Andy K wrote:

My shaver has "NH" on the back, next to the recycle arrows. I assumed
it meant "Nickel Hydride", and not Lithium Ion? Plus, Lithium
batteries are way more expensive (from a manufacturing
standpoint)...why would Braun use them?

Andy, how about the factory Braun service manual? I posted this link
about a year ago:

http://63.230.9.155/braun/Srv_Man/SHAVER/

I replaced the NiMH battery in mine with an equivalent from MCM
electronics, and it has worked fine for over a year now. According to
the cross reference, your 6550 crosses to a model 5704. These are PDF
files, by the way. You'll also find service manuals and parts lists for
Braun electric toothbrushes and household appliances by clicking on the
parent directory. Hope this helps.

James