Being the curious type, I took a look at the Thermoil website. Here is the explanation they give about why it works.
During charging, the chemical process taking place breaks down water into hydrogen, an explosive gas, and oxygen, which corrodes battery posts, cable ends, and any surrounding metallic parts. Continual overcharging increases the internal temperature of the battery, accelerating the destruction of the battery plates and leading to premature failure. The balanced Battery De-Mister® mixture in the battery increases the re-combination of the hydrogen and oxygen gases over a conventional battery and greatly extends the intervals between water additions. When an oil battery is overcharged, the hydrogen and oxygen gases migrating to the electrolyte surface are slowed by the oil, greatly reducing the acid mist and fumes generated in the charging process.
The oil condenses the hydrogen and oxygen gases, returns the water molecules back into the cell and virtually eliminates the sulfuric acid mist forced out the vent during the charging process.
With virtually no mist and much less gassing, the battery top remains free of acid deposits, thus the chemistry of the electrolyte remains the same while greatly reducing water consumption, toxic fumes, explosion and thus eliminating post and cable corrosion while reducing current leakage across the battery cover. Frequent cleaning of the battery, cables and battery tray are no longer necessary to keep the damaging effect of the sulfuric acid from ruining the surrounding equipment.
Let's take this apart, and following in the tradition of the various fact checking organizations we'll assign it from 1 to 4 Snake oil bottles.
During charging, the chemical process taking place breaks down water into hydrogen, an explosive gas, and oxygen, which corrodes battery posts, cable ends, and any surrounding metallic parts.
We'll give this 1 Snake Oil Bottle. Yes, passing a current through a battery while charging does break the bonds between the hydrogen atoms and the oxygen atom. Yes, oxygen does cause corrosion when it is ionized and yes hydrogen is explosive, actually it just burns very rapidly. The problem with this statement is when oxygen formed it is unstable until combines with something, during charging the oxygen ions combine with each other to form stable O2 molecules. The air we breathe is about 20% O2. The corrosion we see on batteries is mostly due to the sulfur compounds that escape from the battery.
Continual overcharging increases the internal temperature of the battery, accelerating the destruction of the battery plates and leading to premature failure.
Another 1 Snake Oil Bottle for this statement. Yes, overcharging will damage batteries, a contemporary multistage battery charger can prevent this. It is misleading to assert that Thermoil will somehow prevent over charging.
The balanced Battery De-Mister® mixture in the battery increases the re-combination of the hydrogen and oxygen gases over a conventional battery and greatly extends the intervals between water additions.
This gets the full 4 Snake Oil Bottle treatment. Yes, combining hydrogen and oxygen does create water, it also creates a lot of energy, it is an exothermic reaction, that's why hydrogen explodes and burns. The problem is that to get the reaction started it is necessary to provide some initial energy to break the H2 molecules and O2 molecules into less stable H+ and O- ions, once those bonds are broken, the H+ and O- ions combine to form water and release a lot of energy which breaks more bonds and repeats. This doesn't just happen with H2 and O2 molecules trapped under a oil film. (This Youtube channel has some cool videos demonstrating properties of hydrogen and the other elements:
https://www.youtube.com/channel/UCtESv1e7ntJaLJYKIO1FoYw)
When an oil battery is overcharged, the hydrogen and oxygen gases migrating to the electrolyte surface are slowed by the oil, greatly reducing the acid mist and fumes generated in the charging process.
This deserves 1 Snake Oil Bottle for being misleading. The oil may
slow the rate at which the gases bubble out of the electrolyte, but the amount gas formed will be a function of the current passed through the battery while charging. The oil will probably slow the bubbles because the oil molecules are larger and "stickier" than water molecules.
The oil condenses the hydrogen and oxygen gases, returns the water molecules back into the cell and virtually eliminates the sulfuric acid mist forced out the vent during the charging process.
Another 4 Snake Oil Bottle Claim. Condensation generally means changes the physical state of a gas into a liquid. Water vapor condenses at 212* F (also the boiling point), hydrogen condenses at -453* F. That is just not going to happen in a battery on a boat.
With virtually no mist and much less gassing, the battery top remains free of acid deposits, thus the chemistry of the electrolyte remains the same while greatly reducing water consumption, toxic fumes, explosion and thus eliminating post and cable corrosion while reducing current leakage across the battery cover. Frequent cleaning of the battery, cables and battery tray are no longer necessary to keep the damaging effect of the sulfuric acid from ruining the surrounding equipment.
Another 4 Snake Oil Bottle award. This simply restates more emphatically earlier statements and contradicts itself. An earlier statement claims the corrosion is caused by oxygen escaping, now the company claims (at least more correctly) that the off gassed sulfuric acid causes it. You can't have it both ways. Perhaps more importantly, this is statement claims to "cure what ails you" making promises that can't be substantiated.
The makers of Thermoil may believe in their product and they may believe that it really works, but they don't provide any substantiation to their claims other than some distorted logic and scientific sounding gobbledygook. Folks like this prey on the gullibility of others and laugh all the way to the bank.
The scientific claims they make and the lack of substantiation are bad enough, read the history of Thermoil that they are indeed a persecuted company, the target of a conspiracy by the battery manufacturer's to put them out of business. Then there is the 100% guarantee, guarantee of what? And when it doesn't work as advertised, how do you prove it didn't work?
Finally, the website lists the company headquarter's address as 4510 West Pine St., Appleton WI. Check it out.
