In my conversations with product engineers and designers, I often come across people who feel confident that their lithium-ion batteries are safe because they passed standards-based safety tests. If that is indeed the case, then why did major global companies experience thermal events even after having passed compliance tests? And that brings up a bigger question – are standards-based tests such as UL safety tests sufficient to guarantee lithium-ion battery safety?
Standards-based testing for lithium-ion batteries is required but not sufficient to guarantee battery safety and reliability. Some key points that I would make in support of my argument:
1. Standards-Based Testing is Focused on Abuse Tolerance
The vast majority of standards-based testing is focused on abuse tolerance such as overcharge, crush, impact and external short circuit tests. However, the majority of field failures happen under normal operating conditions and are rarely related to cell abuse. This actually points to how successful standards-based tests have been in preventing failures from abuse.
2. The Rate of Field Failure is Statistically Low
On average field failures for lithium-ion batteries occur at the rate of 1 to 10 parts-per-million (ppm), although failure rates for specific cases can be higher. For instance, Sony’s 2006 laptop battery failures were about 1 in 200,000 which ended up triggering 10 million battery recalls. More recently, Samsung had to test 200,000 phones and 30,000 batteries in order to find the root cause of their thermal events. It is simply not feasible to find failures that occur at such a statistically low rate by testing a hundred or two hundred cells. (Unfortunately, due to their high energy density and flammable electrolyte, all it takes is a few lithium-ion battery thermal events and some news coverage to bring a lot of unwanted publicity).
3. Internal Shorts Aren’t Mitigated by Battery Safety Systems
Though many field failures can be attributed to internal shorts, internal shorts are not mitigated by internal battery safety systems. While a battery management system (BMS) helps the battery stay within its operating boundaries, it does not protect against internal shorts or improve the quality of cells beyond how they were originally manufactured. Our recent webinar on BMS highlights this point very well.
4. A Sound Manufacturing Process is the First and Most Important Line of Defense
Those of you who know me personally have heard me say ad nauseam that battery safety and reliability are very closely tied to the manufacturing process. The majority of lithium-ion battery failures are related to poor manufacturing quality, while others are primarily related to cell design and lack of tolerance for the battery in the final product, as we saw in the Samsung Galaxy note 7 failures.
A sound manufacturing process is indeed the first and most important line of defense in battery safety and reliability. You can have batteries with latent manufacturing defects that pass quality control checks and essentially "masquerade as good batteries." However, as you age or cycle batteries, the manufacturing defects get exacerbated due to cell expansion and contraction effects.
How do you know you have good manufacturing quality?
1) Conduct supplier manufacturing and design audits
2) Look at the supplier’s Statistical Process Control (SPC) data
3) Conduct teardown analysis and CT scans on a handful of cells
Should you perform one or all of these checks instead of relying on your cell supplier’s assurance? That depends on your level of risk tolerance. Some questions that help you assess risk are:
1) How high is the cell energy density?
2) How high is the cell rate of discharge?
3) Is the product is worn on or close to the body?
Based on your risk assessment, you may decide to use a "trust but verify" approach or a "do not trust until you can verify" approach.
5. Standards Organizations and Cell Manufacturers Don’t Know Your Application
Only you know your application and are responsible for evaluating battery behavior under the product application conditions. Standards-based tests or supplier cell specification do not evaluate the stresses that your product environment imposes.
For example, if you have a high rate discharge application or your product causes the battery to experience electrical and mechanical stress, the cell supplier’s spec sheet and testing will not address these specific issues. The Samsung note 7 failure case, where the product design imposed mechanical stress on the cell, illustrates this point very well. Individual cells may have tested fine, but their behavior under product constraints and the interplay of cell electrical behavior and mechanical stress was not really evaluated.
In summary, standards-based testing is required, but not sufficient guarantee battery safety and prevent field failures on its own. Effectively mitigating battery failure requires a well-controlled battery manufacturing process that produces high-quality cells, along with application-specific testing —a critical component in battery reliability.
To learn more about how to ensure the reliability of lithium-ion batteries in your applications, join us on October 10th, 2019 for our webinar Mitigating Risks of Lithium-Ion Pouch Cells: Common Mistakes and How to Avoid Them presented by Vidyu Challa by clicking the registration link below!