What's the best battery pack out there?

i am wanting to use my iPhone 4 as my golf GPS as well, but one of the main issues is that the constant use of GPS drains the battery very quickly. Hence, I'm looking for one of those clip on battery things that I can use during rounds and not eat too much into my main battery. I don't really caer what it looks like. Here are a couple I've found:


http://www.solidsignal.com/pview.as...m_source=google_base_14_iPod_iPhone_Batteries

http://www.bhphotovideo.com/c/product/673783-REG/Kensington_K33459US_Mini_Battery_Pack_and.html