Digital Talking Book Machine (cont)
The battery pack comprises 6 Nickel Metal Hydride (NiMH) cells, a self resetting fuse and
a 70°C thermostat for safety, and a thermistor for end of charge detection. The nominal
capacity of the battery is 2000mAH which will provide approximately 35 hours of play time
(at normal speed and normal volume settings). To preserve the service life of the battery,
a charge cycle is initiated only if the remaining capacity is estimated to have fallen to
less than 75 percent of full capacity. (Note: Unlike many simple consumer devices, the battery
is not recharged on application of AC power).
The player continually monitors the charge state of the battery pack. The battery’s self
discharge is estimated and is used to modify the capacity measurement.
The player reports the remaining capacity in number of hours of play time available unless
this is greater than 75% of the full capacity in which case the player reports “greater than H hours”
(where H is 75% of the number of hours that the player can play on battery). When the player
estimates that the capacity is than half an hour, the announcement will be “Battery Low”. If
the battery pack has been depleted so that the voltage is less than 6 Volts, and the player
is powered on using battery power, the message “Battery Low, Powering Off” is played and the player
will immediately power off. Due to the uncertainty of the capacity measurement the player underestimates
the actual capacity.
It is estimated that the battery pack will have a service life in excess of 600 charge cycles.
Under moderate battery usage (4 hours per day for 5 days per week), the battery life should
exceed 4 years. At the end of its service life the capacity of the battery pack will be 75
percent of its initial value. A count of the number of charge cycles for the current battery
pack is kept by the player. This may be read by library staff through a diagnostic function.
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