Hi JimLee, internal battery and extender are both DIY, 10s3p and 10s2p for the case of the extender I have used this bottle:
I have used XT60 connectors
I would like to introduce the mtb I made for my wife
Normal version: 540 Wh battery 16.80 kg total weight with pedals
Long journey version: 900 Wh battery 18,45 Kg total weight with pedals