NiCad thermal runaway

Mech, Spring, 2004 by Wallace Williams

I want to focus on battery safety and thermal-runaway procedures. Three mishaps recently have been reported on NiCad batteries that overcharged in the aircraft during flight. Fortunately, aircrew noticed the condition and took appropriate actions to prevent fire or toxic fumes in the aircraft.

How many maintainers would know what to do if a runaway were to happen on deck and during a maintenance evolution? Not many, so I want to share information with the fleet on how to handle this situation.

An excerpt from an article printed in the summer 2003 issue of Mech (reprinted with permission from Touchdown-the Australian Navy Aviation Safety and Information Magazine) gave a good explanation of thermal runaway: "[It] is a condition in which the current for a fully charged nickel-cadmium battery rises out of proportion to the impressed-voltage level. This condition is caused when heat from oxygen recombination, an inherent property of most rechargeable batteries, causes the battery's voltage to drop, as it gets hot."

In event of a thermal runaway, electrical power should be isolated, and no attempt should be made to handle or move the battery for at least 30 minutes. After this period or when the battery is cool enough, carefully remove the vent caps of the affected cells. Make sure you wear a facemask, gloves and protective apron when doing this work.

During a safety survey, I always look at the posted thermal-runaway procedures and often find that squadrons inadequately address the potential situation.

I tell each activity they should use ORM to help them to develop tailored procedures. Start with a review of the battery manual (NA 17-15BAD-1, Chapter 4), and use that reference as a basic template. Each aircraft's maintenance manuals also should be reviewed to evaluate the type of batteries and their location--whether in the aircraft or on the charging bench. Procedures then must be tailored for any particular situation.

The basic procedure is to isolate the battery electrically, allow gases to vent, remove it if possible, cool it if possible, minimize damage to surrounding components, and then notify the base fire department. Once the battery is cooled and removed from the aircraft, disposal issues must be addressed.

These thermal-runaway procedures must be easy to follow and must be used to train personnel. Following the five-step process of ORM will help you develop safety procedures and SOP.

Senior Chief Williams was a maintenance analyst at the Naval Safety Center. He recently transferred to the Fleet Reserve.