To add to the above, it's not always immediately obvious that there's a problem with the battery. Case in point, my work laptop started to have an issue where the mouse cursor would move by itself. Thought it was a bit odd, but didn't do anything about it for a week or so. Then, as it started to happen more I took a look at the laptop itself - note that I normally keep it plugged in to a USB docking station on my desk:
.
Took it back into the office post-haste and the IT guy took the battery out:
Keep in mind, other than the random mouse movement this
laptop portable bomb was working perfectly and it and I had just made a return trip to the USA a few weeks earlier! I shudder to think what would have happened if it had been in the hold and we'd hit a bit of turbulence out over the north Atlantic.