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Backup battery
When the power failure or change of the establishment place of the clock, the AC power supply is cut. It is bothersome to adjust time again when the power supply is turned on once again.
Thereupon, the clock circuit is operated with the charging style battery even if the AC power supply is cut.
A small nickel cadmium battery is used. Therefore, the electromotive force of the battery is gone right away when the whole circuits of the clock are backuped.
The circuits that backup are only the circuits that count the time. The TV tuner, indicator circuits do not backup.
The clock can not to synchronize to the high accuracy frequency of the broadcasting station while the AC power supply is suspending, because even the TV tuner has stopped. Even with, if it is not the power supply suspension in a long time there is not the problem of the error of a clock.
It is able to backup for 10 hours almost.
Charging of the battery
The charging metheod of the nickel cadmium battery with the usage like this time carries out 'trickle charging'. Excessive charging is prevented, with the method that long time charging is done with the little charging current.
The charging current makes 0.033 CmA or less. C is the authorized capacity of the battery and it is 700mAh this time.
It charges with the currents of 0.033 x 700 = 23mA or less.
One piece of the nickel cadmium battery is 1.2V. It becomes 7.2V because it makes and uses 6 pieces a series this time. The voltage that makes from the AC power supply is 12V. Therefore, it becomes the charging current of ( 12V - 7.2V ) / 470ohm = about 10mA when it charges through the resistance of 470 ohm.
The consumption electric power of the resistor is I2R. Therefore, the resistor of 1/8W is sufficient because it is ( 0.01A )2 x 470ohm = 0.047W.