Fulfilling Automotive Promise of EV, Hybrid Battery Technology

With the federal government nearly doubling fuel economy standards to 54.5 mpg by 2025, electric vehicle and hybrid technology is set to play a vital role – if lingering battery life and overheating issues can be resolved.

With battery packs on EV and hybrid vehicles only storing the energy of about 1-2 gallons of gasoline, more needs to be done to harness every milliamp of electricity.

The challenge is, what is your MPG-e, or electric miles per gallon? Any losses in getting battery energy to the motor will compromise EV or hybrid range and viability.

The problem has been that within batteries and electrical connections, traditional fasteners have difficulty maintaining conductivity and connectivity with EV and hybrid battery terminals because they tend to lose clamp load. After extended car vibration and thermal cycling, traditional fasteners lose about half their original clamp load.

Inside EV and hybrid batteries, several packs are typically linked to each other in a series. If a connection is weakened by losing clamp load, you lose not just one battery cell but the whole series of battery cells.

When EV and hybrid fasteners lose clamp load, their batteries lose electrical conductivity, heat can build up and electric arcing can occur.

To assure adequate clamp load and joint integrity in battery packs, battery terminals and the battery box itself while improving connectivity and battery life, engineers are finding a solution in innovative fastening and assembly technologies.

Traditional locking fasteners do not address a basic design problem with the standard 60-degree thread form: that the gap between the crest of the male and female threads can lead to vibration-induced thread loosening, inadequate clamp load and overheating.

These challenges can be met with a locking fastener with a 30-degree wedge ramp added at the root of the thread that mates with standard 60-degree male thread fasteners.

The wedge ramp allows the bolt to spin freely relative to female threads until clamp load is applied. The crests of the standard male thread form are then drawn tightly against the wedge ramp, eliminating radial clearances and creating a continuous spiral line contact along the entire length of the thread engagement. This continuous line contact spreads the clamp force more evenly over all engaged threads, improving resistance to vibrational loosening, axial-torsional loading, joint fatigue, and temperature extremes.

Since the re-engineered thread form has up to 30% more retention of clamp load underhead pressure than traditional threads, the actual faces of the battery terminal are pressed together for better conductivity. On battery terminal posts, for example, there’s an increase in electrical current available to flow through the connection.

The increase in retained clamp load and conductivity could help not only with EV and hybrid batteries but also with terminals connecting leads together. It could help with everything essentially from individual battery cells to large grounding terminals to any electrical connections carrying high capacity charges throughout EV or hybrid systems.

The locking fastener with its 30-degree wedge ramp has been validated in published tests at MIT, the Goddard Space Flight Center, Lawrence Livermore National Laboratory, and British Aerospace and has been used in aerospace batteries for a decade.

In automotive, it has long been used in applications ranging from ring gears, torque converters, and chassis assembly to exhaust manifolds and axle, turbine, or transmission housings, as well as EV and hybrid battery applications.

Regardless of battery type, the design challenge is to ensure that more current gets from point to point as efficiently as possible in EV and hybrid vehicles, without risk of fasteners coming loose throughout their service life.

That goal is within reach for designers now.

Kevin Peacock is an application engineer for Stanley Engineered Fastening in Madison Heights, MI. For detailed test data on the Stanley Spiralock fastener, including comparative graphic loading characteristics and photoelastic analysis/load vector comparison animation, visit spiralock.com.