Batteries are wonderful things. They use chemical energy to store, and then release, electrical energy (yes, there are some specialized, non-chemical batteries such as mechanical flywheel units, but that's a whole different story).
But it's not just energy, it's also about power. (Quick refresher here: power is the time rate at which energy is transferred or used.) And in batteries, both are important factors. When we look at product run times, we look at both the current consumption of the product, which relates to total energy stored, as well as power consumption, which relates to how fast the battery and its subsystem need to deliver and replenish that energy, and how fast the resultant heat must be removed. (More correctly, this current or power consumption should be looked at as dissipation, as the current and power are not "consumed"; they are turned mostly into heat–but the difference between those meanings is not the point here.}
Some of today's batteries have such high energy density (amount they can store per unit volume) that when they release it too suddenly--whether due to an internal fault or an external cause–they can develop and deliver enough heat energy to start a fire. It's very unnerving when your laptop PC, even if not plugged into an AC line or charger, starts to self-ignite, that's for sure.
Testing and abusing batteries for gross failure and resultant fires and explosion is a tough job, but someone's got to do it. Take a look at this recent article in The Wall Street Journal for a look at how Sandia Labs is doing it, "Where Batteries Go to Be Tortured". Sounds like real fun, I am sure many of us are a little jealous!♦