try here…
i use an iphone and there are numerous case/chargers and external charge packs that increase cell life. right now i have an external unit i charge on the usb of my pc and connect to my phone and it adds about 6-8 hours of use. i’ve also had charger cases that has a battery in the case that boosts usage to about twice the norm.
http://www.batteries4less.com/contents/cellular-chargers/cellular-chargers.html
oops
sorry i used the wrong link
http://www.amazon.com/Energizer-Energi-Portable-Charger-Verizon/dp/B003Y3BD3S
simple
when i go on multi day trips with friends i take a cooler radio that is 4 6.5 speakers, 250 watt amp, led lights, 300 laser projector ( for those foggy nights).
i can go four days of partying hard radio all the way up at night and of course charging the ipod or iphone while its playing.
i use a deep cycle marine battery that you will buy for your boat. the only reason i use such a large battery is for the long trips. i could actually use a smaller battery. since you are only charging your phone and have the space in a canoe all you have to do is buy a lawnmower battery and a cigerette lighter outlet attached to gater clips. this total set up will run you about $50 and wont even weigh close to what the large marine battery weighs.
try buying the lawnmower battery from wal-mart and charge your phone with it for a week. see if it will do what you want and go from there. its rechargeable and effiecent. plus put it in your lawnmower when done probably needed a new one for the year anyway.
hope this helps.