Several years ago, some laptop fires scared the public and prompted widespread recalls by some of the most reputable lithium- ion (Li-ion) cell manufacturers. The long-lasting repercussions in the battery industry ranged from cell supply shortages to public misconception about the safety of portable electronics.
Individual companies have taken many steps to ensure that these events do not recur, and the industry has joined forces to unify safety standards. The IEEE Standards Association, the standards-setting body of the Institute of Electrical and Electronics Engineers (IEEE), has published two standards relevant to the design and manufacture of portable battery systems: the 1625 Standard for Rechargeable Batteries for Portable Computing and the 1725 Standard for Rechargeable Batteries for Cellular Telephones.
1625: A CLOSER LOOK
Originally published in 2004, an update to IEEE 1625 was published late in 2008, partly as a response to the laptop fires. IEEE 1625 guides the industry in planning and implementing controls for battery design and manufacture. Safety and the prevention of dangerous malfunction were at the forefront of discussions among the document’s contributors.
Thi s voluntary s tandard defines approaches for evaluating and qualifying portable computing batteries, verifying their quality and reliability, and educating and communicating with end users. But the battery industry can utilize these guidelines as a resource to ensure a safe and reliable end user experience across many diverse types of products. The standard outlines a very conservative approach to design, however, and it will be a challenge for most designs to comply with 1625 in its entirety.
Engineers may be familiar with the original 1625, but the changes deserve a timely review to understand the most useful and broadly relevant guidelines. The standard guides the system and subsystem designers through five major areas—system integration, cell, pack, host device, and total system reliability. This organization of the document remains largely unchanged in the revised version, but many of the details have new, very stringent requirements.
Although this standard affects the designers of the overall system and its various subsystems, such as the cells, let’s focus on the design of the battery pack itself discussed in section six. Complete compliance may be difficult; for example, the required vent direction leaves very few options for the designer, but there are four points that I consider the most broadly relevant and noteworthy.
FOUR KEY ELEMENTS
First, cell specifications are notoriously difficult to obtain, and performance data under specific usage is even harder to come by. Since many of the high-profile batterypack failures originated at the cell level, many of the new requirements outlined in the document were intended as cell manufacturing and specification guidelines.
The addition of a cell specification sheet, provided by the cell supplier, with specific usage information, is crucial for the safe and reliable design of the battery pack. This is to ensure that the pack designer receives complete information about the cells.
Second, the IEEE 1625 standard is harmonized with the relevant standards from compliance or certification bodies such as Underwriters Laboratories. Regulatory requirements referenced in the standard include UL 2054, UL 69050-1, IEC 62133, and the transportation tests and requirements for shipping outlined in the UN manual of Tests and Criteria (ST/SG/ AC.10/11).
Third, the very last clause in the section is particularly notable. It states that the batteries should be shipped at a state of charge of 50% or less in order to limit the energy that’s available to drive faults.
And finally, a Design Failure Modes and Effects Analysis (DFMEA) approach is a typical example of an industry-wide methodology that can be used to highlight and prioritize the possible roots of faults and hazards and is a best-practice procedure to reduce and minimize potential latent problems. FMEAs of both design and manufacturing process are mandatory for medical device batteries. In fact, it is now required that this type of analysis is performed on any new battery pack design that is intended to comply with IEEE 1625.
The design methodologies described in IEEE 1625 are based on some difficult lessons learned, and they ensure the safety and reliability of designs even when the speed of design is paramount. The standard covers design approaches that ensure reliable operation and minimize the occurrence of faults leading to hazards, but in some cases compliance will be difficult. From the beginning of the design of a portable device, it is necessary to examine all facets of the design, including the battery pack, to ensure the reliability of operation and the safety of the end user.
ROBIN SARAH TICHY is a technical marketing manager with Micro Power electronics inc. She has a doctorate of philosophy from the university of texas for her work in solid-oxide fuel cells.