May i suggest a DC power supply, with 8(?) 9V outputs, something like the spyder (http://www.geofex.com/Article_Folders/Spyder/spyder.htm), but with a supply of laptop (19V, 2/3A), splitted into isolated 9V /100 or 200 mA. One output could be 9/12/18 selectable, another could simulate "bad battery" through a little pot.
You'll need transformers for it to step down the voltage from the grid to 9v. I have a little thing that I'm working on where I have two transformers that step down to 2x12v each and then I run it through a bridge rectifier, 9v regulator and a couple of capacitors for filtering. It's really simple and I did it all on vero. I haven't hooked it up yet but I will soon. It's pretty much bulletproof and the 4x9v regulated outputs are all isolated completely from each other.