chargers are only available for use from a 120-volt, 50- or 60-Hz power source. The charger will operate from an inverter with sine wave output. The power drain of the charger is quite low: six watts for the small charging station (which can recharge up to four AA or AAA cells), and 28 watts for the larger station (capable of charging any combination of up to eight D, C, AA, or AAA cells). The charging time for AA and AAA cells is three to five hours, and eight hours for C or D cells. The design of the two charging stations and the mechanical design of the Renewal cells will prevent recharging of normal alkaline or other primary cells in the charger. It is not difficult, however,to modify the label/covering on standard AA- and AAA-size alkaline cells to allow them to be recharged in the Renewal charger, but the results with the normal cells will likely be unsatisfactory.
Depth of discharge
The ultimate life of the Renewal cells follows the same rule as that of other rechargeable cells, including lead-acid deep cycle types. The less the depth of discharge allowed in each use cycle before recharging, the greater the number of use cycles available. Rayovac data shows that when the percent discharge is limited to 25%, more than 100 discharge/charge cycles may be obtained. The initial energy storage capacity of the D-size cell is 5.5 amp-hours. During the 25-cycle expected life of the cell, the total energy output would be 90 amp-hours. Limiting the discharge to 1.375 amp-hours would equal a 25% of capacity discharge. For a flashlight, this operating regime would allow 2.5 hours of continuous use before recharging. If the expected 100-use cycles were achieved, the total amount of energy recovered over the life of the cell would be in excess of 125 amp-hours.
Use of the Renewal alkaline cells in a handheld radio transceiver may appear attractive, but since these devices are designed for use with NiCads, which have an initial voltage of 1.25 volts, using the same number of alkaline cells will result in a higher initial voltage. In addition, the sloping voltage curve of the alkaline cell will cause transmitter power to drop off to a greater degree as the battery pack loses power than with the relatively constant voltage available from a NiCad battery. It is likely that if the radio were only operated with low power from the transmitter (say, one watt), a set of alkaline cells, either standard type or Renewal, would perform very well.
The economics of Renewal cell use are interesting. The PS1 charging station, which can recharge up to four AA or AAA cells at one time, has a list price of $14.99. The average cost of six AA alkaline cells is $4.50, that of six AA Renewal cells is $9.00. Rayovac data claims that six AA alkaline cells used to power an electronic game provide an operating life of 4.5 hours. Their data shows that six AA renewal cells, used for 25 discharge/recharge cycles will power the game for 55 hours. The per-hour cost of power using the standard cells is $1.00. The cost of the Renewal cells is a little more than $0.16 per hour. The cost of the charger must be added to the hourly cost of the cells. From this data, the charger would be paid for after 18 hours of use of the game. (The cost of the electricity used to operate the charger is discounted in this example. If each charge cycle required five hours, a total of 125 hours of charging would be required during the life of a set of cells. With a power consumption of six watts, the charger would have consumed 750 watts of power accomplishing the 25 charge cycles. Assuming a cost of $0.08 per kilowatt-hour for power, the cost of power for charging would be $0.06.)
The PS2 charger, which can recharge up to eight D, C, AA, or AAA cells in any combination has a list price of $29.99. The list price of a package of two D or C size or four AA or AAA Renewal cells is $5.99. The prices quoted are manufacturer’s recommended retail; discounts are likely to be available.
Individual cells of any type are most often used in sets of two, three, four or, occasionally, as many as eight. Regardless of whether the cells are one-time-use or rechargeable types of any construction, it is worthwhile to always keep sets of cells together. If the four cells that power a radio are removed while the radio is not in use, keep the cells together so that they will be used as a set in the future. Since the cells are connected in series when in use, a single low-output cell can materially decrease the ability of the others to power the device. The sequence in which the cells are inserted into the battery compartment is not important, only that the sets of cells that have been used together are kept together. This is especially important when using rechargeable cells, including NiCad and Renewal cells.
There are uses for all three types of primary cells that have been covered in detail here. And there are also applications where either NiCad or the new Renewal-type cells can offer the best overall power performance. Regardless of the type of cell chosen, storing primary cells at low temperatures will pay dividends in extended shelf life.
Contributing editor Chuck Husick is a sailor, pilot, and Ocean Navigator staff instructor.