Battery Pack Safety—Electrolyte Spillage and Electric Shock

erates the reactions between the negative electrodes and the battery electrolyte. Select abuse-tolerant materials and protecting cells within the battery against overcharge and overdischarge to mitigate the hazards of exposing battery materials to high temperatures.

Overcharge and overdischarge protection may be achieved through adjusting the battery cell chemistry to minimize the effects of overcharge and overdischarge using protective battery electronics. In addition, the battery cell chemistry may be adjusted to protect the electrolyte from cell oxidation during battery overcharge. By introducing an electrolyte additive, the reaction will reversibly oxidize above the normal maximum positive electrode potential and below the potential at which the bulk electrolyte material oxidizes. Overcharge and overdischarge protection may be improved by ensuring that each cell contains a chemically balanced amount of positive and negative electrode materials. Battery protection electronics also provide cell protection against battery overcharge and overdischarge by using a combination of battery pack fuses and internal safety mechanisms. Using smart battery protection, the electrical safety mechanism operates during accidental overcharge, when gas evolves. Although normal cell reactions do not generate excessive amounts of gas, pressure build-up causes venting due to gas formation. This in turn leads to failure of the traction battery due to loss of electrolyte. The mechanism operates upon raising internal cell pressure until a vent opens. This in turn breaks the battery circuit.

In a battery pack, an individual battery using an organic electrolyte may cause hazardous electrolyte spills in the event the cells are damaged. The design of the cell and battery container requires seals that can prevent electrolyte spills. Optimizing the amount of battery electrolyte can limit the severity of spills that occur in addition to battery seals. The amount of electrolyte required to conduct ions throughout the life of the cell must also account for the electrolyte decomposition during cycling.

BATTERY PACK SAFETY—ELECTROLYTE SPILLAGE AND ELECTRIC SHOCK

The EVs currently produced worldwide carry a large number of traction batteries onboard. Therefore, a large amount of electrolyte is in either liquid or gel form. In the event of an EV accident, a rollover or crash, there is an associated hazard associated with exposure to such a large amount of electrolyte. This hazard further extends to vehicle occupants,

neighboring vehicles, bystanders, and emergency and clean-up personnel. Some of the important issues that must be addressed in understanding what types of traction batteries are expected to be in production use over the next 5 to 10 years, including their form (liquid or gel type electrolyte), chemical properties of the traction batteries, and associated battery pack temperatures of the various electrolyte solutions are:

• What is the nature of the electrolyte solutions in terms of their pH—namely are they acidic, alkaline, or water reactive solutions? • Where are the battery packs located in the EV? • What are the safety problems associated with the electrolyte contact in the event of a rollover spillage to EV occupants, rescue teams, or clean-up personnel? • Can battery electrolyte spillage result in potential fire hazard or thermal electrolyte burns? • Can the battery electrolyte spillage result in toxic or asphyxiant vapors? • Under what conditions can an electrolyte spillage serve as an electrical conductor or short circuit, thereby creating a fire hazard? • What are the potential safety consequences of having spilled electrolyte from an EV crash mix with a different electrolyte or vehicle fluid including gasoline, diesel, engine coolant, or oil?

Furthermore it is important to:

• Determine the amount of electrolyte spillage allowed after a crash or rollover. • Determine the requirements for the spillage of high temperature liquid coolants from the EV batteries. • Determine what locations of the traction battery pack minimize the battery electrolyte spillage. • Determine if the traction battery pack should use a dual-walled design such that in the event of a rollover, damage of the outer wall of the battery pack will not result in electrolyte spillage. • Determine if there should be sufficient labeling inside the battery pack—the EV—to better assist emergency rescue teams at the scene of the EV crash. • Determine the electric shock hazards associated with an EV. Since most EV powertrain systems operate under relatively high levels of electric power, 600V, 550A maximum. There is a potential for electric shock to persons associated with EV repair and maintenance personnel.