Trickle-charging is how the thermal runaway begins
Looking at the CT scans from both investigations you can see puffed out cell cases in the undamaged battery packs. This is an indication of overcharging in Li batteries.
Looking at the Secura patent for battery charging you will notice two things of interest. The method was based upon a Ni-Cd battery charging profile, and the procedure assumes a trickle-charging phase of indefinite end following the fast charge phase.
Looking at the FDR data from the Boston event you can see that the battery charger was indeed holding a constant 32 volts and trickle-charging the APU battery at 1-2 amps for about 17 minutes after the APU start until the fire started.
Looking at the charging process for Lithium batteries from GS Yuasa and every other Li-battery manufacturer in the world and you will find that there is no trickle-charging. Instead you will find a specific charging procedure that consists of a constant current (at ~1C) charging phase up until the cell voltage hits a specified cut-off value, then followed by a constant voltage phase in which the cell is held at this specified voltage until the current drops to C/20, at which point the cell is fully charged, charging is complete, turn off and disconnect the charger.
Looking at the data sheet for the LVP-65 cells from GS Yuasa used in the APU and Main battery packs you will find that they have a nominal capacity rating (1C) of 65 Amp-hrs, nominal voltage of 3.73, maximum charging rate of 1C amps, and the cell characteristic curves use 4.025 volts as the CC/CV specified charging cut-off voltage. The C/20 cut-off charging current is 65/20 = 3.25 amps. There is no procedure called out nor should there be any additional charging below 3.25 amps after reaching the 4.025 volt level.
Attempting to trickle-charge a Lithium battery, especially one that is already fully charged, is a sure-fire way to cause thermal runaway...
Last edited by kenneth house; 30th Mar 2013 at 20:13.