The official FCCID manual is available here: User-manual-5878746.pdf.
The manual mentions "keeping dry," but it does not stress that the Micro-USB port is susceptible to pocket lint. If your headset charges intermittently, use a wooden toothpick to gently scrape lint out of the port. This resolves 90% of "dead battery" complaints.
Sites like , Made-in-China , or Global Sources often host product manuals for their listings.
If the standard search fails, use specific operators. Type this directly into Google: "BT-5908" filetype:pdf or intitle:"BT-5908" manual