AFGS�87219B APPENDIX A
e. The crew should be provided with means to disconnect the battery so it can be electrically isolated in the event of battery failure. This provision also helps to prevent excessive and inadvertent discharge of the battery on the ground. It may be necessary, however, for some circuits to remain connected to the battery at all times.
f. Effective on-board charging is usually required to keep secondary batteries adequately charged. For lead-acid batteries this can be accomplished simply by "floating" the battery on a constant-potential DC bus. This not only provides for continuous maintenance of charge, but also allows the battery to supply power instantly to the bus when required. Charging of nickel-cadmium batteries is not so simple since they are very sensitive to applied voltage. If the voltage is too low the battery will not become fully charged. If it is too high, the battery will become overcharged with excessive heating and loss of electrolyte. Furthermore, the optimum charging voltage varies inversely with temperature. This can lead to a catastrophic failure (thermal runaway) when a battery overheats and then draws ever-increasing current as its terminal voltage decreases further. Consequently, it is not good practice to float a nickel- cadmium battery directly on a high current DC bus. Better control of the charging current is needed.
g. A generally accepted procedure for charging nickel-cadmium batteries is to supply a constant-current charge until a predetermined, temperature-compensated battery voltage is reached. At that point charging is terminated or reduced to a small trickle- charge rate. The "constant" current may be a continuous direct current or a series of pulses of a controlled average value. In addition, charging is terminated when battery temperature becomes excessive. This method of charge control requires sensing of battery temperature which is not provided by MIL-PRF-81757. Therefore, temperature sensors must be added to the standard nickel-cadmium battery for this purpose.
The following factors require careful consideration in designing the battery installation:
1) Because of the requirement for frequent maintenance, the battery should be readily accessible for inspection and removal.
2) The potential hazard of a battery failure should be recognized and provided for.
Failure effects can range from excessive gassing to complete thermal destruction.
3) The possibility of hydrogen gas evolution requires adequate ventilation of the battery to prevent explosive concentrations from developing.
4) The sensitivity of batteries to temperature and other environmental factors should be considered.
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