There are plans for sale all over the internet for building a homemade hydrogen generator for your car. The biggest concern I have is that some of these plans specify using a glass mason jar for the container. Glass! does that sound safe to hold explosive hydrogen gas in a glass jar?
There are hundreds of videos on YouTube uploaded by folks who are sharing their experiences on building hydrogen generators. I saw one yesterday where a hydrogen fuel cell generator exploded. The maker of the video explained that he was testing it out on the kitchen table when it just exploded.
Luckily, this particular hydrogen electrolysis generator was constructed from PVC and not glass.
So Tip #1 is: Never, never make a hydrogen HHO booster container out of Glass. Even if you construct a bubbler for added safety, there will still be explosive hydrogen gas in the top of the generator container. Electrical connections can become loose and create enough heat to ignite the stuff. (While I am on the subject, I also saw another video of a guy installing a HHO hydrogen booster in his trunk. Horrible idea. )
There are many many different designs sold. One thing they all have in common is they all need some type of electrode material to produce the hydrogen electrolysis. I have seen some that use twisted stainless steel wire, and some that use stainless plates. Some draw only 3-5 amps and some close to 20 amps. The word from auto mechanic forums and blogs is that drawing too much amperage will kill your alternator prematurely. Who needs that?
So Tip #2 is: Make sure to purchase plans that use a design that draws minimal amperage like the design that uses twisted stainless steel wire. Because buying alternators is about as much fun as buying gasoline.
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