Example biosensor devices having wake-up batteries and associated methods are disclosed. One example device includes a biosensor that has a first electrode for insertion into a subcutaneous layer beneath a patients skin, and a second electrode coupled to the first electrode for insertion into the subcutaneous layer, and a first battery to apply a voltage across the first and second electrodes, the first battery at least partially electrically decoupled from the electrodes. The device also includes a second battery having an anode material coupled to the first electrode for insertion into the subcutaneous layer, and a portion of the second electrode. The second battery is activatable upon immersion in an electrolytic fluid. The device also includes a wake-up circuit to receive a signal from the second battery and, in response, to electrically couple the first battery to the first and second electrodes to activate the biosensor.