Many responses to that thread about the exploding UF3000s suggest that the series implementation may have been the cause of the disaster. I have been following this type of stuff for years and would be at odds with that position. While the series implementation may have played a role, it would require something else to have failed before the series implementation could have the negative effect.... Allow me to explain:
A configuration of series wired lithium chemistry cells that are properly implemented and protected is only more dangerous than a single cell by a factor of the increased number of cells that are in front of you, having nothing to do with whether they are in a battery configuration or not. The most dangerous situation that can occur with a set of cells in a series configuration is when there is nothing in place to prevent over-discharge and more importantly reverse charge of a particular cell in the battery. A set of cells with un-equal capacity can wind up with one cell dropping to 0V, and subsequently being reverse charged by the other cells in the circuit. Reverse charge condition has been the #1 cause of CR123 primary cell explosions since CR123s have no form of protection.
In other words, the only way in which the "stacked battery" situation can be called the reason for the disaster is if we assume that one or more of the protection circuits in the cells failed. Leading to a cell that saw a reverse charge condition (perhaps many times before the cell finally blew up).
Here's the list of possible reasons that the explosion happened: (there may be more)
1. Protection circuit failed to prevent over-discharge and reverse charge of one of the cells. UF cells are notorious for inconsistency from cell to cell so this is very possible, but the root cause would be a failed PCB combined with enough cell inconsistency (in other words, cell quality related). The series configuration played it's role, but can not be directly blamed because there is nothing unusual about series configurations of cells in and of itself.
2. The cells were repeatedly subjected to over-charge. (but but, they're protected so that can't happen right? WRONG: most PCBs don't trip the over-charge limit till in excess of 4.30V). All indications are that the charger that was used to charge those cells was possibly an early generation WF-139 which continues to trickle charge at a rate high enough to continue to increase cell voltage after the "light turns green." Sounds like the practice of leaving cells on the charger over night may have been commonplace for the user. A higher quality charger may very well have prevented this, however, it should be noted, that high quality name brand cells that are found in most consumer electronics must pass strenuous abuse testing. A modern name brand cell would be much more apt to handle the improper over-charging for many years without incident. In this case, we are back to pushing for quality, better quality cells and better quality charger would reduce the risk of having such an incident by a large margin.
3. There was nothing wrong with the charger or the protection circuits, one of the cells simply "went off" for no other reason than poor quality control and impurities within the cell leading to oxygenation and/or metallic lithium formation. In this case, the argument again says "buy higher quality cells."
Considerations:
A protection circuit only prevents immediate dangers caused by unusual circumstances like a short circuit or an improperly configured hobby charger. A protection circuit can not and does not protect the user from on-going repeated mild abuses that can still occur within the bounds of the protection limits. I see a lot of threads popping up around here regarding "protected" cells as the holy grail of cell safety. In truth, the only cells you're going to find with protection circuits are lithium cobalt cells. An unprotected well made LiMn or LiFePO4 cell would be safer for many PV applications (especially the smaller cell applications, specifically those smaller than ~17500 size).
Eric
