Operation of aircraft batteries beyond their ambient  temperature or charging voltage limits can result in excessive  cell temperatures leading to electrolyte boiling, rapid  deterioration of the cells, and battery failure. The relationship  between maximum charging voltage and the number of cells  in the battery is also significant. This determines (for a given  ambient temperature and state of charge) the rate at which  energy is absorbed as heat within the battery. For lead-acid  batteries, the voltage per cell must not exceed 2.35 volts.  In the case of NiCd batteries, the charging voltage limit  varies with design and construction. Values of 1.4 and 1.5  volts per cell are generally used. In all cases, follow the  recommendations of the battery manufacturer. The battery charging system in an airplane is of the constant  voltage type. An engine-driven generator, capable of  supplying the required voltage, is connected through the  aircraft electrical system directly to the battery. A battery  switch is incorporated in the system so that the battery may  be disconnected when the airplane is not in operation. The voltage of the generator is accurately controlled by means  of a voltage regulator connected in the field circuit of the  generator. For a 12-volt system, the voltage of the generator  is adjusted to approximately 14.25. On 24-volt systems,  the adjustment should be between 28 and 28.5 volts. When  these conditions exist, the initial charging current through the  battery is high. As the state of charge increases, the battery  voltage also increases, causing the current to taper down.  When the battery is fully charged, its voltage is almost equal  to the generator voltage, and very little current flows into the  battery. When the charging current is low, the battery may  remain connected to the generator without damage. volt AC supply to the desired level before it is passed through  the rectifier. If a constant current charging system is used,  multiple batteries may be connected in series, provided that  the charging current is kept at such a level that the battery  does not overheat or gas excessively. The constant current charging method is the preferred method  for charging NiCd batteries. Typically, a NiCd battery is  constant current charged at a rate of 1CA until all the cells  have reached at least 1.55V. Another charge cycle follows at  0.1CA, again until all cells have reached 1.55V. The charge  is finished with an overcharge or top-up charge, typically for  not less than 4 hours at a rate of 0.1CA. The purpose of the  overcharge is to expel as much, if not all the gases collected  on the electrodes, hydrogen on the anode, and oxygen on  the cathode; some of these gases recombine to form water  that, in turn, raises the electrolyte level to its highest level  after which it is safe to adjust the electrolyte levels. During  the overcharge or top-up charge, the cell voltages go beyond  1.6V and then slowly start to drop. No cell should rise above  1.71V (dry cell) or drop below 1.55V (gas barrier broken). Charging is done with vent caps loosened or open. A stuck  vent might increase the pressure in the cell. It also allows  for refilling of water to correct levels before the end of the  top-up charge while the charge current is still on. However,  cells should be closed again as soon as the vents have been  cleaned and checked since carbon dioxide dissolved from  outside air carbonates the cells and ages the battery. When using a constant-voltage system in a battery shop, a  voltage regulator that automatically maintains a constant  voltage is incorporated in the system. A higher capacity  battery (e.g., 42 Ah) has a lower resistance than a lower  capacity battery (e.g., 33 Ah). Hence, a high-capacity battery  draws a higher charging current than a low-capacity battery  when both are in the same state of charge and when the  charging voltages are equal. The constant voltage method is  the preferred charging method for lead-acid batteries. Constant current charging is the most convenient for charging  batteries outside the airplane because several batteries  of varying voltages may be charged at once on the same  system. A constant current charging system usually consists  of a rectifier to change the normal AC supply to DC. A  transformer is used to reduce the available 110-volt or 220- Battery inspection and maintenance procedures vary with  the type of chemical technology and the type of physical  construction. Always follow the battery manufacturer’s  approved procedures. Battery performance at any time in a  given application depends upon the battery’s age, state of  health, state of charge, and mechanical integrity, which you  can determine according to the following: •  To determine the life and age of the battery, record the  install date of the battery on the battery. During normal  battery maintenance, battery age must be documented  either in the aircraft maintenance log or in the shop  maintenance log. •  Lead-acid battery state of health may be determined  by duration of service interval (in the case of vented  batteries), by environmental factors (such as excessive  heat or cold), and by observed electrolyte leakage (as  evidenced by corrosion of wiring and connectors or  accumulation of powdered salts). If the battery needs to  be refilled often, with no evidence of external leakage,  this may indicate a poor state of the battery, the battery  charging system, or an overcharge condition. Constant Current Charging  Battery Maintenance Constant Voltage Charging (CP) Battery Charging  9-24