J&R offers the Energizer PowerSleeve for iPhone 4, model no. AP1201, for $29.99 with $2.99 for shipping. (Buy.com charges a penny more.) That’s the lowest total price we could find by $5, although we saw it for $8 less in March. This non-slip and fingerprint-resistant silicone case charges an iPhone via its built-in 1500 mAh rechargeable battery; it detects when the phone is low on battery life, then automatically charges it.