The guys at Rodeo bike are really nice too. They have a shop in Bellwald just at the bottom of the bike park and one on the valley floor in Brig. They rent out Spesh bikes, so maybe they'll rent you a battery too. https://rodeobike.ch/ I also vouch for a visit to Bellwald. If you're staying in just 2 locations...
I wouldn't do Zermatt either, but there is plenty to do in the Valais region. Good starting point is Visp, Sierre or Martigny. Lots of small cable cars will get you half-way up the mountain. You can then add more vert by pedaling. We've got looooong descents after 👌 Lemme know if you choose Valais, I can give you info...