Victron Energy's VE.Bus Smart Dongle is really two products in one:
First, it is the bluetooth accessory for Victron's range of VE.Bus inverter/chargers: MultiPlus, Quattro, MultiPlus-II, MultiGrid, MultiGrid-II, and the range of VE.Bus Phoenix Inverters. With the VE.Bus Smart Dongle correctly installed, you can use Victron's free app, VictronConnect, to monitor the connected device's operation status and also access limited remote control functions: change the AC Input current limit (if applicable) as well as switch between Off, On, Invert-Only and Charge-Only modes (some features are model- and system-specific).
Second, the VE.Bus Smart Dongle works as a voltage- and temperature-sense device for the connected inverter/charger. This works with all models when properly installed and connected.
For very basic monitoring, this device is a great option for small systems on which a GX device may not be practical or necessary.
