Until today, I had only read about other people’s woes with battery bulge/bloat/swelling. My HTC Hero from 2009 was not charging properly and a full charge drained out in 45 minutes. Popped out the casing to find a swollen, malformed Franken-battery.
According to this site, battery bloat is “primarily due to the irreversible chemical reaction which a lithium-based cathode undergoes upon charging. The cathode will permanently liberate elemental lithium and form a metal oxide composed of lithium’s ‘partner’ metal. In most lithium ion batteries in the market, the partner metal is cobalt, so overcharging it will form cobalt oxide. Cobalt oxide has a chemical structure radically different from the original lithium cobalt oxide form, which weakens the structure and causes battery bloat. In addition, cobalt oxide is a very toxic substance.”
Yet another source has an abstract which mentions moisture as causing battery bloat — a distinct possibility in humid Singapore.
But don’t you go hating on cobalt and water vapour just yet. With a relatively ancient four year old smartphone, I could actually open the back cover, take the battery out (before it could explode in my pant pocket) and order a replacement for US$10. Try doing that with the newer HTCs or any of the iPhones…