Does anyone know what the maximum voltage is that you can apply to the external power block of the Adafruit Motorshield?
I've got two 9v batteries wired in series for a combined 18.2v, will this work or fry my shield?
What does the adafruit product page say?
Use a NiHm battery pack. They didn't even specify voltage.
Says 13.5V for the h-bridges here (https://www.adafruit.com/products/1438)
9V batteries are never going to be able to produce enough current for motors in the first place.
Use an SLA or LiPo battery for good current handling.