I've just read on another forum that the aux battery for the Gain and I assume other bikes using the same system is soon to be released. Some details here
Word is that its going to cost £510 , the capacity is 208 Wh, I'm guessing there must be quite a profit margin in that price.
I think the Bosch 500 Wh power tube battery is around the £650 level so not sure how they justify £510 for a battery with less than half this capacity.
The £510 is only what I have read on another forum it has not come from Orbea so the cost could be less than this.