Battery Power Supply for NEMA 17 Steppers

[Tater]

Hi all,
I'm looking to build a simple robot, with 2, maybe 3, NEMA 17 stepper motors, controlled by an Arduino. I have attached arough sketch of the setup that I have had working, albeit poorly which includes;

• Nema 17 Stepper
• Big Easy Driver http://www.schmalzhaus.com/BigEasyDriver/
• a couple of 9V Batteries in parallel
• Arduino UNO

I have also attached the data sheet/schematic for the drivers and motors. I know 9V batteries are not optimal and do not last a significant amount of time, maybe a couple of test runs.
The big easy driver based on Allegro A4988 says it accepts 8-35V but when I tried switching to an 18V drill battery or 15V SLA battery the driver chip got super hot (like 150^oC) (I know I've fried that one).

I did some testing when the setup was working with the 9V battery and getting at max 500mA from the battery, even when adjusting the current adjust pot on the driver board.

What does it really mean by accepts 8-35V? I just would like to know how to go about powering this setup correctly.

Any help and tips would be greatly appreciated.

Edit; I used this guide https://learn.sparkfun.com/tutorials/big-easy-driver-hookup-guide as my basis.

[lapm]

Those 9 volts batteries are going to be depleted fast. Nema 17 size stepper motors eat current... If i were building robot that needs independent power, i would lok for Li-Ion batteries. They are cheap considering current capasity and rechargable with proper charger...