Best guess is it requires the original battery to talk with or it won't operate. There's a number of OEM systems like that; basically the owners are out of luck in these situations, unless they buy more OEM supplied batteries.
Some of them you could parallel more cells with the originals, but some of them may detect the sudden change in capacity and lock out the battery thinking something has gone wrong.
Some OEM batteires have been known to brick themselves when any of the cell groups become disconnected from the BMS, so swapping the electronics over to a new larger pack isn't easy, even if it doens't detect a change in capacity and react to it. One would have to connect the new pack in parallel with the original first, at each balance connection and hte pack mains. If desired, the original cells could then be removed.
I don't know of any that actually lock up the bike itself if a non-OEM battery is used, but it's always possible for some OEM to do that at some point.
I don't have any details on any of these systems, as I haven't dealt with any of them, but there are a number of threads on ES about battery rebuilds for various OEM bikes. Dunno if there's one for the Max.
The actual comm buss is probably "CANBUS", seems to be growing popular. But that's just a physical interface, and doesn't constrain them to any specific data or command set, so if you don't already know how to talk on their bus, and what responses to make to inquiries from controller/etc., you dont' have any way to find out other than snooping the bus and canning the interchanges under various conditions, and then storing those in an MCU lookup table for use in your own BMS for a new pack.
It might not be all that complicated, and you might be able to just leave the comm buss dsconnected. But I doubt it will work without it.
If you're lucky, you can just parallel the new main pack with the OEM pack's main leads, and let the OEM pack take care of all the comm, and let the new pack just provide more capacity. Most likely, this will leave the capacity meter display on the Max still only showing the original capacity, if it's actually coutning the charge passing thru it's BMS, and it won't show anything from the paralleled pack.
