Hydrogen Vent FAQ

Hydrogen FAQ

The process of charging lead acid batteries involves passing electric current through water, contained in the electrolyte inside the battery. A natural by-product of this process is the splitting of the water into its basic components, Hydrogen and Oxygen which can build up to explosive levels if it is not ventilated properly. Explosive mixtures can be prevented if the battery enclosure is designed to take advantage of the principles of natural convection and ventilation. The patented H2Vent™ systems from Zomeworks have been developed for this purpose.

Frequently Asked Questions:
What is the danger of explosion during battery charging?
What is Zomeworks’ involvement with hydrogen venting?
When does hydrogen burn? When does it explode?
How much hydrogen does a battery emit?
What is thermal runaway?
Why not use a fan?
Why not use high and low vents?
Why not use a direct vent tube to the outside?
How much ventilation is needed?
What is the passive hydrogen vent?

 


What is the danger of explosion during battery charging?

Battery rooms and cabinets are notorious for explosions when hydrogen created by electrolysis and mixed with oxygen is ignited by a spark.

The proliferation of back up batteries at communications sites has spread the hazard from the private concern of the battery’s users to the public at large. Battery cabinets, vaults and rooms are now scattered like time bombs all around the world, many where an explosion could injure or kill unaware bystanders. The danger is not imagined. Many vaults have exploded and recently a communications shelter in Yuma, Arizona exploded shattering the windows of a neighboring house.

Manufacturers of both batteries and battery shelters tend to downplay the danger of explosion due to hydrogen buildup during battery charging. Manufacturer’s literature commonly underestimates the worst case for dangerous hydrogen accumulation, and often displays reassuring calculations proving that no danger really exists. But dismissing such a critical safety issue is not a safe or responsible way to deal with it. Instead, we should be prepared to face the likely possibility of hydrogen buildup, clearly identify the conditions when the risk is highest, and design systems that protect us from explosive levels in a fail-safe way.


What is Zomeworks’ involvement with hydrogen venting?

As the manufacturer of the Cool Cell™ passive temperature regulating battery shelters, the safe and reliable control of the environment inside the shelter is a primary concern. At Zomeworks many tests and experiments have been done to develop a clear understanding of the forces at work inside the battery box. By measuring hydrogen concentrations, air flow rates and temperatures over a wide variety of “real world” conditions, we have identified the potential flaws and pitfalls for all the common ventilation methods. Test explosions have been set off to confirm the severity of the danger at various hydrogen concentrations. Our chosen approach is based on years of hands on experience and verification.


When does hydrogen burn? When does it explode?

The Lower Explosive Limit (LEL) for hydrogen is commonly accepted to be 4% by volume. For example, the air in a box with a volume of 100 cubic feet that contains 4 cubic feet of hydrogen gas would be expected to ignite when exposed to a spark or open flame. Our own experiments with hydrogen show that such a diluted mixture does not explode, but rather burns off with more of a fizzle than a bang, and so does not represent the severe explosion hazard that one might expect. It isn’t until the hydrogen concentration climbs above the 4% LEL to 6 – 8 percent that it pops when it is ignited. When the concentration climbs to 10 – 20 percent the explosive power becomes very impressive, able to blow the doors, roof, or walls off a battery shelter.


How much hydrogen does a battery emit?

A charging system that is functioning perfectly will emit only small amounts of hydrogen. For example, specifications from the Johnson Control line of batteries shows that when a floating charge is maintained, 4 – 10 cc/hour/battery of hydrogen is released. When a higher charge is used for equalization, 10 – 20 cc/hour/battery can be expected depending on the size of the battery.

In a shelter that contained 30 of these batteries charging at the higher rate, the hourly accumulation of hydrogen would be 600 cc/hour or .021 cubic feet per hour. At this rate it would take 8 days for a 100 cubic foot shelter to reach the LEL danger zone if the space was sealed airtight. But at this low rate of hydrogen generation, the hydrogen will typically diffuse and leak out without reaching critical levels, as long as the shelter is not truly airtight or has any kind of functional ventilation system.

While a normally charging battery poses little danger, a much more critical situation can occur when the battery charge controller fails to regulate the charging rate properly, and proceeds to “boil off” all the water in the battery. During this kind of failure the rate of hydrogen production can be as high as 3.4 milliliters per minute per Watt supplied by the charger as the water in the battery is converted into Hydrogen and Oxygen by rapid electrolysis. For example, a 1000 Watt charger stuck in “full output mode” can generate 3.4 liters per minute or 7.2 cubic feet per hour. In this example a 100 cubic foot airtight shelter would reach the combustible LEL in about half an hour and would be ready to explode with real force in about an hour or two.

Even though a runaway battery charger may be a rare event, it is not only possible but a likely mode of failure in both conventional and photovoltaic battery charge control systems. When it does happen, it creates a potential for deadly explosion which must be anticipated and disarmed by proper ventilation in the same way that a pressure relief valve is used to disarm the explosive threat of a runaway water heater or boiler. Just like a pressure relief valve, the hydrogen safety vent must be designed to handle the worst case without fail, not just during normal operation, but under any environmental conditions, whenever it occurs.


What is thermal runaway?

Batteries in enclosures can go into thermal runaway, which very rapidly produces hydrogen, whenever several conditions are simultaneously present.

1) The batteries are exposed to high temperatures inside the enclosure (above 100 F) for a sustained period, either because the enclosure is unprotected, or because the cooling system is inadequate or has failed.

2) The batteries continue to be charged at high voltage and current because there is no temperature compensation in the battery charging circuit, or because said circuit has failed.

3) Batteries have aged and become dehydrated.

Solar heating will cause the internal temperature of a plain metal battery box to go above 100 F in most of the country during much of the year. According to Bellcore, internal enclosure temperatures will increase to between 21 F and 27 F above the external daily high temperature, depending on the U. S. location. Thus, if the external daily high temperature reaches only 80 F, then the internal temperature of an unprotected enclosure will surely rise above 100 F, setting the stage for thermal runaway.

Temperature compensation in battery charging circuits is a relatively new technology. Where it has been utilized, it has proven to be generally effective, but, as with any system subjected to environmental extremes, not completely reliable. And, in most cases, a potential failure is not easily detectable until it has already occurred. Although thermal runaway remains relatively rare, it occurs with sufficient regularity to be of grave concern.

Once thermal runaway has begun, hydrogen can accumulate in an unventilated enclosure to a concentration above the lower explosive limit (4%) in a few minutes. While the explosive effect of such a concentration is small, if hydrogen continues to be released, it can accumulate to far more dangerous levels. The concentration can keep rising to the point that ignition will completely destroy the enclosure and its contents (above 10%) in less than an hour. Given the large number of enclosures in populated areas, the potential for collateral damage to people and property is unacceptable.


Why not use a fan?

A simple fan produces a constant flow of air, but the need for ventilation in a battery shelter can vary by a factor of 300 or more when thermal runaway occurs. Even if a sophisticated control system were implemented that controlled the fan speed in proportion to the presence of hydrogen, such a system would be susceptible to mechanical and electrical failure. A failure that coincided with thermal runaway conditions would prove disastrous. In such a scenario, the failed fan or control system could actually become the source of the spark or heat that ignites the explosive hydrogen mixture. Clearly, this is not a Fail-Safe solution for hydrogen ventilation.

A constant fan supply of outside air creates an additional problem of thermal control inside the battery shelter. On cold winter nights, the ventilation air can cause the batteries to become too cold, and lose much of their energy storage capacity. In the heat of summer, the outside air could cause the batteries to overheat, which causes them to age more rapidly. This defeats one of the main purposes of using an enclosed battery shelter; to provide the batteries with a comfortable operating temperature range that extends their useful service life. Of course, more heating and cooling equipment and controls could be added to counteract the temperature extremes introduced by the fan. But this, too, would be susceptible to failure and could not be made to be Fail-Safe.


Why not use high and low vents?

High and low vents are unreliable and therefore hazardous. One expects them to expel lighter hydrogen rich air through the high vent while fresh air flows in through the low vent. This will only take place when the hydrogen rich air is as warm or warmer than the outside air. When the hydrogen rich air is colder than the outside air, the flow is stifled and the hydrogen can easily reach explosive concentrations. High and low vents also admit hot air during hot weather and cold air during cold weather, eliminating the advantage of an insulated battery enclosure, exposing the batteries to damaging temperature extremes.


Why not use a direct vent tube to the outside?

Some batteries can be connected directly to a tube that can then pipe the hydrogen directly outside. This approach can be very effective, providing the tubing is properly installed and is never disconnected from the battery. Since there is always a possibility that a battery could become disconnected from its vent tube, either by accident during maintenance, cracks due to aging, or by thermal expansion over time, this solution does not guarantee that hydrogen will never escape into the air inside the shelter causing a disaster. Ventilation of the air space in the cabinet or shelter is the most cert


How much ventilation is needed?

The goal of any hydrogen ventilation system is to keep the concentration below the LEL of 4%. When no Hydrogen is present, no air should be moving through the ventilation system in order to conserve heating/cooling energy and to reduce temperature extremes. During normal battery charging, up to 20 cubic centimeters per hour per battery may be released. To achieve a 1% hydrogen concentration, this must be mixed with 99 times its volume of air, or 1,980 cc/hour/battery. To maintain a concentration below 4%, over 480 cc/hour/battery of air must be mixed with the hydrogen.

During thermal runaway, the need for ventilation depends on how many Watts the battery charger can pass into the batteries. Each Watt can produce up to 3.4 cc/min or 204 cc/hour, requiring about 20,200 cc/hour per watt of air to maintain a 1% mixture, or 5,000 cc/hour per watt of air to stay below a 4% concentration.


What is the passive hydrogen vent?

The ideal hydrogen ventilation system would react only to the presence of hydrogen, and not provide constant ventilation air when it is not needed to avoid the extra heating and cooling problems that this would create. It should not require a fan or other mechanical or electrical components that are prone to random failures. It should not react to air temperature, only the presence of hydrogen.

The Zomeworks patented H2Vent™ passive hydrogen ventilation systems were designed to meet these specifications. This system is available on all Cool Cell™ passive temperature regulating battery enclosures. The H2Vent™ passive hydrogen ventilation systems can also be designed into other battery charging cabinets, vaults, rooms or shelters. For more information contact Zomeworks Corporation, 1 (800) 279-6342.