HHO is best produced at lower voltages, you are correct.
For this reason multi- cell units are used. However, multi cell does not have to mean multiple generators.
Using the correct set-up a multi-cell can be easily built into one HHO Generator.
The lower voltage will reduce the amount of heat-waste energy.
We have been producing these systems since 2005 and have great success with them. Be aware, there are several copies out there, however these Hydrogen generators are not made using the correct metals, surface preparation etc.
In short, any Supplemental Hydrogen generator making HHO at 12 - 14 V is generating a lot of waste heat and will only be marginally effective.
You can check our site at www.HHO-PROS.com/
P.S. We are revamping the faq, you will find some gems in there as well.
P.P.S. PWM is the brainchild of stanley meyers, his theory was proven to be a hoax and it really P's me off that his material is still out there confusion 100's of well meaning people, who are throwing good money after bad buying the ebooks on his stuff.
see wikipedia at http://en.wikipedia.org/wiki/Stanley_Meyer on "Also, if the device worked as specified, it would violate both the first and second laws of thermodynamics, allowing operation as a perpetual motion machine. Meyer's claims about his "Water Fuel Cell" and the car that it powered were found to be fraudulent by an Ohio court in 1996."