Teardown Tuesday: Going to the source

Ennie has a neat hear rate monitor watch that decided to crap out on her. Let me start by taking a step back -- her previous watch had a non-user-replaceable battery; WTF? -- I know. Her spinning instructor managed to replace it once and then something decided to crap out on it. After that I got her a new one for a Christmas a year or two ago (see -- there is a connection to Christmas after all!)The assumption going into this was that this was going to be as traumatic to replace the battery.Thankfully, a tiny turn of a coin later and the battery was freed from its confines inside the watch. I looked next to where I was and lo, there was my multimeter. I tested the voltage and it read 2.8 V.Here's where we get into the nitty gritty.What does 2.8V mean? Nominally the battery is a 3V Lithium cell. 2.8V seems pretty darn close, no?The source, in this case, is the datasheet.The cell I pulled out was a Panasonic, so I'll link to Panasonic's datasheet for the cell.Basically, it turns out that the discharge curve for a cell like this is pretty interesting. Sure, it starts off at around 3V... the thing is that is pretty much stays there for the whole time. Until it gets to a cliff when the charge runs flat.At 2.8 volts at no load it's well and truly dead.It took a while to get this rigged up so I could measure and take pictures of this tiny cell.In the rigging process I accidentally shorted the cell for a moment. That pushed it even further over the edge.Up against a >1M-ohm impedance for the voltage it is still only reading 2.7V. Two hours previous it was down to 2.6V; it took that long for it to recover.What I'm getting at is that before you start to make assumptions on how things are supposed to react, it's a worth while pursuit to go to the source and look at the datasheet for whatever might be piquing your interest. Even something as mundane as a battery has a datasheet.Engineering is everywhere. Learn to love it!