Battery on the Moto X ( if used as a music player ) wil outlast most DAPs on the market bar Sony.
Sound quality is excellent too, read my post to see what I've compared it to
Only drawback might be memory capacity of mSD card as I only tried it up to 128gb so unsure if could use anything larger.
To be honest if I was thinking of getting all my music into the car it would be via a portable USB drive into my car stereo via USB ( if available ) and power it with a USB hub if necessary.
Edit: battery usage in the car isn't a problem as it can be kept in charge the whole time via USB if power output is there.