Código para obtener el nivel de batería actual:
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//Battery level percentage
IntentFilter ifilter = new IntentFilter(Intent.ACTION_BATTERY_CHANGED);
Intent batteryStatus = context.registerReceiver(null, ifilter);
int level = batteryStatus.getIntExtra(BatteryManager.EXTRA_LEVEL, -1);
int scale = batteryStatus.getIntExtra(BatteryManager.EXTRA_SCALE, -1);
float batteryPct = level / (float)scale;
final String text = String.valueOf(batteryPct) + "%";
Toast.makeText(context, text, Toast.LENGTH_SHORT).show();
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referencia: http://developer.android.com/training/monitoring-device-state/battery-monitoring.html#CurrentLevel
