That guy is a rip-off for what he's charging. I did the same thing myself and it works flawlessly for much cheaper.
Anker Astro Pro 2: (20,000 mAh charger)
Amazon.com: Anker® Astro Pro2 20000mAh Multi-Voltage (5V 12V 16V 19V) External Battery Charger for iPad Air, Mini, iPhone 5S, 5C, 5, 4S, Samsung Galaxy S5, S4, S3, Note 3, HP, Dell, Acer, Asus, Toshiba Netbooks, Notebooks and many other devices (Appl
AC adapter for the connector piece: (this tip is the perfect size for the output of the anker astro pro 2)
18V AC Adapter for JBL on Stage II iPod Docking Charger Power Supply Cord | eBay
and a cheap knockoff of a surface charger for the magnetic tip: (need to splice this to the jbl tip)
P4PM New DC 12V 2A Car Charger Power Supply Adapter for Microsoft Surface RT | eBay
How long does that power your SP2?