Ok so I remember we have discussed this before on the forum but I can’t find it at the moment, but it was someone trying to pair the older platform to a newer platform (V50 to B50 ?) which couldn’t be done, although they were able to link from the same family (say V50 to V90)
I think here it’s simply because of the rechargeable voltage is different from the 675 battery, plus the rechargeable does have other features that the UP doesn’t, so wouldn’t make a good match as a set.