I don't own the Soundsync, and only know what is written above. Suggestion: You'll need a passive pass-thru battery, Anker does not make, which will power the Sound Sync regardless of what power is coming or not from the vehicle. You'd still need the pass-thru battery to recharge without itself needing to be replugged.
Pass-thru batteries are more expensive and so tend to be smaller capacity.
Anker's older batteries had pass-thru but it aged them so Anker removed it in newer version.