BATTERIES I n recent years, there has been a substantial increase in the number of battery stores and charging rooms incorporated into buildings used by a variety of occupiers. The purpose of these rooms that potentially contain wet-cell batteries could be to offer emergency power in the event of a mains failure. Because most power failures are temporary, backup batteries can be a good solution to keep critical systems running without the need for generators. Many other applications use wet-cell batteries for example, in aviation, public transport, warehousing, university research and development departments, and the Ministry of Defence. There are two main types of battery solutions for the storage of power wet-cell lead-acid and lithium ion. The latter are used in todays electric vehicles and mobile phones, and have the advantage of a deeper discharge of up to 70% of the full charge, as opposed to 50% for a lead-acid battery. However, they are more prone to overheating. Wet-cell lead-acid batteries are easier to recycle and cheaper than their counterparts, so they tend to be used for industrial and commercial applications. Lead-acid batteries produce hydrogen (H2) and oxygen (O2) two parts H2 to one part 02 during a normal charge cycle. These gases are vented to air in the room. In the case of valve-regulated batteries, venting can occur when a battery becomes warm or is overcharged. TAKING CHARGE A build-up of hydrogen in battery stores or charging rooms will create an extremely flammable atmosphere, so detection systems should be competently installed and commissioned, says Medems Chris Dearden | GAS DETECTION 70% Lithium ion batteries have the advantage of a deeper discharge of up to 70% of the full charge, as opposed to 50% for a lead-acid battery Rising temperature and other factors will increase the amount of gases produced and discharged to air. Hydrogen dangers Hydrogen gas is colourless, odourless, lighter than air and highly flammable, so it must not be allowed to accumulate in concentrations greater than 1% of the volume of air in a space. The lower explosive limit for hydrogen is 4%, and the upper explosive limit is 75%. A build-up of H2 in a battery room installation will create an extremely flammable atmosphere. With added O2, the risk of a powerful explosion is significant. Adequate ventilation is important to stop gas build-up, but consideration should be given to installing a hydrogen gas detection system to give an early warning of a change in gas levels or failing ventilation. Gas detection The installation of a gas-detection system for H2 enables the permanent monitoring of a given space for the target gas. This will guard against equipment and battery failure which could lead to significant levels of H2 being produced in a short period and minimise the risk of both explosion and exposure to users. A pre-alarm level of 1% with a high alarm level of 2% ensures time is given for action to be taken to stop a dangerous situation from developing. Placement of the detector heads should be at high level near a ceiling because H2 is lighter than air, and www.cibsejournal.com January 2019 43 CIBSE Jan19 pp43-44 Medem batteries.indd 43 21/12/2018 15:10