Just a thought, I converted my standard non active dock into an active dock by trimming the bottom of the dock with a dremmel multitool to enable the usb flap to open. You can then use a standard car charger lead to keep the battery charged. When it rains disconnect the lead and shut the flap and the tomtom runs on its battery. Took a bout 5 mins. to do and saved me £70.
Simples !