You have to use the high power 675 implant as you stated. There is currently no rechargeable option. One battery should last 2-3 days, less if you do lots of streaming from your iPhone.

I use these for my Osia and they work great (these are the ones Cochlear recommends): https://www.amazon.com/gp/aw/d/B000SZNSR4?psc=1&ref=ppx_pop_mob_b_asin_title

If you use Facebook there is a very active Osia group I recommend you join and feel free to ask my any questions, I got mine this summer!