we have a few LED string lights that use 3 AA batteries and eat through batteries about once a week. I would like to replace the AA batteries with a USB battery pack I have which has about 8,000 MAH. Since AA batteries are 1.5v and standard USB is 5v, is this as simple as cutting an old USB cord and finding the correct positive / negative cables to wire to the empty AA battery enclosure? I am not sure if the +/- .5v would make a difference? Is there an easy way to find out which wire from the old USB cord is + and -? Am I nuts or will this work?