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The battery’s burned insides indicate it operated at a voltage above its design limit, a Japanese investigator said Friday. That’s a clear sign of an out-of-control chemical reaction, explained Reginald Tomkins, a professor of chemical engineering at New Jersey Institute of Technology.

Hmmm...

It sounds like Boeing is covering their butts.

I've been using these batteries for almost 10 years and when the batteries are manufactured, the nominal voltage is 3.7 volts per cell. The maximum per cell voltage in a fully charged state for a properly charged battery is 4.2 volts.

The only time they go into a thermal runaway state is when they are charged to a state above 4.2V per cell or if they are discharged at a rate higher than their designed to deliver.

In both cases (over-charge and over-discharge) they will develop internal shorts which trigger the thermal runaway.

I've never heard of a Li-Ion battery going into a thermal runaway all by itself.

One of the unique features of Li-Ion batteries is the ability to wire them in series and parallel at the same time. This allows the batteries to be formed into packs that deliver a higher voltage and capacity. When wired in parallel, each cell must be balanced before or during the charging process to ensure that each cell's voltage never rises above 4.2 volts.

If one cell resting voltage is higher than those of the rest of the pack, and each cell's voltage is not monitored during charge or if all the cells in a pack are not balanced with each other before charging, then it's possible to over-charge a single cell above it's maximum rated voltage, which will trigger a thermal runaway.

Once a cell goes into thermal runaway it can trigger other cells.