Aisle5Deals offers the uNu Power DX Plus 2,400mAh Rechargeable Battery Case for iPhone 4 and 4S in Black/Silver, model no. DX-04-2400S, for $39.49 plus $3.50 for shipping. That's tied with last month's mention and the lowest total price we could find by $12. It features an integrated LED power gauge and stores power for up to 10 hours of talk time and 9 hours of 3G web browsing (up to 14 hours on WiFi). Deal ends January 20.