Run a Cig lighter plug from battery and just charge phone from car charger.
This is essentially what I run on the bike but when on foot such as hiking it's not really an option. backup batteries can help get me an extra day out of the phone if needed. Turning off non-essential stuff on the phone like bluetooth, wi-fi, ect really helps extend battery life too. The biggest one is turning off the "radio" portion of the phone that actually recieves and xmits to the cell towers. Turn that off and your smart phone will last a long while. Even while playing music.