J&R offers the Energizer PowerSleeve for iPhone 4, model no. AP1201, for $24.99 with free shipping. (Click through to see this price.) That's the lowest total price we could find by $8, although we saw it for $5 less last month. This non-slip and fingerprint-resistant silicone case charges an iPhone via its built-in 1,500 mAh rechargeable battery; it detects when the phone is low on battery life, then automatically charges it.