state of charge, water should never be added while the battery  is installed in the aircraft. Overfilling the battery results in  electrolyte spewage during charging. This causes corrosive  effects on the cell links, self-discharge of the battery, dilution  of the electrolyte density, possible blockage of the cell vents,  and eventual cell rupture. Constant current battery chargers are usually provided for  NiCd batteries because the NiCd cell voltage has a negative  temperature coefficient. With a constant voltage charging  source, a NiCd battery having a shorted cell might overheat  due to excessive overcharge and undergo a thermal runaway,  destroying the battery and creating a possible safety hazard  to the aircraft. Pulsed-current battery chargers are sometimes  provided for NiCd batteries. CAUTION: It is important to use the proper charging  procedures for batteries under test and maintenance. These  charging regimes for reconditioning and charging cycles  are defined by the aircraft manufacturer and should be  closely followed. Aircraft battery inspection consists of the following items: the airplane is in flight. This helps to ensure a continuous  flow of air across the top of the battery through the sump  and outside the airplane. The acid fumes going into the  sump are neutralized by the action of the soda solution, thus  preventing corrosion of the aircraft’s metal skin or damage  to a fabric surface. Modern airplanes are equipped with battery ventilating  systems. The ventilating system removes gasses and acid  fumes from the battery in order to reduce fire hazards and  to eliminate damage to airframe parts. Air is carried from a  scoop outside the airplane through a vent tube to the interior  of the battery case. After passing over the top of the battery,  air, battery gasses, and acid fumes are carried through another  tube to the battery sump. This sump is a glass or plastic jar  of at least one pint capacity. In the jar is a felt pad about 1  inch thick saturated with a 5-percent solution of bicarbonate  of soda and water. The tube carrying fumes to the sump  extends into the jar to within about  1 ⁄ 4 inch of the felt pad.  An overboard discharge tube leads from the top of the sump  jar to a point outside the airplane. The outlet for this tube is  designed so there is negative pressure on the tube whenever  Ventilation Systems Installation Practices Aircraft Battery Inspection 1.  Inspect battery sump jar and lines for condition and  security. 2.  Inspect battery terminals and quickly disconnect plugs  and pins for evidence of corrosion, pitting, arcing, and  burns. Clean as required. 3.  Inspect battery drain and vent lines for restriction,  deterioration, and security. 4.  Routine preflight and postflight inspection procedures  should include observation for evidence of physical  damage, loose connections, and electrolyte loss. •  External surface—Clean the external surface of the  battery prior to installation in the aircraft. •  Replacing lead-acid batteries—When replacing  lead-acid batteries with NiCd batteries, a battery  temperature or current monitoring system must be  installed. Neutralize the battery box or compartment  and thoroughly flush with water and dry. A flight  manual supplement must also be provided for the NiCd  battery installation. Acid residue can be detrimental to  the proper functioning of a NiCd battery, as alkaline  is to a lead-acid battery. •  Battery venting—Battery fumes and gases may cause  an explosive mixture or contaminated compartments  and should be dispersed by adequate ventilation.  Venting systems often use ram pressure to flush  fresh air through the battery case or enclosure to a  safe overboard discharge point. The venting system  pressure differential should always be positive  and remain between recommended minimum and  maximum values. Line runs should not permit battery  overflow fluids or condensation to be trapped and  prevent free airflow. •  Battery sump jars—A battery sump jar installation  may be incorporated in the venting system to dispose  of battery electrolyte overflow. The sump jar should  be of adequate design and the proper neutralizing  agent used. The sump jar must be located only on the  discharge side of the battery venting system.  •  Installing batteries—When installing batteries in an  aircraft, exercise care to prevent inadvertent shorting  of the battery terminals. Serious damage to the aircraft  structure (frame, skin and other subsystems, avionics,  wire, fuel, etc.) can be sustained by the resultant high  discharge of electrical energy. This condition may  normally be avoided by insulating the terminal posts  during the installation process. Remove the grounding  lead first for battery removal, then the positive lead.  Connect the grounding lead of the battery last to  minimize the risk of shorting the hot terminal of the  battery during installation. •  Battery hold down devices—Ensure that the battery  hold down devices are secure, but not so tight as to  9-26