One Tethercell per device!
Tethercell is extremely simple to install. First, remove a single AA-battery from whichever device you choose. Next, make sure that Tethercell already has a AAA battery installed within it, then replace the removed AA battery with Tethercell. No matter how many AA batteries are required to run the device, you only need one Tethercell. After the Tethercell is snug in the battery compartment, turn on the device, fire up the Tetherboard app after downloading it free from the App store and you're ready to connect from any iOS or Android device that is Bluetooth Smart compatible.
The Tethercell Mobile App
You can think of The Tethercell App as your Tethercell control center. From the Tethercell App you can easily connect to and control all active Tethercells in range straight from this intuitive interface which is freely downloadable for supported devices. Tethercell's range is approximately 100 feet, line of sight (outdoors), and approximately 50-60 feet indoors.