So it looks like you can just buy a Spectra S2 plus a $30ish rechargeable battery pack like this:
https://www.amazon.com/gp/product/B00ME3ZH7C?colid=YVPNAC4B2U7O&coliid=IGYA7P8LHS5J1&ref=br_mb_ov_item_detail
Is there something I'm missing, or why would it make sense to pay big upgrade fees for the S1 then?