Without proper ventilation, an outdoor battery enclosure can accumulate explosive concentrations of hydrogen. The explosive power of hydrogen rich air can blow out the doors, walls, and roof of an enclosure. No company that deploys outdoor battery enclosures in public places can afford to ignore the risk of inadequate hydrogen ventilation.
Batteries in enclosures can go into thermal runaway, which very rapidly produces hydrogen, whenever several conditions are simultaneously present.
Solar heat loading will cause the internal temperature of a thermally unprotected enclosure to go above 100 deg. F in most of the country during much of the year. According to Bellcore, internal enclosure temperatures will increase to between 21 deg. F and 27 deg. F above the external daily high temperature, depending on the U. S. location. Thus, if the external daily high temperature reaches only 80 deg. F, then the internal temperature of an unprotected enclosure will surely rise above 100 deg. 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 as little as 35 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 over just a few hours. The concentration can keep rising to the point that ignition will completely destroy the enclosure and its contents (above 10%). Given the large number of enclosures in populated areas, the potential for collateral damage to people and property is unacceptable.
High and low vents are unreliable and 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.
All Cool Cell™ passive temperature regulating battery enclosures are fitted with one of two patented methods of hydrogen ventilation. These are the Convection H2 Vent™ (U.S. Patent No. 5660587) and the Diffusion H2 Vent™ (U.S. Patent No. 5603656).
Generally, the Convection H2 Vent™ is used in applications where multiple strings of batteries are housed in the Cool Cell™, providing the potential for a rapid accumulation of a large amount of hydrogen. The Diffusion H2 Vent™ is installed in Cool Cells™ designed for fewer, smaller batteries.
Like the Cool Cell™ itself, both H2 Vents™ are completely passive devices. There are no sensors, fans, or other electrical or moving parts. Both H2 Vents™ are virtually failure proof, a critical specification in this application.