Newegg has a small rechargeable power supply that works with all versions of iPhone and all iPods with the 30-pin dock connectors. The Miccus ChargeBlock CBFI-01 has an 800 mAh rechargeable lithium ion battery that lasts 500+ charges. It charges with the included mini-USB cord, and outputs 5V/+/-0.25V. It has automatic on/off charging that prevents overcharging your device battery. The Miccus ChargeBlock has LEDs that indicate charging status and shows the amount of power it has remaining. The Miccus ChargeBlock CBFI-01 is $19.99, with free shipping in the continental US.
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Now that’s most useful and love that it’s compact and cheep!
I bought two of these and they are awesome. One of these fully charged gives me a half charge on my dead iPhone 4. Lifesaver! These are very compact and don’t stick out of your phone too much. You can charge and continue talking without problems. If you need a full charge on your phone, i suggest looking for a battery pack with at least a 1200mah capacity.