Which battery should I use to power arduino+motor shield + two motors

I am using two motors ( from old two drill machines - same kind, these are 3.6v motors ), connected to arduino + arduino motor shield seeed studio v2.

When I measured the both motors (indvidually) using a multimeter in series with your motor and an about 3.6V power supply, I found that the both are varying from 1.46A-1.47A, (I supposed it as 1.5A)

I tried to use 18v/1.2a battery that was laying at home (from old machine ) what I have noticed is that motors are running for 3-5 seconds than stop, one of the red lights start blinking in the motor shield, then it stops for 3-5 seconds then it works again and loop.

Notes:

  • The motors are working when I connect them directly to 4.5 or 6v battery. But when I try to assemle everything , it doesn't work (I am adding this just to say that motors are in good health).

  • I tried 14v battery I noticed the same thing ( but slower, and this is something logical ), the battery of 9v don't make any move ( even by powering th arduino alone (using 9v) and the motors alone (9v) ).

  • The drill machine is small one (as big as a human hand). Motors from electric drills are designed as high-speed high-torque motors, but I don't think it's too high in my case to be a huge problem.

  • I spent a week looking for answer before posting this thread, I tried every single solution I came across on the web.

What are things that I should consider here, any thoughts.

What did I miss? And what kind of battery should I use ??