Chapter 7 Sources of Electricity 109 ■ High energy density per unit volume. ■ Wide range of operating temperatures. ■ Compact, thin size. Compact silver-oxide batteries have the highest electrical volume and leakage resistance of any battery of that size. Th ey are commonly used in watches. Two types of silver-oxide batteries are made for use in watches. One type uses caustic potash for electrolyte. Th e other uses caustic soda. Th e caustic potash battery has the symbol W on the bottom of the battery. It is for high-drain use, where more power is needed. It is used in wristwatches with liquid crystal displays and multifunction analog watches. Th e caustic soda battery has the symbol SW on the bottom of the battery. It is for low-drain use. It is used mostly in single function analog watches. Figure 7-7 shows a cutaway of a silver-oxide cell. 7.1.3 Secondary Cells A secondary cell can be recharged or restored. Th e chemical reaction that occurs on discharge may be reversed by forcing a current through the battery in the opposite direction. Th is charging current must be supplied from another source, which can be a generator or a power supply. Figure 7-8 shows one type of battery charger used for recharging auto- mobile and motorcycle batteries. An alternating current, which will be studied in a later chapter, must be rectifi ed to a direct current for charging the battery. Lithium-Ion Cell Lithium-ion cells, or Li-ion cells, use an intercalated (inserted between or among existing elements) lithium compound for an electrode instead of metallic lithium as used in non- rechargable lithium cells. Batteries using Li-ion cells have been in use since the 1990s. Li-ion batteries are lightweight and provide high-density power supplies. Lithium-ion polymer cells, also known as LiPo cells, are constructed with the same electrode and electrolyte materials as a Li-ion cell. Th e main diff erence is in the construction of the lithium-polymer case. A fl exible, foil case is used in place of the traditional hard plastic case found in Li-ion cells. Th e lithium-polymer foil case results in a reduced overall weight of approximately 20%, as well as a much thinner battery. Th e light weight and thinner construction make it ideal for portable electronics, such as cell phones and computers. Charging from a USB Port Many devices are designed to be charged from a USB port. USB ports can provide 5 V at approximately 500 mA. Charging from a USB port is only practical for small Li-ion batteries and is not possible for large Li-ion battery packs. Typically, a host laptop or portable electronic device will automatically disconnect the USB port if the battery is drawing over 500 mA. (+) Top cap Insulator Top plate (and gasket) Insulator Can Jacket (PVC) Separator Lithium anode Cathode ([CF]n) (–) Terminal Insulator Panasonic Battery Sales Division Figure 7-6. Cross-sectional view of a cylindrical-shaped lithium battery. Cell can Gasket Anode Anode cap Cathode sleeve Electrolyte in absorbent Cathode Barrier Panasonic Battery Sales Division Figure 7-7. Cutaway view of a silver-oxide cell. Mariusz Szczygiel/Shutterstock.com Figure 7-8. This type of charger is called a trickle charger. It slowly brings a battery back to full charge. Copyright Goodheart-Willcox Co., Inc.