I use one of these phone mounting brackets with two USB ports to hold my Blackberry Z10, which has Mireo offline SatNav on it (full European maps, voice directions, speed alerts, etc). Charging seems to more than keep up with the power draw from the GPS (which is always pretty substantial), and the mounting is plenty solid enough (it can be hard finding something that works, at reasonable value).
"Dual USB Ports Car Cigarette Lighter Mount Holder Bracket Kit For BlackBerry Z10"
http://www.ebay.co.uk/itm/380721321170?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649
I'm thinking of getting one for the rear 12v socket as well tbh (I'd like a beefed up one in the same style for my PlayBook if I can get one, as that's really solidly built, and 4 USB ports is better than 2 which is better than none).
They do mounts for different phones, and delivery was pretty good (about 10 days from Hong Kong - other parts of China can be a few extra weeks). Seems excellent quality for the price - £7.99 including delivery.
I noticed the blue light on the mount stays on for a bit, but I take the phone off and take the lead out whenever I get anywhere, so there's no current going through it before it auto switches off.
Hope that helps.
PS. After China boycotted the EU for flights,etc., over the attempted carbon tax nonsense, pretty much any Chinese retailer on ebay with a feedback over 99% (and lots of it) after the terrible delays that caused, is a pretty good sign of a good retailer. That one I linked has 99.2% on 32,760 sales that had feedback, and I have used them a couple of times prior, with no problems.Obviously once their parcels are with the carriers, it is out of their control.