Power factor can also be represented as a percentage. Using  a percentage to show power factor, the circuit in the previous  example would have a power factor of 80 percent. It should be noted that a low power factor is undesirable.  Circuits with a lower power factor create excess load on  the power supply and produce inefficiency in the system.  Aircraft AC alternators must typically operate with a power  factor between 90 percent and 100 percent. It is therefore very  important to carefully consider power factor when designing  the aircraft electrical system. Aircraft batteries are used for many functions (e.g., ground  power, emergency power, improving DC bus stability,  and fault clearing). Most small private aircraft use lead- acid batteries. Most commercial and corporate aircraft use  nickel-cadmium (NiCd) batteries. However, other lead  acid types of batteries are becoming available, such as the  valve-regulated lead-acid (VRLA) batteries. The battery best  suited for a particular application depends on the relative  importance of several characteristics, such as weight, cost,  volume, service or shelf life, discharge rate, maintenance,  and charging rate. Any change of battery type may be  considered a major alteration. Aircraft batteries are usually identified by the material used  for the plates. The two most common types of battery used  are lead-acid and NiCd batteries. Dry charged cell lead-acid batteries, also known as flooded  or wet batteries, are assembled with electrodes (plates) that  have been fully charged and dried. The electrolyte is added  to the battery when it is placed in service, and battery life  begins when the electrolyte is added. An aircraft storage  battery consists of 6 or 12 lead-acid cells connected in series.  The open circuit voltage of the 6 cell battery is approximately  12 volts, and the open circuit voltage of the 12-cell battery is  approximately 24 volts. Open circuit voltage is the voltage of  the battery when it is not connected to a load. When flooded  (vented) batteries are on charge, the oxygen generated at  the positive plates escapes from the cell. Concurrently, at  the negative plates, hydrogen is generated from water and  escapes from the cell. The overall result is the gassing of the  cells and water loss. Therefore, flooded cells require periodic  water replenishment.  [Figure 9-32] VRLA batteries contain all electrolyte absorbed in glass-mat  separators with no free electrolyte and are sometimes referred  to as sealed batteries.  [Figure 9-33]  The electrochemical  reactions for VRLA batteries are the same as flooded  batteries, except for the gas recombination mechanism that  is predominant in VRLA batteries. These types of battery  are used in general aviation and turbine powered aircraft and  are sometimes authorized replacements for NiCd batteries. When VRLA batteries are on charge, oxygen combines  chemically with the lead at the negative plates in the presence  of H$_{2}$SO$_{4}$ to form lead sulfate and water. This oxygen  recombination suppresses the generation of hydrogen at  the negative plates. Overall, there is no water loss during  charging. A very small quantity of water may be lost as a  result of self-discharge reactions; however, such loss is so  small that no provisions are made for water replenishment.  The battery cells have a pressure relief safety valve that may  vent if the battery is overcharged.  9-21 Figure 9-33.  Valve-regulated lead-acid battery (sealed battery).  Figure 9-32.  Lead-acid battery installation. Aircraft Batteries Types of Batteries Lead-Acid Batteries Dry Charged Cell Lead-Acid Batteries Valve-Regulated Lead-Acid Batteries (VRLA)