Serial PWM motor controller

[Kevin Sevcik]

It looks like minimum power dissipation on a P3 is around 20W. Max is 27W. Additional draw from the drives, etc would probably end up around 20W, so figure 40W to 47W from the PC, and just assume 100% efficiency for the power supply. That's about 4A draw, at a minimum, assuming a very plain video card. If you're using a car battery, they have a reserve capacity rating that says how long they'll last with a 25A load. 80 minutes is typical, so you'd get something like 8 hours out of that on a good day. Of course real world loads will be a good bit higher when you're running motors, etc. If you're really looking for long runtimes, I'd invest in a deep-cycle or SLA battery, as those are designed for lower draw levels and more capacity. Car batteries are primarily design to push a very large amount of current for less than a minute without significantly shortening the battery's life.