I have built a fully kitted out car just to find out that its slightly too heavy. I am powering my arduino off 9V battery, and the L239D off of three 1.5V AA batteries providing 4.5V to the EXT_PWR. My easiest solution is to reduce weight, mainly the batteries. If I used a single 9V battery into the L239D, would it step down the voltage to 4.5V for my motor? I assume not. I only have one motor which is another reason for the inadequate power. My last option would be to get a more powerful motor, as I would have to remove many things, some that have been permanently fixed.
Could I also use button cell batteries? For either the arduino or L239D? Their current output seems to be too low, so I doubt it. The run time I am looking for isnt long, 10-20 minutes ideal. Would I be able to use a small lipo as external power for the L239D?