Chapter 7 Sources of Electricity 113 4OH– O2 + 2H2O + 4e– ...(1) Hydroxide Oxygen Water Electrons ions Positive Th is oxygen passes through the separator to the negative. After this, an absorption reaction takes place at the negative and absorption occurs. O2 + 2H2O + 4e– 4OH– ...(2) Oxygen Water Electrons Hydroxide ions Negative Nickel-Metal Hydride Cell Nickel-metal hydride (NiMH) is a rechargeable battery very similar to the NiCad battery. Th e NiMH battery uses nickel for the positive plate and a hydrogen- absorbing alloy for the negative plate. Hydrogen-absorbing alloys are made from combinations of nickel and iron (Fe), magnesium (Mg), and lanthanum (La). Th e alloys can absorb large volumes of hydrogen equal to approximately 1000 times their weight without an increase in volume. Th is means the batteries can be sealed without the danger of explosion caused by expansion of materials. A NiMH battery can have two to three times the storage capacity of a NiCad battery of equal size. Th is has made it an excellent battery for hybrid vehicles and portable computer applications. Th e discharge voltage level is 1.2 volts, which is the same as for NiCad batteries. Th e NiMH battery can be recharged over 500 times. Th e combined characteristics of the NiMH makes it an ideal replacement for existing NiCad battery applications. NiMH batteries should never be completely discharged. If they are, one or more of the cells can reverse polarity, which will result in permanent damage to the battery. Electronic devices that use NiMH batteries typically have a special circuit designed to detect low voltage levels and then disconnect the battery circuit before damage can occur. 7.1.4 Batteries in Series and Parallel Often, a single cell is called a battery. By strict defi nition, however, a battery consists of two or more cells connected together. Th ese cells are enclosed in one case. Series Connection In the study of electricity, it is important to understand the purpose and results of connecting cells in groups. First, consider the series connection. In this method, the positive terminal of one cell is connected to the negative terminal of the second cell. Th e voltage can be calculated by the number of cells being connected in series: ET = EC × N where N = Number of cells EC = Voltage of one cell ET = Total voltage In Figure 7-12, four cells are connected in series. Th e output voltage will equal: = 6 V ET = 1.5 V × 4 Copyright Goodheart-Willcox Co., Inc.