Alternator/Generator Failure Operating on the Main Battery Loss of Alternator/Generator for Electronic Flight  Instrumentation Techniques for Electrical Usage Master Battery Switch Depending upon the aircraft being flown, an alternator failure  is indicated in different ways. Some aircraft use an ammeter  that indicates the state of charge or discharge of the battery.  [Figure 11-4]  A positive indication on the ammeter indicates  a charge condition; a negative indication reveals a discharge  condition. Other aircraft use a loadmeter to indicate the load  being carried by the alternator.  [Figure 11-4]  Sometimes an indicator light is also installed in the aircraft to  alert the pilot to an alternator failure. On some aircraft, such  as the Cessna 172, the light is located on the lower left side  making it difficult to see its illumination if charts are open.  Ensure that these safety indicators are visible during flight.  When a loss of the electrical charging system is experienced,  the pilot has approximately 40 minutes of battery life  remaining before the system fails entirely. The time  mentioned is an approximation and should not be relied upon  as specific to all aircraft. In addition, the battery charge that  exists in a battery may not be full, altering the time available  before electrical exhaustion occurs. At no time shoul d a pilot  consider continuing a flight once the electrical c harging  system has failed. Land at the nearest suitable airport. While en route to the airport of intended landing, reduce the  electrical load as much as practical. Turn off all unnecessary  ele ctrical items, such as duplicate radios, non-essential  lig h ting, etc. If unable to turn off radios, lights, etc., manually,  consider pulling circuit breakers to isolate those pieces of  eq u ipment from the electrical system. Maximum time of  useful voltage may be between 30 and 40 minutes and is  influenced by many factors, that degrade the useful time.  One techn ique for conserving the main battery c harge is  to fly the  aircraft to the airport of intended  landing while  operating  with minimal power. If a two-position battery  master/alternator rocker switch is installed, it can be utilized  to isolate  the main battery from the electrical  system and  conserve power. [Figure 11-5] With the increase in electrical components being installed  in  modern technically advanced aircraft, the power supply  an d the charging system need increased attention and  Figure 11-5.  Double rocker switch seen on many aircraft. Figure 11-4. Ammeter (left) and loadmeter (right). 6 7 5 4 3 IN. Hg. + 60 - 60 30 30 0 A M P V A C ALT AMPS  0  30  60  Ammeter  Loadmeter  11-5