The first HHO systems that I saw were made with jam and preserve jars that had electrodes inserted, where the jars were filled with water and the electrodes connected to a 12 volt power source , and one had to add a catalyst , like baking soda, to create electrolysis, which, in turn separates the water molecules, or the hydrogen from the oxygen. In turn, again , the system sent the hydrogen through the air system of the vehicle as a supplement to the gasoline or diesel.
The systems seemed to work pretty well with providing better fuel mileage, more power , and cleaner emissions. But there were a few problems that needed attention, like, because the electrodes were submerged in water all the time some of the electrical power would leak away in the form of heat. The system would get hot, adjustments had to be made fairly often to maintain the constant correct voltage to supply enough LPM ( liters per minute) to maintain a 14.7 to 1 air to fuel ratio. It did involve a lot of trial and error, but Hydrogen on Demand technology was very experimental.
Today new systems have been developed that are efficient , with electric components that maintain constant voltage, where by , the system provides the correct LPM, Liters per Minute, 24 hours a day, if you wish. More safety features, have been added like back flow prevention , fusible links and relays, and a proper catalyst. Advanced electronics for tuning and that allow one to view the performance stats all the time is a plus.
Dry cells are where electrolysis happens. Interesting enough, only part of this cell is dry. Instead of the electrodes being completely submerged in the electrolyte solution, the electrolyte runs from a separate reservoir through a series of metal plates that are charged to provide the hydrogen supplement . It is a closed system and leakage is not a factor provided it is installed correctly. Also provided that you purchase this system from a reliable company. In fact many hydrogen on demand systems look like factor equipment installed, and they are easy to maintain. Mostly all one really needs is to fill the reservoir periodically . Remember, this system never stores any hydrogen. The hydrogen supplement is made only as the engine requires it.
The cost of these systems generally run between $400 and $600. That is from reliable companies and because they manufacture warranty is available. With the savings in fuel and maintenance the system pays for itself over short periods of time.
The Dry Cell Systems do a better job of providing the large benefit for your car, the environment, and your wallet.
For more information, a good place to get educated would be HHO Kits Direct . This site is easy to navigate.