I'm afraid the Bosch system is over-integrated/locked up, with Bosch ensuring lack of spare parts. Not quite sure how that squares with the EU reparability directives but there you go.
You could try Jimmy at ebikebatteries (
https://ebikebatteries.co.uk/), but he is just scaling down, and probably couldn't have helped anyway.
I'm afraid a new battery is likely to be your only hope. If the old one is still good apart from the BMS you will have some decent quality used cells to sell on, but probably not much more than that.
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If I'd known more when I bought our Bosch powered Motus I wouldn't have bought it, for that and related reasons. It's been excellent apart from very poorly designed connections for the battery and the display; but I'm dreading the time it goes seriously wrong or I need a new battery.