Polymer fuses: Why nothing much happens when you drop a cell phone in the toilet

Here's some deep background on the picture I took for the PowerSource blog post Baby it's cold outside: Is your cell phone battery pack smart enough to know it?. I was somewhat concerned that when I placed the cell phone into the bowl of water thatsomething unpleasant might happen. I've read a lot lately about rogue lithium ion battery packs, and while water's not an ideal conductor, it is a conductor nonetheless and conceivably it could short out the phone's battery pack. My lovely assistant (ie, husband Doug, also an engineer) scoffed, as lovely assistants are wont to do. Nevertheless, I dropped the cell phone in the water while standing outside rather than inside.

Immediately the phone went "*tick". — a small, odd sound that caused both me and my lovely assistant to instantaneously levitate backwards about three feet. And that was it – nothing else. [sound of scoffing increases here.] So I filed it away for further pondering: The lithium ion battery pack did virtually nothing when dumped in water with a full charge. And of course I then remembered all the stories I've heard of cell phones inadvertently dropped into public toilets with no catastrophes reported, not even quiet "*ticks". (Other than the fact that the beloved cell phone was, well, in a public toilet, and quite likely in a liquid that had a much better conductivity that fresh water.)

Ok, now fast forward to a conversation I had yesterday at APEC with Usha Patel, sales director for Raychem, the circuit protection company. It seems that every cell phone out there has a fuse on the battery pack to protect against conditions such as inadvertent immersion in a conductive liquid, or simply getting a metal splinter across the leads. Battery pack fuses are generally polymer positive temperature coefficient (PPTC) devices that come in various shapes and sizes. One configuration is a flat strap that can be spot welded across the cells in a pack. If such a pack is immersed in water, and the water's resistance is low enough to produce a short, the PPTC should protect it, but this assumes that the cell itself doesn't admit any water. If the water gets in and provides a low-resistance path, all bets are off: The water will provide a shorting path to the internal circuitry and anything can happen.

Another configuration is a disc-shaped PPTC fuse crimped into the header of the cells themselves, and may provide more protection if the cell construction makes it so the fuse is protected from the external environment, such as nasty low-resistance water leaking in.

The more closely you look at lithium ion cells and battery packs, the clearer it becomes that all cells and packs are not created equal.

(btw, there's been more development work being done on polymer fuses:Although PPTCs have been used for years in relatively low-voltage DC applications, such as battery packs, until now they haven't been suited for line voltages. Patel showed me a polymer switch for AC line voltages – the PolySwitch LVR series – for 120VAC and 240 VAC for up to 2A at 20 degC. This is a big improvement over the present bimetal switches, which can have a limited life compared to polymer, as well as larger dimensions. The PolySwitch prices start as low as $.25 each.)