Safety in Battery Design

pack, the electronic control module, the motor, the battery charging port, and other high-voltage components. Running parallel to these high-voltage cables is an additional pilot circuit that acts as a simple continuity loop. The pilot circuit is integral to the high-voltage charging cable such that it is not possible to disconnect. In the event that a break occurs in the charge cable it happens by doing the same to the pilot cable. If an accident occurs that results in the high-voltage and causes the pilot cable to disconnect, the pilot circuit records the loss of electrical continuity. It will automatically disconnect the high-voltage cabling from the battery pack. The location of this pilot circuit disconnect system is also vehicle-specific but is typically found in proximity to the vehicle battery pack.

Many vehicle manufacturers employ a combination of the disconnect systems for both redundancy and safety purposes. Whether a ground monitor, inertia switch, or pilot circuit is used, it is important to know that these devices isolate the rest of the vehicle only from the traction battery pack voltage. However, lethal levels of electric current may still be present in the battery pack. It is of utmost importance that an EV battery pack be treated with the same caution and respect as a full gasoline fuel tank in an internal combustion vehicle.

All current OEM EVs also have special manual disconnects that decouple the battery pack from the remainder of the vehicle wiring and systems. The locations of these disconnects are very vehicle-specific and are intended to be used mostly by vehicle service personnel during periodic maintenance procedures. Newer electric bus models now have a manual disconnect located on the driver’s control panel. This allows for additional safety in the event that the inertia switch, ground monitor, or pilot circuit fail to disconnect the battery pack from the vehicle wiring systems.

SAFETY IN BATTERY DESIGN

Battery electrolyte decomposition can be hazardous to the EV operator. Overheating of the traction battery pack accelerates the electrochemical reaction that causes electrolyte decomposition. During the first charging cycle, the process of initial formation of the interfacial films leads to the electrolyte reduction. This reduction may continue in to the subsequent charging cycles with certain combinations of the negative electrode and the electrolyte materials.

In addition, electrolyte decomposition leads to phase changes, which can also pose hazards to the EV operator. The organic liquids identified

for the possible use of the VRLA and NiMH battery electrolytes have boiling points in the range of 60 to 250°C under standard conditions. The boiling point of any proposed electrolyte is a key factor, as vaporizing the electrolyte will damage the cell integrity.

For example, vehicle passengers can be exposed if battery containment fails and the electrolyte leaks. Battery pack electrolytes are more likely to leak when a new cell is damaged. New cells contain more electrolyte than previously used cells, because some electrolyte is consumed during cycling. Exposure can also occur during processing of used traction batteries. Overheating, overcharging, and overdischarging can cause decomposition or phase changes in the electrolyte, posing hazards of exposure to the electrolyte decomposition or gaseous electrolyte compounds.

Exposure to other cell materials can also occur during the manufacturing of the batteries. Once battery cells are completed, exposure to aluminum, copper, and nickel will be unlikely. Battery overcharging and venting can cause exposure to the fumes from the decomposition of polypropylene or polyvinylidene fluoride.

Acid spills from the battery pack are also an important factor for battery pack design. A typical flooded Pb-acid battery has 15 to 19 gallons of corrosive electrolyte. A set of 24 batteries contains 360 gallons or 3,600 pounds. Since the electrolyte is corrosive, UBC 307.2.3, The Uniform Fire Code, Article 64, and the local codes that reference the fire code require that the entire battery be surrounded with an acid containment system, especially when the battery exceeds the acid volume limit of more than 100 gallons.

This system adds another $1,700 to $3,000 for each rack of the flooded Pb-acid batteries. In comparison, the VRLA battery has no free electrolyte and thus requires no additional systems. This further translates to additional savings of $1,700 to $3,000.

The VRLA battery pack design has a higher initial cost. However, when the cost of maintenance, ventilation, installation, etc., is factored into the overall cost of battery pack, the VRLA based battery pack is 21 to 36% cheaper than the flooded Pb-acid battery during its entire life. The worst-case scenario of 21% is based on replacing the VRLA battery after 15 years in comparison with 20 years with the flooded Pb-acid battery.

Electrical Safety

Using electrical safety as an example, the EV connector must be polarized and configured so that it is noninterchangeable with other electrical devices such as electric dryers. The method by which the EV

charging equipment couples to the EV can be either conductive or inductive, but must be designed so as to prevent against unintentional disconnection. Additionally, the new electrical codes require that EV charging loads be considered continuous; therefore, the premises wiring for the EV charging equipment must be rated at 125% of the charging equipment’s maximum load.

All EV charging equipment must have ground-fault circuit interrupter devices for personnel protection. Rainproof the battery system, including the battery pack, for outdoor compatible equipment. An interlock to de-energize the equipment in the event of connector or cable damage must be incorporated. Furthermore, a connection interlock is required to ensure that there is a nonenergized interface between the EV charging equipment and the EV until the connector has been fastened to the vehicle.

A ventilation interlock is also required in the EV charging equipment; this interlock enables the EV charging equipment to determine whether a vehicle requires ventilation and whether ventilation is available. If ventilation is included in the system, the ventilation interlock will allow any vehicle to charge. However, if ventilation is not included in the system, the mechanical ventilation interlock will allow vehicles equipped with nongassing batteries to charge, but not vehicles equipped with gassing batteries.

Mitigation of Intrinsic Materials Hazards

The intrinsic material hazards of some of the traction battery designs can be mitigated through battery design and workplace procedures.

Using integrated circuits to monitor battery cells may assist with, both electrical and thermal management. In case of the battery pack, the individual battery temperature and current is monitored by using battery monitoring systems (BMONs).

As mentioned earlier, intrinsic material hazards increase when VRLA and NiMH batteries are exposed to elevated battery pack temperatures. This can cause hazardous conditions such as exothermic and gasproducing reactions. In an EV, heat from the battery itself, and other components can lead to elevated battery pack temperatures. The thermal management system mitigates the hazards caused by elevated battery pack temperatures. Research trends suggest that thermal runaway with heat sensitive (shut-down) separators will be able to stop electrochemical reactions.

The electrical system abuse poses material hazards. Short circuit of cells raises the battery temperature. The battery temperature rise accel-