Yes, I know these guys, I have done quite a lot of research so far! I find their system is pretty expensive and I don't think their battery has a built-in heater, which means no solar charging below 0 degrees when it is sunny, which would be quite often for me in winter. Quite interested to know what else you discussed with them. Did you talk about the EHU built-in charger and whether it works fine with the Lithium battery?
Something else that might interest you (and also others trying to find out about upgrading to lithium on a T6.1)...I had a catalog from my local AMAG (the official VW dealer in Switzerland) that had a 100Ah LiFePo4 battery (with heater, I think) upgrade with the same Voltronic charger for "only" 1750chf. This surprised me a lot since AMAG usually rip you off something proper. Anway, I emailed them because I was convinced it could not be correct. Below is their reply in German and google translated to English:
The battery is our own, which we have manufactured for assembly and use in California.
A charging booster from Votronic is installed
Normally the isolating relay is removed. For the T6.1, the software 0503 must be installed on the KFG (i think KFG just means the van).
The original charger is not replaced, just a pin is connected in a different location (talking about the EHU charger here).
The voltage and current work on the display, but the capacity is often incorrect. But we have our own “solarpeak” Bluetooth app
There is too little space for a Victron charging booster. In principle, you can also use a Victron charging booster. (I asked if they could use a victron booster instead).
Original in German:
Die Batterie ist unsere eigene, welche wir für die Montage und Verwendung im California herstellen lassen.
Es wird ein Ladebooster von Votronic eingebaut
Normalerweise wird das Trennrelais ausgebaut. Beim T6.1 muss die Software 0503 auf dem KFG installiert werden
Das originale Ladegerät wird nicht ersetzt, lediglich ein Pin wir an einem anderen Ort angeschlossen
Die Spannung und der Strom funtkioniert auf der Anzeige, bei der Kapazität stimmt es oft nicht. Wir haben aber eine eigene «solarpeak» Bluetooth App
Der Platz für einen Victron Ladebooster ist zu knapp. Grundsätzlich können Sie aber auch einen Victron Ladebooster verwenden.
Further research on "Solar peak" turned up this site and so this must be the battery:
https://solarpeak.ch/Solarpeak-LiFePo4-128V-100Ah-200A-BMS-winter
It is also interesting to note that they do NOT change the EHU charger, just change a pin (no idea what this means). They also need to change the software, but it is not clear why. This was quite interesting for me and is the first time I have had some solid info from a VW approved source.