Hi John,
For a device that uses 40 watts like the Kangertech K-Box, we recommend using a battery with a higher amp limit. The Efest Purple IMR18650 2500mAh 3.7V 35A High Drain Battery will work great!
http://www.smoktek.com/Efest-Purple-IMR18650-2500mAh-37V-35A-High-Drain-Battery--Flat-Top_p_1021.html
Best regards,
Brad