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Storage Cell Fundamentals
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The popular terminology for storage cells is somewhat confusing. Storage cells are almost always referred to as "batteries" in common usage, but this is not technically correct.
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The storage cell described in the previous section is just that, a "cell", not a "battery". It consists of one cathode and one anode in an electrolyte. A storage cell with specific electrode materials and electrolyte has a certain output voltage, and to get higher voltages with that specific technology, they must be electrically connected together in series as a "battery".
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Flashlight cells are just that, cells, but the lead-acid battery used in an automobile consists of several cells packaged and chained together, so it is indeed a battery. A single cell of a lead-acid battery has a voltage of 2 volts, and so a 12-volt lead-acid battery has six cells in series.
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This document will use the term "storage cell" or just "cell" by default, and reserve the term "battery" for when it is specifically appropriate. However, this is just to be precise, since at least in the US both cells and batteries are called "batteries" wherever they are sold and referring to them as "cells" or anything else will just cause confusion.
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* There are two classes of storage cells: nonrechargeable or "primary" cells, for example typical cheap throwaway flashlight batteries, and rechargeable or "secondary" cells, for example an automotive lead-acid battery.
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Storage cells can also be classified as "wet cells", which have liquid electrolytes; "dry cells", which have electrolytes in the form of a paste; and "solid electrolyte" cells, which as their name indicates use a completely solid electrolyte.
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There are also standardized form factors for certain classes of storage cells, such as AAA and AA penlight cells; C and D flashlight cells; and the standard nine-volt brick-shaped "transistor radio" battery. Output voltages are also more or less standardized for these products. However, storage cells are otherwise not highly standardized items, as shopping for a watch button-style cell quickly proves.
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Many storage cells can maintain their output voltage at a reasonably constant level over a fairly wide range of output currents. In electrical engineering terms, they are said to have a low "internal resistance". Those that have high internal resistance cannot operate at high current loads, since the voltage at their terminals drops below useful levels. Cells with high internal resistance will also burn up a high proportion of their stored electricity with their own resistance at high current loads, draining them prematurely.
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This means that another parameter for storage cells is the maximum useful current output. Storage cell makers may also provide curves giving the fall-off in voltage with increasing current drain, from maximum voltage to the "cutoff voltage" specified for the cell.
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By the way, the low internal resistance, or equivalently current capacity, of big automotive batteries makes them potentially dangerous. While their output voltages are so low that getting a shock off them is not a problem, if the output of a large automotive battery is shorted to the chassis ground the large currents flowing through the short can cause an almost explosive flash and severe burns. It is not usually a good idea to wear a watch with a metal band while servicing a vehicle, since the vehicle's chassis is ground and a short from a "hot" wire could easily lead to a nasty accident.
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The total energy capacity of a storage cell is measured in the number of hours it can supply a given level of current, or "ampere-hours". This is a straightforward figure of merit for storage cells based on the same technology, since they will all have the same voltage.
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However, ampere-hours can be misleading for comparing different storage cell technologies, as the voltages may differ and the power output of a storage cell with a lower voltage is lower for the same level of output current. For this reason, the unit of "watt-hours" is used to compare energy storage capacity between different storage cell technologies.
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A related rating is the "specific energy" of a storage cell, which gives the storage capacity of the cell relative to its mass. For example, a storage cell could be said to have a given number of watt-hours per kilogram. A related measure is the "energy density" of the cell, which gives its storage capacity relative to its volume, for example in watt-hours per liter.
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Specific energy and energy density are used in comparisons between different classes of batteries, particularly for automotive propulsion applications. Electric-powered automobiles have always suffered from the limited energy capacity of electric storage cells compared to gasoline and other chemical fuels, and so obtaining storage cells with greater specific energy has been one of the most important goals of electric-automobile designers.
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* As the previous section mentioned, rechargeable storage cells can be run backwards and more or less restored to their original, charged state. The "more or less" is important. The restored state is not a perfect replica of the original state, and so rechargeable storage cells degrade slightly every time until their storage capability fades out. For this reason, rechargeable storage cells are also also described by the number of "charging cycles" they will tolerate. The number of cycles tends to be lower with greater average depth of discharge. Manufacturers may also provide curves showing how the storage cell's capacity slowly falls as the number of cycles increases.
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Another parameter specific to rechargeable cells is "efficiency", or the ratio of power available when the cell is fully charged to the power required to recharge it. Other battery parameters include, of course, the physical dimensions and mechanical specifications of the battery; its shelf life; its expected service life, or how long it can be expected to survive in normal operation; and environmental limits on its operation, particularly temperature specs.
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By the way, since the rate of chemical reactions increases at higher temperatures, it is customary to store flashlight cells in a refrigerator to prolong shelf life, though this is becoming less important as improved cell technologies have long shelf lives.
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* For an example of storage cell specifications, the data sheet for the Duracell MX1500 AA-size alkaline cell provides a mechanical diagram with dimensions in millimeters, along with weight in grams and volume in cubic centimeters. The operating temperature range is specified as -20 to 54 degrees Celsius. Nominal output voltage is specified as 1.5 volts, with curves giving:
- Output voltage versus time for different levels of power drain.
- Hours of service versus power draw for different levels of output voltage.
- Total amount of energy delivered for given levels of power drain.
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* Vendors have now gone beyond simply selling batteries and now sell complete battery-based power modules that can be designed into portable equipment. Such a power module consists of a pack containing cells, power output and recharging control, and control electronics.
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The control electronics will include a small cheap digital microcontroller with electrically-programmable ROM to store battery parameters. The module communicates with the rest of the system through a two-wire serial-interface bus called the "SMBus", devised by Intel Corporation. Such schemes are now being standardized by an open specification called the "Smart Battery System (SBS)" that specifies the functionality, interfaces, and software protocols of the battery pack.
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