Battery Recombination Caps


Our new FLUX™ Recombination Vent Cap virtually stops water loss! Now you can avoid the chore of adding water to your battery for months or even years!

Previously, you may have chosen a VRLA battery based on its advantage of low maintenance. But with the advent of the FLUX™ recombination cap, more choices are now available. The FLUX™ vent is a game-changer because it almost eliminates maintenance on a vented battery. It now makes sense to choose a VLA battery which has the advantage of lower costs and longer life.

This amazing vent cap saves time and pays for itself! Simply replace the existing battery caps with our FLUX™ vent. The water that normally escapes in the form of gas is now recombined back into water and retained in the battery. The FLUX™ vent virtually eliminates the emission of hydrogen and toxic acid fumes. Lower utility bills and equipment costs can result because there is no need to provide mechanical ventilation where the batteries are installed in living spaces or enclosed areas.

Quantity Discounts

Part NumberQuantityPrice
"500+Contact Us
"500+Contact Us

To buy, select a FLUX recombination cap model in the drop-down box below. Then enter the desired quantity and select the Add to cart button. Then select the Checkout button that appears at the top left.

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FLUX™ Recombination Cap Features:

  • Extremely efficient catalyst stops water loss!
  • Eliminates flammable hydrogen gas.
  • Water Miser™ technology added.
  • Standard bayonet-style installation.
  • Recommended for batteries up to 550 AH
  • Safety flame arrestor tested by independent lab.
  • Increases reliability and reduces maintenance costs.
  • Made in the USA.

How much water does it save?

Wow! That was our reaction when we saw the test results. The batteries in the independent lab test were subjected to a full discharge/recharge cycle once per day to document how much water is saved. Since the greatest water loss occurs when a battery is being charged, if you do not charge your batteries every day, the water savings you experience will likely be much greater. This is demonstrated in the chart below. It compares how much water is saved in relation to how often the batteries are charged. For example, if the batteries are charged every day (24-hour cycle), the batteries could last 9.8 months without needing water. If the batteries are charged every other day (48-hour cycle), you could wait 19.7 months before adding more water. Depending on how often your batteries are cycled, it is conceivable you would not need to add more water for almost five years! However, the amount of water saved is directly related to many factors such as temperature, battery size, how often the battery is subjected to charge/discharge cycles, etc., so the water savings you gain may less or greater than what is shown in the chart.

Water savings chart for Battery Recombination Caps

Recommended Usage:

  • Max. charging voltage: 2.4 Volts +/- 1%
  • Recommended for stationary batteries only. Not intended for batteries subjected to rough motion.
  • Perfect for L16, 6-Volt, 8-Volt, 12-Volt, GC2 batteries, and more.

Safely Eliminates Hydrogen Gas

All lead-acid batteries produce hydrogen gas during recharging. This gas is extremely flammable and will likely explode if ignited. If the flame enters the battery itself, the hydrogen can ignite and cause an explosion, propelling battery pieces and acid in every direction. The FLUX™ recombination cap neutralizes the hydrogen and renders it harmless by combining it with oxygen and converting it into water. In addition, every FLUX™ battery cap contains a flame arrestor for added safety.

How does it work?

There are basically two ways in which a battery loses water. The first way is through evaporation exacerbated by warm ambient temperatures and internal heat generated by the battery itself. Although the electrolyte is usually a mixture of both sulfuric acid and water, only water is lost because the acid does not evaporate except for a few molecules detectable as an odor. Therefore, only distilled water should be added when refilling a battery.

The second way that a battery loses water is caused by electrolysis which occurs when a battery is being recharged. Electrolysis is the breakdown or splitting of water into hydrogen and oxygen gas caused by the passage of an electric current. Normally this gasified water is allowed to escape from the battery and must eventually be replaced. If this is not done, the internal plates will dry out, and in time the battery will be ruined.

The FLUX™ recombination cap recaptures both the evaporated water vapor and also the water that was split into hydrogen and oxygen gas. It performs this in a two-stage operation.

The first stage employs our Water Miser technology which acts as a prefilter to condense the water vapor into droplets, thus returning much of the water that would otherwise be lost back into the battery.

The second stage uses a catalyst to recombine the gas back into water. Therefore, virtually all of the water that is normally lost is now recaptured and returned into the electrolyte reserve.

Flux Recombination Cap diagram

You may view the above diagram as a PDF file, and it explains how the recombination cap works. Please remember to check the available space above your batteries and then compare it with the dimensions shown above to ensure there is sufficient room before ordering. The dimensional drawings for the FLUX-1000 and FLUX-2000 are also available.

It will fit most battery brands, including Crown Batteries, Interstate Batteries, US Battery, Rolls Batteries, Trojan Batteries, Tulip Batteries, Exide Batteries, and more. The FLUX™ battery recombination vent cap will replace any standard quarter-turn bayonet-style vent cap, see example pictured below. To verify if it will fit on your battery, just compare your existing vent cap measurements with the example shown below.

Total Height2.90" [7.37 cm]3.97" [10.01 cm]
Installed Height (above battery)1.88" [4.77 cm]2.95" [7.49 cm]
Diameter1.85" [4.70 cm]1.85" [4.70 cm]
Weight1.80 oz [51.01 gm]2.71 oz [76.80 gm]
0-300 AH (Mobile)YESNO
301-500 AH (Mobile)NOYES
0-2000 AH (Stationary)NOYES
Group Size GC Series 6V / 8V / 12VYESNO
2-Stage Recombination ProcessYESYES
Bidirectional ValveYESYES
Water Miser Technology AddedYESYES
Recombination Catalyst CoreYES (1X)YES (2X)
Safety Spark ArrestorYESYES
Tested by Independent LabYESYES
Rubber Washer IncludedYESYES
Patented DesignYESYES

History of the Recombination Battery Vent Cap

The development of battery recombination caps has a long history. Ever since the lead-acid battery was invented, there have been numerous attempts to remedy the problem of electrolyte loss caused mostly by electrolysis and to a lesser degree by evaporation. Electrolysis occurs during charging and consumes the water in a battery by converting it to hydrogen and oxygen gas. Normally this gas is allowed to escape from the battery by venting itself through the battery caps. The idea to convert this gas into water by using a catalyst or other means has been an elusive goal.

In 1911, Thomas Edison originated the idea of recombining the hydrogen and oxygen gas within a lead-acid battery.[i] His invention involved igniting the gas mixture by heating it with a wire. This process converts the hydrogen and oxygen into water. The recombined gas forms water vapor which condenses into droplets that return into the electrolyte tank, thus reducing the labor and time needed to refill batteries with water.

Methods to limit electrolyte loss further progressed when the first lead-acid gel battery was invented in 1934 by Elektrotechnische Fabrik Sonneberg.[ii] Adding silica to the sulfuric acid produced a gel electrolyte less prone to evaporation. However, the main advantage of electrolyte in gel form is that it made the battery less prone to leakage when tipped or handled roughly.

The idea to recombine battery gases took a leap forward in 1957 when Otto Jache invented the first true VRLA (valve-regulated lead-acid) battery.[iii] This type of battery is completely sealed except for a small pressure relief valve that plugs into a small hole in the battery case. The hydrogen and oxygen gas are retained within the battery where the two gases are combined. The valve is a safety feature allowing the trapped gas to escape if the pressure within the battery becomes too high. Any gas that exits the battery results in a tiny amount of permanent water loss. Fortunately, this rarely occurs during normal operating conditions and most of the gas recombines to form water. Most VRLA battery valves are commonly set to release pressure at a certain PSI to ensure that full recombination occurs.

A number of battery recombination caps have been invented with varying degrees of success. In 1926, Claud Everett sought to prevent the loss of electrolyte and the resulting wet condition on the top surface of the battery.[iv] He designed a battery cap filled with glass wool on which escaping moisture would condense. When plugged into the battery vent hole, this reduced the amount of electrolyte lost due to evaporation. However, it did not actually bind the hydrogen and oxygen together nor did it prevent the gas from exiting the battery. Therefore, his invention had limited success in reducing electrolyte loss.

In 1929, Dean Davis tried to improve on Thomas Edison’s idea. He designed a battery plug that also utilized a wire to change the gas into a liquid.[v] The wire catalyst was powered by the battery itself and employed a mercury switch to electrify the wire. When the gas pressure within the battery increased, this caused the mercury to rise and make contact with the wire, thus completing the circuit. To help absorb internal moisture, spun glass was inserted in a separate compartment in the recombination plug just below the catalytic chamber.

In 1933, Harry Guthrie of the U.S. Navy designed a “self-watering” battery that used a catalytic material instead of using a heated wire.[vi] His intention was to rejoin the battery gas at a lower temperature. That may have offered little advantage because the recombination activity itself generated heat. To solve this problem, he devised a method to cool the area in close proximity to the catalyst. At the same time, he was faced with another dilemma. When the catalyst becomes wet, the gas combining ability of the catalyst is adversely affected. To rejuvenate the catalytic material, heating elements were used to dry out the material to restore its ability to initiate the recombination process.

In 1946, Palmer Craig invented another sealed battery with a means to merge the emitted gases to form water.[vii] This would be especially useful on submarines or other venues where the gas cannot be allowed to escape but must be stored or otherwise disposed of. The patent application mentions a way to remove excess water from the catalytic material, and two methods for doing so are mentioned. The first one involves a mechanical means of compressing the saturated recombination media to remove the majority of the water. This involves manually pressing a plunger located on the top of the battery. Pressing on the plunger compresses the catalyst material and squeezes out most of the liquid. The remaining moisture is then removed by means of an electric heater, presumably by plugging in wires and attaching them to the battery, flipping on a switch, and then switching it off after enough time has elapsed. Since all of this requires manual intervention, the usefulness of this invention as a timesaver may be questionable since its main objective was to reduce the amount of time spent replenishing the water inside the battery.

Another milestone was reached in 1949 when Palmer Craig invented another battery cap.[viii] It was designed to replace the OEM battery cap for the purpose of reducing or eliminating the need to refill the battery with water. The chamber below the catalyst was filled with glass wool and was intended to form a water trap. An upper chamber was filled with another material to assist the catalyst.

As previously noted, when the catalytic material becomes wet with recombined moisture, the recombination ability of the catalyst was interrupted. A method to dry out the catalytic material is outlined in the inventor’s claim by means of an electrical heating conductor wrapped around the catalyst container and controlled with a switch. To help prevent the catalyst from becoming wet, the top of the recombination cap was shaped like an inverted cone. As a result, any water droplets that form on the surface directly above the catalyst are directed outward instead of falling onto the catalytic material, thus helping to keep it dry. The recombined water then flows down the side of the vent cap wall and underneath the catalyst holder where it returns into the battery.

Another object of this invention was the rapid conduction of heat away from the catalyst to the outer wall of the battery cap where it dissipates into the surrounding environment. This was done by means of highly conductive materials such as copper or other suitable media.

In 1960, a group of inventors sought to design an improved recombination cap.[ix] They designed a permeable container to house the catalyst material. This container was water-resistant but still allowed battery gases to enter and interact with the catalyst. Any water that condensed on the outside of the container during the recombination process would simply roll off, thus helping to keep the catalyst dry. In addition, holes were provided in the side of the vent cap to supply additional oxygen to enhance the recombination activity. The holes also allowed outside air to reach the side of the catalyst container which helped to dispel the heat. Just below the catalyst is a chamber that contains a material to absorb impurities formed within the battery.

In 2016, Michael Doyle and Drew Barrett applied for a patent for an improved battery recombination cap. The application was granted in 2018 and was assigned patent number 10,601,010.[x] The design was intended to overcome the shortcomings of other brands currently on the market. Notably, one had a tendency to catch fire and melt. Hence there was a need to invent one that will not cause this issue. This invention also includes a safety flame arrestor that will prevent the hydrogen gas within the vent cap from being ignited by a nearby spark or flame. More information is available in the brochure.

The original intent for this recombination cap was for usage on stationary batteries such as solar battery arrays and backup batteries. However, one RV owner installed these on the 6-volt batteries in his motorhome and was very happy with the results. He drives the RV between four and five days per week, so the batteries are used on a regular basis. About 15 months after installing them, he checked the water level and said he could not tell if the battery had lost any water. He said that if he had to guess, they may have lost about 1/8 inch of water at most, and he was confident the batteries could go at least two years without adding water.


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Additional information

Weight N/A
Dimensions N/A
Model #

FLUX-1000, FLUX-2000

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