Not sure what you are planning to do.
Most batteries (if you don’t buy them as “soft packs”) come with a PCM. I’ve linked to a few PCMs here in the forum a while ago. If you want to do it, better do it right. Replace the whole battery compartment with your pack that uses two cells (or a bigger one) you know about and how they work and use PCMs configured for them and that give the phone and the charging hardware in the phone correct feedback. Make the standard battery the fallback once your pack is empty, so the user can just swap it back in.
I once asked FP about the specs (Qualcomm charging). No, I never got an useful answer. And I think they are pretty concerned about their “IP” and the “IP” of their ODM. But maybe they are not allowed to talk about it. You can find a few specs online.
Anyway, I wouldn’t start it without FP’s full support as far as they can support you without violating contracts. If they want a hardware community around their phones, this information is critical.
I fear that in the end modules with not be made by users but by licensed partners. But that’s just my speculation. I have nothing to prove it. All I know is that I got no information ever.
Maybe asking during the AMA will help?