PowerSource

And the plot continues to thicken with Apple joining Dell and issuing recalls of Sony-cell-based laptop battery packs. But there are clues that Sony's cells are not the only contributing factor in the recalls. Note that the battery packs' problem of overheating to the point of catching on fire seems to occur only when the units are plugged in, which implies that it's during recharging. Is it possible that Dell and Apple's aggressive charging techniques exacerbated the contamination problem with the Sony cells?

Keep in mind that these battery packs are based on cells which Sony supplies to many laptop manufactures – Sony is one of the top suppliers of 18650 lithium ion battery cells, yet the problems are apparently limited to Dell models, and some discontinued Apples. (Lenovo, for example, has said, nope, we're not seeing it.) So what clues can we glean as to what's really going on? And what lessons can we draw as design engineers? And I don't think the lesson to be learned is "The problem is the cell supplier's," but rather, "When you try to wring every last bit of performance from your charging circuitry you can stress those cells in ways that have distressing results."

First, a quick run-down of what a battery pack is and isn't. It's not a collection of consumer-market batteries. A battery pack is based on lithium ion cells that come in an industry-standard package, the 18650, so-called because it's an approximately 18mm diameter x 65mm-long cylinder. The 18650 cell looks on the inside like a layered jelly-roll consisting of the anode and cathode layers separated by a plastic layer that's permeable to lithium ions that move between the anode and cathode and generate a current. Says Dr. Robin Tichy of high-end battery pack designer Micro-Power, "The separator is a flexible polymer membrane and like any other plastic [as it heats] it has a softening point where it's more likely to be breached. So a contaminant that’s hard and sharp would be more likely to puncture it at higher temperature."

Now take a look at Sony's press release, issued after the Apple recall, which states, "The potential for this [battery pack over-heating and/or catching on fire] to occur can be affected by variations in the system configurations found in different notebook computers."

Dell advertises its super-fast recharge time for laptops. With a too-aggressive recharge scheme that too-rapidly heats cells cells with a probably small – though admittedly out-of-spec – level of contamination you could have a perfect storm that causes battery cells to "rapidly disassemble" as they say in the battery business.

But wait, you say, "Haven't there been several news reports lately about laptops catching fire while in airplane cargo holds? Those puppies sure aren't plugged in and recharging." That's a different scenario: The battery packs implemented in these cases have been aftermarket packs, which if you've been reading previous blogs and articles (and following the discussion threads) you'll remember are prime candidates to rapidly disassemble. Here's a quote from the Wall Street Journal, "The trend with smoke or fires would be some type of after-market use, like with spare batteries," says Bill Wilkening, who enforces hazardous-materials regulations at the Federal Aviation Administration. Product regulators say replacements marketed as compatible with many different devices frequently aren't. Some of them, including a significant number of counterfeits, lack circuitry that protects the battery cell from overcharging."

Several IC vendors have recently introduced battery charging chips that distinguish themselves by their individual charging algorithms. Whether you roll-your-own charge/recharge subsystem, or go with on of these new chips, make sure you thoroughly understand the impact this will have on your battery pack. Your company's liability goes far beyond a simple battery pack replacement.