Mini-USB cable will only do data, but charges other devices?

[EyeB]

think it's for older computers that may not supply enough power to charge it, so instead of charging it drains the battery even quicker because it keeps trying to charge but cant.

I get this problem in my car when I'm trying to charge phone while playing slacker and using garmin gps lol. I get a notification window saying charge is not enough to charge phone and to quit some programs while using my fm transmitter

I also got this message when I tried to make a solar power backcover for phone, the phone would be kept on thinking it would charge, but the panels provided less power than phone was using to keep it in charging state lol