You also likely don't want to use System.getBatteryStatus() as you'd have to continually call this function at specific intervals.

The better way to do this would be to register for System Level events using System.addEventListener/System.removeEventListener

You should be able to drop this code into an Activity script and run on a remote. Note that you can also send data to an onscreen label with ProntoScript name of 'BATTERY'.

System.setDebugMask(9);

function myBatteryListener(aValue) {
System.print("batteryEvent:" + aValue);
var w = GUI.widget("BATTERY");
w && w.label = aValue;
}

System.addEventListener("battery",myBatteryListener);

//And, you'd likely want to remove the listener when exiting the activity to clean //things up.

// Here, CF.activity() and 'this' are the same.
this.onExit = function() {
System.removeEventListener("battery",myBatteryListener);
}