KOH Electrolyte for Dry Cells
Before You Get Started
First off, as a safety precaution, you should always be wearing safety goggles and rubber gloves when handling Potassium hydroxide (KOH). Now, it is very important that you use steam distilled water only. Look closely at the label. Filtered drinking water is not good enough, neither is reverse osmosis (sometimes referred to as distilled). Do not use tap water, mineral water, spring water, well water, rain water or purified water. It must be "distilled" by the method of “steam” distillation. Understand that any mineral in the water will not only act upon the continuity between the anodes in the cell but can also have adverse reactions on the surfaces of the plates.
Mixing the Electrolyte
We will want to measure the Potassium hydroxide (KOH) solution as a "% by weight" ratio. 1 gram of KOH is equivalent to 1 mL of distilled water (since 1g = 1mL). As an example, lets say we want a 20% solution: You would use 200 grams of KOH with 800 ml of water. This would make a 20% solution of KOH by weight, out of 1000 total equivalent units of measurement. Now, the proportion for most applications in moderate weather is approximately 30 grams of KOH with 1.0 L of distilled water. This will make a 0.03% ratio of KOH by weight (not a typo; you'll only need a very small percentage ratio for normal outside temperatures). Another way to look at this same ratio would be: about 1/2 cup of KOH with 1 gallon of distilled water (which might be easier for people in the U.S., since distilled water is sold by the gallon). Keep in mind that small adjustments may be necessary. This beginning ratio is for systems with 5 neutral plates between each "hot" plate, which is our standard configuration. This recipe is often a little weaker than needed to reach your desired amperage, but we start out at this ratio because it's easier to add a little KOH to the solution in the reservoir than to dilute the water in the reservoir which will already be filled.
KOH Freezing Temperatures
In freezing climates you will most likely need to add additional KOH to your solution to lower the freezing point of water considerably. Your system will run at a much lower duty cycle and may be a little less efficient, but that's better than having your system freeze up and not being able to use it at all. Mixing 31% KOH (by weight) with distilled water will allow your electrolyte to stay fluid down to -65 C (-85 F) degrees F. Most climates won't require more than 28% which is good down to -40 C (-40 F). The temperatures at which various KOH concentrations will freeze can be found in the charts below (One chart is for Fahrenheit and the other is for Centigrade).
Note: These charts show the freezing temps down to a theoretical maximum. In actual practice you will find that you don't get much benefit going higher than 28%. The reason is that the solution becomes saturated with chemical, and after that point, no further chemical will dissolve. That means that more chemical will just be floating in your electrolyte, clogging up any check valves or small orifices. The amount of chemical that will dissolve increases with increases in temperature. Therefore at very low temps, less chemical will dissolve and therefore provide less protection from freezing. So above 28% you really don't gain anything.
Also, realize that as the water is consumed making HHO, the concentration of the remaining electrolyte gets stronger. When you use 1/2 the water, the concentration will be double. This may get to the point where there are so many KOH solids in your mix that your system can get clogged up. So when you are using heavy concentrations like this, you should keep your reservoir topped off more often.