Lithium Batteries.
Inside the battery there is monitoring circuitry to monitor each cell's terminal voltage. If the terminal voltage deviates slightly from the designed terminal voltage, either above or below by approx 0.05Volt, Lithium metal is deposited out of solution onto one of the electrodes. As internally the battery is moist, as it has to be (Electrolyte), even if only a gell, the Lithium metal reacts violently resulting in a fire. This why a Lithium cell shuts down when there is a slight failure, rather than dying slowly as in NiMh or NiCd batteries. Also if the battery gets too hot in operation, it gets to a point where thermal runaway occurs, simply making itself hotter by itself. Lithium metal deposition once again will occur resulting inb a fire.

Jim.