Clients original installer has disappeared, he has a Multiplus II installed as a battery back up system and now wants to hook up solar panels, I thought that I would need a GX device, although, according to the above diagram, all I need is an mppt.
Again a Deye/Sunsynk installer, so any advice would be much appreciated
just get a GX device, it just ties everything together.
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You can do it without, but you ideally want communication between the Multiplus, MPPT and battery and for that you need a GX.
At the price of a Cerbo GX these days, it is almost not even worth mucking around with a Venus PI, just buy the proper thing.
Remember the communication cables, what battery do the client have, can it communicate with Victron?
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He has lead acid ( silver calcium )
Since you asked me personally to look at it… 
I looked at it yesterday and the advice you got is fine already… there wasn’t really a need for me to reply
Yes, you can throw in just an MPPT and configure it for the voltage requirements of the lead acid bank. It will work just fine. You won’t get any nice reporting though, the user will have to manually connect to the thing with VictronConnect to check how much it charged today, the voltage, etc etc.
The customer will be happier in the long run with the additional GX box added.