CowBoom offers the Energizer PowerSleeve for iPhone 4, model no. AP1201, for $19.99 with $3.74 for shipping. That's ties our mention from last week as the lowest total price we could find by $12. 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.