I have recently bought a USB car charger from Halfords. Order No: 20578730. SKU: 275045. It will not charge my phone (Samsung Galaxy Ace) in the car with or without the engine running. The USB charger can supply more power than the phone needs to charge. I am using the lead supplied by Samsung which charges fine when plugged into the computer.
I've had the same problem. Although it has a 1 amp fuse (and the Galaxy Ace apparently needs no more than 0.7 amp) it seems this does not mean the charger is actually supplying that much.