G'day and sorry for my delayed response.
The BMV-600 transmits with a TTL serial signal level of 3.3V. It does not receive any data at all, or rather, don't send any data to it. It can't do anything with any data sent to it and a 5V level signal sent to it probably won't do it any good.
The Arduino can fully interpret a 3.3V level signal as a high and therefore no level shifting will be required to talk to an Arduino. I have connected the Tx of my BMV-600 directly to the Rx of the Arduino. I am pretty sure that the same goes for the BMV-700 or BMV-702.
I realise that my very early posts in this thread discussed level shifting when communication with a proper RS-232 port on a PC. I built the level shifting circuit to interface the BMV-600's 3.3V serial to RS-232 in much the same way that Victron's $100 cable would do. I needed this facility to read the data stream into the Victron software and then into a terminal in order to begin figuring out how to get the Arduino code to work.
One of my posts on the first page of this thread has the Xively code attached to the post which you may use as an example for your project if you so wish.
This project seems quite popular in your part of the world. There is a Kiwi chap who has been running a version of my code for the last few years in his home solar setup.
