What’s covered, what’s not, and sensible options to close the gap
It is a dramatic understatement to say that the world of electronic devices continues to go more portable. The mantra of smaller, lighter, and faster shows itself through the myriad of devices we use as we move through our world outside of traditional work and home environments. Enjoying such freedom also means not having to plug in for our power.
To realize this aspect, most of our portable devices utilize the wonder of rechargeable lithium ion batteries as their primary power source. Lithium offers a game-changing blend of extended runtime, light weight, low cost, and long overall life desired by users of all types, but the advantages come at a price. As frequently immortalized in internet videos, lithium ion batteries that have been designed incorrectly, manufactured improperly or subjected to abuse can pose harm to both persons and property. Understanding the risk of such events and their prevention is critical. Realizing this within the product development realities of cost control and schedule compliance requires maximizing the value of testing investments.
Mandatory lithium battery transportation testing per the UN Manual of Tests and Criteria Section 38.3 provides a good start. To understand the inherent benefit in UN testing, we must first look at those risks commonly associated with lithium batteries. From there, we will review the UN test profile and overlay it with our listing of common risks to expose what gaps exist. To address those gaps in the risk profile, we will propose selected additional testing options that offer high value at reasonable cost and minimal test duration.
Common Lithium Battery Risks
What are some common rechargeable lithium battery risks? To answer that question, we must first acknowledge that some risks are inherent to the cell itself while other risks come about when the cell is integrated into a battery pack. The list in Table 1 is far from all-inclusive, but highlights what many would consider typical.
It is critical to note that the cell itself is always the source of greatest risk. When bad things happen outside the cell, there is a chance to mitigate potential safety events through protection circuitry or other passive protective devices (PTC, fuses, etc.). When problems happen inside the cell, such options generally do not exist, particularly in lower-cost, less-complex applications common to portable electronic devices.
Successful completion of UN transport testing found in section 38.3 of the UN Manual of Test and Criteria is required to ship lithium cells or batteries by all common commercial modes in almost every global locale. It requires testing at the cell level, battery pack level and battery pack assembly level (an assembly of previously tested battery packs). Commercially available cells are generally tested by cell manufacturers. Battery packs may be tested by the pack assembler as a client service or independently tested by the supported end-device manufacturer. The standard is self-certified, so third party testing, although common, is optional. Typical market parameters for the U.S. are given in Table 2. Note that these are intended only as “order of magnitude” for general reference.
|16 battery packs
The UN test regime, commonly known as “T1-T8” testing, is extensive in scope and particularly mechanically intense. (Figure 1) A portion of both cells and batteries undergo 50 charge-discharge cycles for aging, whereas others are tested fresh (cycle 1). Tests 1-5 are done sequentially using the same group of samples. Testing consists of a high-altitude simulation is followed by five days of thermal cycling outside of normal li-ion storage temperatures. Mechanical testing is next and is comprised of nine hours of vibration and 18 high energy mechanical shocks. To round out the sequence, the cells and packs are externally shorted at elevated temperature. An additional group of cells is mechanically crushed or indented (impact test), while a separate group is reverse charged (forced-discharge). At the pack level, a separate group of packs is overcharged at elevated voltage and current.
Overlaying our listing of common lithium risks with the UN test profile reveals some positive news – A good portion of the identified risks are evaluated, at least to some degree:
- At both the cell and pack level, the combination of cycling with short-circuit at elevated temperature can help expose internal cell problems such as a propensity for dendritic lithium metal plating leading to internal shorting.
- The sequence of thermal cycling, vibration, and mechanical shock at both the cell and pack level represent a severe assessment of mechanical robustness. Note that after each of these stress exposures, the device is checked for voltage loss (a sign of internal shorting) and mass loss (an indication of a venting event [excessive pressure] resulting from an internal cell problem). Furthermore, the follow-on short-circuit test at the pack level partially evaluates whether the safety circuit remains functional after mechanical stress.
- Crush/impact testing assess cell mechanical robustness to external compression forces
that are expected to generate some level of
- Forced-discharge testing drives the cell into full voltage reversal as might happen in an extreme out-of-balance situation. This test basically turns the cell into a resistor (drops voltage instead of sourcing voltage).
- Overcharge testing simulates a pack being left on a defective charger for 24 hours and then placed in the shipping channel for seven days (there is a seven day wait period after overcharge exposure). This is a test of the pack’s safety circuit design margin to protect the cells against long-duration exposure to elevated voltage and current.
Updating our original list of common risks noting coverage offered by UN testing in shown in Table 3.
To round out the risk profile, what are some testing options that meet the criteria of being low-cost, short-duration and ideally involve only a small number of samples? To the surprise of many, viable options do exist. With proper equipment and expertise, these can be done in-house or alternatively provided by most commercial battery labs. Let’s begin with the cell level.
(<$1k; <1 week; 5 samples)
This test is also known as “thermal abuse” and is commonly referred to as a “hot oven test.” It can be found in various standards such as IEC 62133, UL 1642, and UL 2054. Its purpose is to provide an assessment of the high temperature stability of lithium cells. This involves the response of the separator as well as the active material in the cell. The test consists of placing fully-charged cells in a blast oven at room temperature and ramping the temperature at a rate of 5°C/minute to 130°C and holding that for 10 minutes (some newer standards are pushing this out to 30 minutes). This test will make the cell non-functional by design, but it is how it fails that counts. Failing safe is acceptable. Evolution of flame or explosion is not. This test is a good method to evaluate competing cell options, particularly if the soak period is extended past the specified minimum time, ideally to failure. The reality is that there is a very big difference in the relative stability between a cell that fails at 12 minutes and one that lasts 58 minutes.
(<$1k; <1 week; 5 samples)
This is an intentionally destructive test to assess safety margin. The purpose is to evaluate the cell’s inherent safety when exposed to high voltage and separately to high current. Both tests are typically run at ambient temperature to minimize costs, but could be run at the maximum specified charging temperature. Cell voltage, current, and temperature should be logged.
- Over-Voltage: Lithium ion cells can enter thermal runaway when maximum charge voltage limits are sufficiently exceeded. Thermal runaway is essentially where the cell begins to self-heat at an increasing rate and heat is produced faster than it can be removed. This results in destabilization of the cell and release of energy by fire and/or explosion. It is important to assess at what voltage this begins so that safety circuit thresholds in the battery are set accordingly to stop charge before this occurs. The test consists of charging a fully discharged bare cell at the maximum charge current to 6 Volts.
- Over-Current: As all cells have a finite equivalent series resistance, internal resistive heating is proportional to current, thus higher currents can cause safety concerns. (Note that resistive heating is but one of multiple heating processes that occur in the cell during charging.) The chances of entering thermal runaway are much less than with elevated voltage, but nonetheless warrant evaluation. The test consists of charging the cell at two times the maximum specified charging current to the maximum specified charging voltage. Note that the elevated current only affects the initial constant-current (CC) portion of charge as the current will taper during the final constant-voltage (CV), although thermal momentum may keep temperatures rising throughout.
What about the pack level?
(<$1k, <1 week, 1 sample: non-destructive)
This is a simple evaluation to confirm that the safety circuit protects against over-voltage, over-current, and under-voltage as intended. It is possible that what the designer intended has fallen victim to undesirable variations. These might include part tolerances, supplier variances, manufacturing variation, interactions with other active or passive components in the circuit and even undetected negative impacts of intentional design improvements. This is generally a bench test, but care should be taken particularly when evaluating over-voltage. It is typically run at ambient conditions to save costs, but could be run at the minimum and maximum charge and discharge temperatures for a more thorough assessment.
Component Temperature Test
(<$1.5k, <1 week, 1-2 samples)
This test originates from UL 2054. The intent is to confirm that component thermal specifications are not exceeded during the maximum specified charge and discharge. It also incorporates the impact of the maximum operating temperatures through calculation, but it must be noted that this approach is overly conservative by nature as it treats increased ambient temperature as a linear impact when it is not. The test process begins by instrumenting a pack’s critical components with thermocouples. In a series pack, the series cells (or parallel groups) are instrument for voltage to ensure that cell voltage maximums are not exceeded during charge (imbalance concerns). The pack is fully discharged for the charge test. Similarly, a fully charged pack is used for the discharge test. The same pack can be used for both tests as the testing is non-destructive.
Engineering Teardown of a Battery Pack
(~$500, <1 week, 1 sample)
This begins with a review of the internal construction of an actual pack against its assembly specifications. Are all the safety critical parts included and are they placed where they should be? A key concern is insulator placement. These are usually hand placed and manufacturing variation can lead to an increased potential for shorting inside the pack. Additionally, the mechanical aspects of the pack should be reviewed. Is the design going to break under physical stress? In this vein, it is also common to do a teardown after mechanical stress such as vibration, shock or repetitive drop. In this case, stress marks or other stress indications should be evaluated for their potential to cause latent defects in actual field use.
Short-Term Cycling with Cell Voltage Tracking
(Cost & duration vary with charge and discharge parameters; 3-5 samples)
Recall that the forced discharge test (T8) in UN 38.3 gives some insight into the ultimate outcome of cell imbalance in terms of the cell itself. By cycling the pack while accurately monitoring series cell voltages, one can gain more specific insight as to which cell in the design may be more susceptible to accelerated aging causing capacity imbalance in the pack. In a pack, this may be caused by factors like parasitic resistances that drain a specific cell or proximity to heat sources (like the FET on the safety circuit) that cause hot spots on the cell and accelerated capacity degradation. In actual field use, note that the supported host device may unevenly heat the battery as well. As with most of the tests presented here, it is usually run at normal ambient temperature to reduce costs.
In summary, rechargeable lithium ion cells and batteries are the predominant power source for our increasingly portable world. Despite very active efforts in the energy technology sector to find new portable energy solutions, most would agree that lithium ion batteries will maintain their market supremacy for the foreseeable future. Although the technology itself is mature, it involves very high energy densities and must be designed, manufactured, and employed properly to assure safety.
Understanding the relative risk for a given design to include misuse is important for those providing portable products to the market. Mandatory UN transportation testing covers many of the most common lithium ion risks. Past this, a number of inexpensive, short duration, low sample count testing options have been presented that can help further complete the assessment of risk. Your test lab has a wealth of knowledge about these options and should be both able and willing to provide you guidance on your specific situation.
John C. Copeland is co-owner and technical manager for Energy Assurance LLC, an independent, fully-accredited cell and battery test laboratory. His career has included various positions in quality engineering, reliability engineering, failure analysis, project management, supplier assessment, and quality management in the electronics and portable energy sectors. John holds a BSEE from Auburn University, MSQA from Southern Polytechnic State University, and is trained as a Six-Sigma Black Belt. He can be reached at email@example.com.