DIY Stromer Di2 Charging, Shimano

[500WhatsThat]

Having built DIY ebikes before, I have chargers and batteries along with a box of connectors and wires and am not afraid to use them.
However, the Stromer ST2 is very proprietary in its design & use.
Seems there should be a way to use a standard Lion, NIMH charger with enough specs and pin-outs known for the Stromer system.
Specifically, has anyone figured out how to charge the Di2 Shimano, without the Di2 ($$$) charger? The pins read zero volts so I assume there's a microcontroller handshake needed, or?

Any experience for the main battery would also be greatly appreciated.

Thanks in Advance