Motorola Q

I've been charging my Q in the car with a 4' USB cable that connects to a USB/cigarette lighter charger. I do this because I've got a dedicated outlet in the back seat that reaches easily to the front and don't want the to use the actual cigarette lighter, which requires keeping a sliding tray open and is unseemly. Anyway, this configuration worked just fine with my 8100 and 6700 PPC and charged the battery just fine, but it doesn't seem to be charging the Q. The little display in the corner changes from battery to the power cord, but I don't get any additional bars of power, even when driving around for what should be enough time to do so. Any ideas?