Enduring bonds: optimising EV battery reliability

Author : Richard Warrilow | Founder | Declaration

01 April 2021


The reliability of battery pack & power modules used in EVs depends largely on the performance & integrity of hundreds of wire bonds. Here, electronics engineer, technical writer & founder of technical communications agency, Declaration, Richard Warrilow talks to supplier to the microelectronic & advanced technology research & industrial sectors, Inseto & electronic manufacturing solutions provider, Custom Interconnect Limited (CIL)...

...and explores production challenges and solutions.

full version of this article was originally featured in the April 2021 issue of EPDT magazine [read the digital issue]. Sign up to receive your own copy each month.

The global electric vehicle (EV) battery market is predicted to see a CAGR of a little over 18% running up to 2027, by which time it will be worth more than $133billion, according to data analysis company, Verified Market Research (VMR). This market will be served by a variety of battery types, of which lead-acid and lithium-ion (Li-ion) are but two.

An EV’s battery is its ‘defining component’. It sets the range, which potential customers typically ask about even before price. The role played by the battery, its chemistry and size will be governed by the type of vehicle and, as readers will certainly be aware, a battery’s electrical ‘size’ is expressed in terms of energy capacity (in kWh) and power (in kW).

For example, the role of a 12V lead-acid battery in a solely internal combustion engine (ICE)-powered vehicle is mainly for energising the starter motor. Transient currents (cranking amps) are high, warranting a power of a few kW, and battery energy is circa 1kWh. In sharp contrast, a purely electric vehicle with a lithium-ion (Li-ion) battery pack will have a power requirement of about 100kW and an energy capacity of tens of kWh. The potential difference will be much higher too, typically several hundred volts.

Between these two extremes, we have mild hybrid, full hybrid and plug-in hybrid electric vehicles (MHEV, HEV and PHEV, respectively) which mainly differ by how much mobility is provided by their electric drivetrain components.

Battery packs
For EVs, the battery is technically a ‘battery pack’. Fitting the largest pack possible for a given vehicle body is not practical because of the weight penalty. More prohibitive, though, is cost, as according to a paper written for the 53rd CIRP Conference on Manufacturing Systems in July 2020, an EV’s battery pack accounts for some 30% of its cost. Of that percentage, about 40% is associated with manufacturing.

Not surprisingly, battery pack manufacturers are looking to get the most from well-established and proven manufacturing technologies to make increasingly dense and complex battery packs, while keeping costs as low possible.

A battery pack is made up of multiple battery modules, connected (using bars, bolts and heavy gauge cables) in parallel and series combinations to produce the desired energy and power characteristics. Each module contains
several cells – of pouch, prismatic metal can or cylindrical type – and, in many cases, dedicated power and thermal management systems.

The two most popular methods for connecting cells are laser welding and ultrasonic wire bonding. Of these, the latter is more popular. Jim Rhodes, a Director of Inseto (a technical distributor of equipment and materials for advanced engineering sectors), comments: “Unlike the laser weld process, minimal heat is generated with ultrasonic wirebonding. Indeed, the process is dubbed ‘cold weld’, so there’s no risk of overheating and damaging the cell, which can result in thermal runaway during manufacture...

Is there an after-life?
As we all know from using any product with a rechargeable battery, efficiency decreases over time. Two things determine the lifespan of a rechargeable battery cell:
• Time spent at minimal discharge capacity.
• The number of times the cell is charged and discharged.

For EVs, a battery’s end-of-life (EOL) is considered to have been reached when it can no longer be charged to more than 80% of its intended capacity.

According to an IDTech report (‘Li-ion Battery Recycling: 2020-2040), the rapid adoption of EVs is set to result in 7.8 million tonnes of EOL Li-ion batteries per year by 2040

Read the full article in EPDT's April 2021 issue...

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