I'm surprised the NiCad fires haven't made as big of a stink as this.

A "thermal" runaway in a confined space i.e. a box can result in an explosion!!!

I'm surprised the NiCad fires haven't made as big of a stink as this.

 

While it is true that a nicad cell can experience thermal runaway under certain circumstances, the reaction is far less exothermic than when runaway happens in a li-ion cell. The electrolyte in a nicad cell is liquid - a mixture of water and potassium hydroxide, neither of which are flammable - so the main result of a runaway is the production of a lot of steam. The cell might rupture from the excess pressure, but it is far less likely to actually ignite... by the time all the electrolyte has boiled off, the reaction has usually run its course. I had a nicad cell run away in a SAFT aircraft battery while I was doing a capacity check on the test bench. It vented like a geyser for a few minutes, then it quit.

 

By contrast, the electrolyte in a li-ion cell is highly flammable, and once it starts to run away, the process accelerates with time, getting hotter and hotter, making a fire or explosion almost inevitable.

 

Another big difference is the per-cell energy density. A typical 28 volt aircraft nicad battery contains 20 individual cells, each of which has a terminal voltage of only 1.35 volts when fully charged.

 

The 787 li-ion battery operates at 32 volts, and contains only 8 individual cells, which operate at almost 4 volts when fully charged. In other words, each cell in the li-ion battery contains far more stored energy than any individual cell in a nicad battery.

 

Yes, under the right circumstances, a nicad battery can burn, but it usually requires a major issue external malfunction to initiate the reaction. Either a dead short circuit across the main output terminals of the battery, or a massive overcharge situation, such as when a main engine generator's voltage regulator fails, allowing the DC bus voltage to rise far above the normal 28 volts. These kind of external failures are quite uncommon, and if either were to happen on an aircraft in flight, a burning battery would be the least of the flight crew's problems.

 

By contrast it appears that a li-ion battery can easily experience spontaneous runaway leading to fire or explosion, without warning, and without any obvious external trigger.

The inaugural Denver to Tokyo nonstop flight takes off today when a United Airlines 787 departs DIA at 12:35 MDT. I wish that I was on board.

7 Beta 7 and beta passengers.