| SlowOne |
14-11-2007 08:39 PM |
If the cells are exploding (and they are) then that takes a lot of energy. Is this a viable theory...?
If it takes a lot of energy, then that has to be built up somewhere. If it were to build up slowly, then the can would distort badly first, before splitting. If that build-up were a gas, it would vent in the majority of cases through the top, or peel the cell apart at it's weakest point - probably where the lid is folded into the can. My theory is that it is not a build-up of gas that leads to the failure. If teh build up were expansion of the chemical, then the can would split, and the chemcial would spurt out, and that would be the end of the incident.
If the build-up of energy is very sudden, then the cell will fail catastrophically. The main source of a sudden pressure change would be the direct conversion of the energy in the cell, and that would come from a direct connection of positive to negative - a short-circuit. That suggests that the construction of the cell is failing quickly. The main stress put on that structure is during charging and discharging, and the higher the charge rate, the more the stress. Since we have reports of this happening during, or shortly after, charging, then it suggests to me that the charge rate, and charge condition (balance of the cells, and overcharging due to imbalance of cells at peak) is likely to be the cause of stress. My theory is that the cause of the failure is in the internal structure.
Supporting evidence is that the cells have been stretched to the maximum over the years by changes to internal structure. Those who use 1C to 1.2C charge rates, and that those who balance cells before charging at this rate, have less problems with cell performance - they go much longer before the performance goes off. Most of the reports of incidents talk about re-peaking and/or high charge rates. Also, this is not unknown from the past, and those failures would be consistent with an internal structure failure.
My theory is that the internal structure is weaker than lower capacity cells, that the charging regime is damaging that internal structure, and that the failures are from a very rapid build-up of energy arising from an internal structure failure (short-circuit) releasing large amounts of energy capable of splitting the can catastrophically.
Discuss...
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