Hi, all, the battery of my iPhone 4 can last only one day, i'm planning to buy a external battery in order to get a longer duration, i actually find 2 rechargeable external batteries for iPhone 4, one is solar powered and the other is a normal rechargeable external battery with back case, you can check the links for details:
http://www.tinydeal.com/2400mah-usb-solar-powered-rechargeable-external-battery-case-for-apple-iphone-4-4g-pxe1s6-p-24956.html
http://www.tinydeal.com/2350mah-rechargeable-external-battery-power-supply-with-back-case-for-apple-iphone-4-4g-black-pxe1s6-p-21184.html
I couldn't decide which one to buy yet, their capacity is almost the same, personally, i prefer the solar powered one, but the price is nearly twice than the normal one, do you think it worth to buy? And, will the external battery has a bad effect to my original battery?

I would recommend MiPow Power Tube 3000 (3000mAh). Unlike some knockoff products or universal products claim to be worked for iPhone, MiPow Power Tube 3000 is "Made for iPhone" certified which means you can use it to charge your iPhone without worrying it would break your devices.

http://www.mipow.com/product_detail.php?pid=32&lang=en

Here's my suggestion: A-Solar AM-403 Power Pack for iPhone 4. Nice weather is coming, so you could make the most of it. Good luck!