In order to conserve battery power, transmissions only occurred at a set interval of time. The following power budget was used in order to figure out how often to transmit.
Any device that used current from the buoy's gel batteries must be factored into the power budget. Because the VHF Marine Transceiver is powered by its own battery pack, only devices in Component 1 need to be considered. See the block diagram for device details. Listed below, in Table 1, is each device and the amount of current it uses.
|Table 1: Current Specification of System Devices|
4.0 mA always
|GPS Module||100 mA always|
|Radio Modem||1.5 A during transmit, 10 mA on standby|
Assuming that transmission takes no longer than 2 seconds, take the interval between transmissions to be 30 seconds. Therefore, the duty cycle will be 2/32 for these parameters. The 12V battery is 3.2 A-Hrs. Therefore, to find out how long the system will last, do the following calculations:
Current always drained = (4.0 + 100 + 10)mA = 114 mA = 0.114 A
Current during TX = 1.5A x Duty Cycle = 1.5 x 2/32 = 0.09375 A
Total Current = Current always drained + Current during TX = 0.114 A + 0.09375 A = 0.2078 A
Battery Life = (3.2 A-Hr)/(Total Current) = (3.2)/(0.2078) = 15.4 Hrs
Therefore, if the transmission interval is 30 seconds, the system will last 15.4 Hrs, which is an adequate length of time for one day of ocean current research, which is how long the buoy will be used before its batteries will be recharged.