If a scenario arises in which you cannot recharge your cell phone, but the phone infrastructure is still up-and-running, this Dark Energy Reservoir portable charger is a good choice for your Go Bag.
One spec that stands out – it retains full capacity for 1 year if unused.
It costs $129 and is slightly larger than an iPhone. For most smartphones it can keep them charged for a week. In a survival situation (where you are super conscious of battery use) that could work out to be a month rather than a week. See this list for how many times your device can be charged.