Hummm, haven't tried to charge the cell phone. I did plug in my Nuvi GPS and it worked OK. Zombie has the right idea get a test light or better a meter and check it. If you don't have one Harbor Freight sells a VOM for 3 or 4 bucks that will work just fine to check it out. Then you can see if polarity is wrong. They tell you in the manual that it is very low wattage, can't remember but it's not much.