The external relays are far better for robots. The internal ones supplied are very small and dont handle much current. I had the contacts weld on my first pair of electronize speedos trying to run 200 watt wheelchair motors at 24 volts. I replaced them with external relays 5 years ago and have had no problems since. For external relays I use the automotive ones from Maplin, £1.49 each last time I bought them.