LEt me tell you about my experience with this issue.
Firstly, most of the USB power supplies you can easily get are rated at about 500 mA, (1/2 an Amp). If you look a bit harder you can find ones rated at 1 amp (1,000 mA)
I found a site in the USA that supplied a nice little USB socket about a quarter the size of a matchbox, That just had two wires you connected to the Battery. Unfortunately I Lost all my links so I cannot paste it for you .
Through bitter experience, I have learnt that when using the GPS and Mapping on my Mobile phone, While plugged into the car adapter, The charging current is not enough to meet the demands of the phone while it is doing it's phone thing as well as the mapping downloads and the GPS reception, so eventually the phone would completely discharge. This is no major problem but it is an annoyance thing.
I tried making the screen stay active for shorter and shorter times, but in the end I just gave up. My phone is a HTC Desire, but if you have another phone it may behave differently.
I eventually Purchased a Strike Genius, and initially connected it via my nice little USB power supplies, however it suffered with the same affliction as the phone, It would eventually discharge and stop working, even while plugged in and charging .
I took a look at the power supplied provided with the GPS and saw it was rated at 1.5 Amps. (1,500 mA).
Once I hard wired in the Supplied power supply to a wire that is energised when the bike is running, all my problems went away and the GPS worked fine,
I suppose the moral of the story is that GPS and mapping software is a power intensive thing , so for it to be successful, you need to get a decent power supply that is rated high enough to
keep it happy.
Not sure f that helps or not but it will give you some things to keep in mind
Tom