Bloated batteries sound like fodder for yet another “Made in China” recall story – and it’s not inconceivable that some manufacturing defect could turn out to be the culprit in this case.
But when it comes to lithium ion batteries, it turns out that how you charge one can mean the difference between a well-functioning device and one that bears a resemblance to a certain top criminal under-lord in the galaxy. “I’ve seen lithium-ion batteries that are so swollen you’d be tempted to prick them with a pin,” says Don Sadoway, a professor of Materials Chemistry at MIT who is an expert in advanced battery technologies. The problem has to do with the chemistry of lithium ion batteries, which reacts unfavorably to overcharging.
“There are strict limits on how much current can be put through a lithium-ion cell,” explains Sadoway. “During normal charging, you never see metallic lithium, which is inherently unstable. But during overcharging, the lithium builds up faster than it can dissipate. The result is that metallic lithium plates up on the anode. At the same time, the cathode becomes an oxidizing agent and loses stability.”
Problems while charging
The big danger, says Sadoway, is that this chemical reaction is accompanied by heat and-- as every engineer knows -- warm gas occupies more space, which is what causes the battery to swell. The bulging battery is now, in essence, a ticking time bomb. As a safeguard, today’s battery engineers normally design in internal charging circuits, using charger ICs and vendor-supplied reference designs.
Rather than controlling the charging directly, these internal battery pack circuits prevent an overcharge. And if they detect a limit, they shut down the battery.
Design of these circuits can be tricky, as consumers can accidentally connect the wrong power adaptor to a device. So engineers need to plan for almost any contingency. And if they’ve been designing for nickel-metal-hydride batteries in the past, they need to throw the existing charger out as it will be incompatible with the new design.
Apple learned that lesson the hard way in the early 1990s with a laptop that was designed for use with either a nickel-metal-hydride or lithium-ion battery.
To save space, Sony (the battery maker) put the charging protocols on the motherboard and supplied a single charger. Problem was nickel-metal-hydride charges at a much higher rate than lithium-ion. Ouch.
Some useful articles on battery charging: Lithium-ion Batteries, ECN Magazine 04/01/07 Charger-Circuit Designs Fulfill Consumer Needs ECN Magazine 02/06/07