I have purchased Mophie brand at
Best Buy (making them price match
Amazon). I also purchased Anker at Target. Both worked fine but both were heavier than I would have liked. You need to do the math - google the mAh of your phone, then look at the mAh of the chargers you are considering.
For example, if your phone is 2942 and your charger is 3200, then if you let your phone battery get fully run down the charger will be able to fully charge it once and then charge it a little bit the second time. After that, you would need to charge the charger.