IMHO to assist.
These SatNavs are now built down to a price in a cut-throat market. Some of the devices offer a built in battery to allow you to avoid using the socket for short journeys or out of the car. One of the things they scrimp on are the batteries they fit. Unless you observe the battery charging and storage instructions these small cells often become very short lived.
Make certain the socket is rated for this load as the old ones often are not and have a low amp fuse. It may be worth considering a dedicated cable run to a subtle location out of site if you are going to use it heavily. A new product that was released recently which can help resolve charging issues is
http://www.practicalmeter.com which sits in line with the USB cable and tweaks the connections. It has doubled the charging speed on my iPad in many cases where using a non-Apple power source.
A typical TomTom Go 5" will take a standard load around 1.5a at 5v through the converter. In many cases look for a 12v car converter that is rated at iPad capacity (3amp 12v)
http://www.ebay.co.uk/itm/3-1A-USB-IN-CA...=item2ecb2c8b20 or consider using a standalone battery unit like this (not specifically recommending this item, there are thousands.) 3000mAh would be my recommendation as a minimum.
http://www.ebay.co.uk/itm/12000mAh-Porta...=item1c3979dba1 I agree about using a retailer to review them however I would hesitate on recommending any part of the dixons stores group (Dixons, Currys, PCWorld) as they promote specific products that have a high margin. You may not get a clear view. Most offer the same range of capabilities and the spot price and comfort using the software interface tends to be a key decider once you have decided size and features.
Hope this helps.
