I do also recommend buying a secondary phone battery as a backup.
You will need an external phone battery charger for convenience.
I think that having an infinite battery backup is good, so I have purchased eneloop kbc-e1 boost charger.
It uses 2 eneloop aa batteries, and can charge the batteries also. You can have as many alkaline or eneloop with you as power source.
Get rid of cheap 2aa battery boosters, they are crap.
As an alternative you can get energizer, duracell , camelion 2 battery boosters.
