I did look at that E-Werk but problem is that GPS units USB port is has a rubber cover to keep out the rain, so I could only use while riding in dry weather (it rains a bit here sometimes, even in the summer)
With the Reecharge, I hope, I can charge up during the day and then plug it into the GPS unit during evening/night (in my tent during trips) to charge it up. Only using it to charge as I ride in a real emergency.
I haver 5 bespoke batteries for my GPS so could always just change them over. They usually last about 6 hours each if the screen is not on all the time.
Note: My GPS is a SatMap Active 10 - see
http://www.satmap.com/about.php