Bluetooth needs battery power at both ends, the phone and the Paired-device. Also that BT transmission back to the phone will require quite a lot of battery power as transmitting (Tx) radio signals is a power hungry operation. Receiving (Rx) is about 1/20th to 1/50th of the Tx power.
Depending on what you are trying to achieve, there may be better technology to do what you require. NFC?? RFID??