hereinafter , an embodiment of the present invention will be described in detail with reference to the accompanying drawings . fig1 illustrates an exemplary configuration of a cellular mobile phone in accordance with the present invention . the cellular mobile phone 1 shown in fig1 includes an antenna 2 , a transmit / receive unit 3 , a control unit 4 , a display unit 5 , a battery 6 , and a temperature detection unit 7 . the transmit / receive unit 3 modulates a signal outputted from the control unit 4 to send the modulated signal via the antenna 2 , while demodulating a signal inputted from the antenna 2 to supply the demodulated signal to the control unit 4 . the control unit 4 controls standby and talk modes , and also calculates an available time for the cellular mobile phone 1 and performs a user - data backup process . the control unit 4 includes a non - volatile internal memory 4 a in addition to a volatile memory in which user data is temporarily recorded . the non - volatile internal memory 4 a is a memory in which a backup of the user data may be stored , and may be a flash memory , for example . the display unit 5 displays information such as a telephone number which is inputted from the control unit 4 , and also displays the available time calculated by the control unit 4 . the battery 6 supplies power for the entire cellular mobile phone 1 . the temperature detection unit 7 detects temperature near the battery 6 and supplies the control unit 4 with a signal that indicates the detected temperature . further , a non - volatile external memory 8 is removably connected to the control unit 4 . the external memory 8 serves as another memory in which the backup of the user data may be stored . the transmit / receive unit 3 and the control unit 4 are mounted on a single semiconductor integrated circuit chip . fig2 is a conceptual diagram of a mobile communication network in which the cellular mobile phone 1 shown in fig1 is used . in fig2 , the reference numerals 10 and 11 denote radio channels and radio base stations , respectively . the reference numeral 12 denotes mobile communication control stations , which control the radio base stations 11 . the reference numeral 13 denotes a common channel signaling network , which connects the mobile communication control stations 12 with each other . the reference numeral 14 denotes a home memory , which manages information on numerous cellular - phone subscribers . the home memory 14 can be used by the subscribers taking along the cellular mobile phone 1 even as they move from area to area covered by any of the radio base stations 11 , and functions as still another memory in which the user - data backup may be stored . the battery 6 shown in fig1 has a threshold voltage voff upon which whether the battery 6 is usable or not depends . at the point in time that the terminal voltage v of the battery 6 becomes the threshold voltage voff , there is slight capacity left in the battery 6 . however , once the terminal voltage v of the battery 6 has dropped below the threshold voltage voff , the terminal voltage v decreases rapidly and sharply , which may cause overdischarge ; therefore such capacity cannot be used . specifically , when the terminal voltage v of the battery 6 reaches the threshold voltage voff , the battery 6 is no longer able to supply voltage , which means that the capacity of the battery 6 that has been used until the terminal voltage v of the battery 6 reaches the threshold voltage voff is the available capacity thereof the available capacity of the battery 6 normally changes due to its deterioration and temperature changes . fig3 shows an example of changes in characteristics of the battery 6 which are attributable to its deterioration . the solid line indicates terminal - voltage and available - capacity characteristics when no deterioration occurs , in which case the available capacity is q 1 . the broken line indicates the characteristics when slight deterioration occurs , and the available capacity in this case is q 2 . the alternate long and short dash line indicates the characteristics when severe deterioration occurs , and the available capacity in this case is q 3 . in the graph , q 1 & gt ; q 2 & gt ; q 3 , which indicates that the available capacity of the battery 6 decreases as the battery 6 deteriorates . fig4 shows an example of changes in characteristics of the battery 6 caused by temperature changes . the solid line indicates terminal - voltage and available - capacity characteristics at room temperatures , in which case the available capacity is q 4 . the broken line indicates the characteristics at a low temperature , in which case the available capacity is q 5 . in the graph , q 4 & gt ; q 5 , from which it is found that the available capacity of the battery 6 at a low temperature is smaller that that at room temperatures . hereinafter , available - time display and user - data backup in the portable cellular phone 1 shown in fig1 will be sequentially described in detail . it should be noted that a suitable one of the internal memory 4 a , external memory 8 , and home memory 14 can be selected as a memory in which backup of user data is stored . fig5 is a flow chart illustrating process steps performed in the control unit 4 when an available time is calculated in the cellular mobile phone 1 shown in fig1 . the reference numeral 100 in fig5 denotes the step of measuring the terminal voltage v ( n ) and current i ( n ) of the battery 6 . the reference numeral 101 denotes the step of calculating capacity δq ( n )(= i ( n )× t ) used per unit time t . the unit time t is an amount of time necessary to return to step 100 . the reference numeral 102 denotes the step of calculating remaining capacity q ( n )(= q ( n − 1 )− δq ( n )) of the battery 6 . the reference numeral 103 denotes the step of updating data which indicates the remaining capacity q of the battery 6 in terms of the terminal voltage v of the battery 6 , and correcting the data using the rate k of change of voltage in the battery 6 in accordance with a temperature detected by the temperature detector 7 ( see fig4 ). fig6 shows results of step 103 . in fig6 , v ( 0 ) and q ( 0 ) represent initial values immediately after the battery 6 has been charged . the reference numeral 104 in fig5 denotes the step of calculating remaining available capacity q *( n )(= q ( n )− qoff ) of the battery 6 . in this step , the remaining available capacity q *( n ) is calculated by obtaining threshold remaining capacity qoff of the battery 6 in terms of the threshold voltage voff , at which the battery 6 becomes unusable , from the previously recorded data that indicates the remaining capacity q of the battery 6 in terms of the terminal voltage v of the battery 6 and that has been corrected in consideration of the temperature . the reference numeral 105 indicates the step of calculating an available time t ( n )(= q *( n )/ i ). in this step , necessary current i is obtained based on the display mode ( standby , talking , mail - receiving , mail - sending , internet - connection , or other mode ) and the radio - wave receiving condition at the time of the calculation . fig7 shows a current table which is referred to in step 105 . backup modes shown in fig7 will be described later . the reference numeral 106 in fig5 denotes the step of outputting the available time t ( n ) to the display portion 5 . thereafter , the operations in steps 100 through 106 are repeated . as described above , in accordance with the process steps shown in fig5 , the data that indicates the remaining capacity q of the battery 6 in terms of the terminal voltage v of the battery 6 in the cellular mobile phone 1 is updated when necessary , which allows the cellular phone 1 to accurately calculate and display the available time t ( n ). the radio base station 11 shown in fig2 , instead of the cellular mobile phone 1 shown in fig1 , can also perform the above - described available - time calculation process . fig8 is a flow chart illustrating process steps performed in the control unit 4 of the cellular mobile phone 1 when the radio base station 11 shown in fig2 carries out available - time calculations . the reference numeral 200 in fig8 indicates the step of measuring the terminal voltage v ( n ) and current i ( n ) of the battery 6 . the reference numeral 201 denotes a step in which the measured terminal voltage v ( n ) and current i ( n ) of the battery 6 , a temperature detected by the temperature detecting portion 7 , and the radio - wave receiving condition in the mobile phone 1 are outputted to the transmitter - receiver 3 , thereby providing notification thereof to the radio base station 11 . the reference numeral 202 denotes the step of determining whether the transmitter - receiver 3 has received an available time t ( n ) calculated by the radio base station 11 . the reference numeral 203 denotes the step of outputting the available time t ( n ) to the display unit 5 in the case where it has been determined in step 202 that the available time t ( n ) has been received . thereafter , the operations in steps 200 through 203 are repeated . fig9 is a flow chart illustrating process steps carried out in the radio base station 11 shown in fig2 when the radio base station 11 performs the available - time calculations . the reference numeral 300 in fig9 denotes the step of receiving the information , that is , the terminal voltage v ( n ) and current i ( n ) of the battery 6 , the temperature detected by the temperature detection unit 7 and the radio - wave receiving condition in the cellular mobile phone 1 ., transferred by the cellular mobile phone 1 . the reference numerals 301 through 305 indicate steps which correspond to steps 101 through 105 shown in fig5 . it should be however noted that data that indicates the remaining capacity q of the battery 6 in terms of the terminal voltage v of the battery 6 is recorded in the home memory 14 . the reference numeral 306 denotes the step of transmitting the available time t ( n ) to the transmit / receive unit 3 of the cellular mobile phone 1 . as described above , in the process steps shown in fig8 and 9 , the data recorded in the home memory 14 and indicating the remaining capacity q of the battery 6 in terms of the terminal voltage v of the battery 6 is updated as needed , such that the radio base station 11 can calculate the available time t ( n ) accurately and display the calculated available time t ( n ) on the display 5 of the handheld phone 1 . fig1 is a flow chart illustrating process steps performed in the control unit 4 when a user - data backup process is carried out in the cellular mobile phone 1 shown in fig1 . the reference numerals 400 through 404 shown in fig1 denote steps which correspond to steps 100 through 104 shown in fig5 . the reference numeral 405 denotes the step of calculating battery capacity qbackup (= ibackup × tbackup ) needed to make a backup of user data . in this step , current ibackup and time tbackup required to make the backup are calculated by considering which one of the internal memory 4 a , home memory 14 and external memory 8 is used as a memory into which the backup of the user data is stored , and the wave intensity at the time of the calculation . the magnitude of current necessary in each backup mode with respect to the wave intensity is as shown in fig7 . the reference numeral 406 shown in fig1 denotes the step of making a determination as to whether the remaining available capacity q *( n ) of the battery 6 is sufficiently larger than the capacity qbackup required for making the backup . the reference numeral 407 indicates the step of performing the user - data backup process when the remaining available capacity q *( n ) of the battery 6 becomes substantially equal to the capacity qbackup necessary to make the backup . if , in step 406 , the remaining available capacity q *( n ) is determined to be sufficiently large , the backup process is deferred for a return to step 400 , and then the operations in steps 400 through 406 are repeated . in this manner , the control unit 4 controls frequency of the user - data backup process in order to reduce it in accordance with the magnitude of the remaining available capacity q *( n ) of the battery 6 , so that the consumption of the battery 6 is controlled . fig1 is a flow chart illustrating the details of step 407 of the backup process in fig1 . the reference numeral 500 shown in fig1 represents the step of determining which one of the internal memory 4 a , home memory 14 and external memory 8 is used as a memory in which the backup is stored . the reference numeral 501 denotes a step in which when it has been determined in step 500 that the backup is stored in the internal memory 4 a , a determination is made as to whether the remaining capacity of the internal memory 4 a is larger than the capacity of the user data ( backup data ) to be backed up . the reference numeral 502 indicates a step in which when the determination has been made in step 501 that the remaining capacity of the internal memory 4 a exceeds the backup data capacity , the backup data is outputted to the internal memory 4 a . the reference numeral 503 refers to a step in which when it has been determined in step 501 that the remaining capacity of the internal memory 4 a is smaller than the capacity of the backup data , unnecessary data in the internal memory 4 a is deleted . the process of deleting the unnecessary data in the internal memory 4 a will be described later . the reference numeral 504 represents the step of determining whether the backup becomes possible . if it has been determined that the backup is able to be made , the backup data is outputted to the internal memory 4 a in step 502 . the reference numeral 505 denotes a step in which when the determination has been made in step 504 that the backup is unable to be made , a message which indicates that the backup is not possible is outputted to the display portion 5 . the reference numeral 506 shown in fig1 denotes a step in which in the case where the determination has been made in step 500 that the backup is not retained in the internal memory 4 a , it is determined whether the home memory 14 or the external memory 8 is used as a memory in which the backup is stored . the reference numeral 507 represents a step in which when it has been determined in step 506 that the backup is stored in the external memory 8 , whether the remaining capacity of the external memory 8 is larger than the capacity of the backup data is determined . the reference numeral 508 denotes a step in which when it has been determined in step 507 that the remaining capacity of the external memory 8 exceeds the backup data capacity , the backup data is outputted to the external memory 8 . the reference numeral 509 refers to a step in which in the case where the determination has been made in step 506 that the remaining capacity of the external memory 8 is smaller than the backup data capacity , it is determined whether data in the external memory 8 may be deleted . the reference numeral 510 denotes a step in which when it has been decided in step 509 that the data in the external memory 8 may be deleted , such data in the external memory 8 is deleted . the reference numeral 511 indicates the step of determining whether the backup becomes possible . if it has been decided that the backup is able to be made , the backup data is outputted to the external memory 8 in step 508 . the reference numeral 512 refers to a step in which in the case where it has been determined in step 509 that the data in the external memory 8 should not be deleted , or in the case where it has been determined in step 511 that the backup is not possible , a message which indicates that the backup cannot be made is outputted to the display unit 5 . the reference numeral 513 denotes a step in which when the determination has been made in step 506 that the backup is stored in the home memory 14 , the backup data is outputted to the transmit / receive unit 3 . the backup data outputted to the transmit / receive unit 3 is transferred to and retained in the home memory 14 . fig1 is a flow chart illustrating the details of step 503 , the unnecessary - data deleting step , shown in fig1 . in fig1 , the reference numeral 600 denotes the step of determining whether a plurality of the same telephone numbers have been registered . the reference numeral 601 denotes a step in which when it has been determined in step 600 that the same telephone numbers have been registered , all such numbers except one are deleted . the reference numeral 602 denotes the step of making a determination as to whether the remaining capacity of the internal memory 4 a is larger than the capacity of the backup data . if it has been determined in step 602 that the remaining capacity of the internal memory 4 a is greater than the capacity of the backup data , the process of deleting the unnecessary data in the internal memory 4 a is ended . the reference numeral 603 denotes a step in which when the determination has been made in step 602 that the remaining capacity of the internal memory 4 a is smaller than the capacity of the backup data , it is determined whether the history list of sent mail contains a mail message whose history has not been stored . the reference numeral 604 represents a step in which in the case where it has been determined in step 603 that the sent - mail history list includes a mail message whose history has not been stored , the history of such sent mail message is deleted . this step allows the unnecessary data to be deleted in the internal memory 4 a . if it has been determined in step 603 that there is no sent mail message whose history has not been stored , the process of deleting the unnecessary data is ended . as described above , in the process steps shown in fig1 , if the remaining capacity of the internal memory 4 a is insufficient , the unnecessary data in the user data is found and deleted , such that the memory remaining capacity required to make the backup can be obtained . in place of the cellular mobile phone 1 shown in fig1 , the radio base station 11 shown in fig2 can also perform the above - described backup determination process . fig1 is a flow chart illustrating process steps performed in the control unit 4 of the portable cellular phone 1 in a case where the radio base station 11 shown in fig2 performs a user - data backup determination process . the reference numeral 700 in fig1 denotes the step of measuring the terminal voltage v ( n ) and current i ( n ) of the battery 6 . the reference numeral 701 denotes a step in which the measured terminal voltage v ( n ) and current i ( n ) of the battery 6 , a temperature detected by the temperature detector 7 , and the state of radio waves received by the cellular mobile phone 1 are outputted to the transmit / receive unit 3 , thereby providing notification thereof to the radio base station 11 . the reference numeral 702 denotes the step of determining whether the transmit / receive portion 3 has received “ continuous - use permitting information ” transmitted by the radio base station 11 . if it has been determined in step 702 that the continuous - use permitting information has been received , the process returns to step 700 , and then the operations in steps 700 through 702 are repeated . the reference numeral 703 denotes a step in which in the case where it has been determined in step 702 that the continuous - use permitting information has not been received , a determination is made as to whether the transmitter - receiver 3 has received a “ backup command ” sent from the radio base station 11 . if the determination has been made in step 703 that the backup command has not been received , the process returns to step 702 . the reference numeral 704 indicates a step in which when it has been determined in step 703 that the backup command has been received , backup operation is performed . the details of the backup operation are as described in fig1 and 12 . fig1 is a flow chart illustrating process steps performed in the radio base station 11 shown in fig2 when the radio base station 11 performs the user - data backup determination process . the reference numerals 800 through 804 shown in fig1 indicate steps which correspond to steps 300 through 304 shown in fig9 . data that indicates the remaining capacity q of the battery 6 in terms of the terminal voltage v of the battery 6 is recorded in the home memory 14 . the reference numerals 805 and 806 shown in fig1 refer to steps which correspond to steps 405 and 406 shown in fig1 . the reference numeral 807 shown in fig1 denotes a step in which when the remaining available capacity q *( n ) of the battery 6 becomes substantially equal to the capacity qbackup necessary to make the backup , the backup command is transmitted to the transmitter - receiver 3 of the cellular mobile phone 1 . the reference numeral 808 denotes a step in which when it has been determined in step 806 that the remaining available capacity q *( n ) is sufficiently large , the continuous - use permitting information is transmitted to the transmit / receive unit 3 of the cellular phone 1 so that the backup operation is delayed . it should be noted that in the process step of deleting the unnecessary data in the internal memory 4 a , although the registered same telephone numbers and the sent - mail history that has not been stored are deleted in this order in fig1 , this order may be changed in accordance with user settings .