Thanks guys. My needs are for battery simulation to test both batteries and charging circuits. The instruments I am looking at are the Keithley 2281S and ITech IT6432. The Keithley has the brand recognition. The ITech is less expensive, can sink more current, which might be important to me, and has two channels. The Keithley is a two-quadrant PS (1.5 quadrant given that it can't sink as much as it can source?), but the ITech appears to be a four-quadrant supply.
I had to do a lot of research to discover that this is what I needed and thought that a "Fundamentals Friday" style video would be enlightening. The tear-down videos are interesting and educational, but they don't really go into details about what they are, how they work, when you might need one, and the proper way to use them.