amberwolf said:
Three Turnigy 6s packs that failed for Pdf all had the same cell fail--2nd from the end.
See that's interesting - getting 3 duds is one thing and statistically it just reflects a certain degree of poor QC or a bad batch etc and a bit of bad luck in getting as many bad ones as you do.
But having the same cell die in different packs without reason is significantly less likely to happen. If it was always the outer cells to die you could blame external damage, or if it was always the inner most ones you could blame heat but the fact that different people are having the same individual cell die in multiple packs is quite strange. If it was a BMS you'd blame that, but most of us don't use them.
...If anyone with statistics experience wants to work some numbers, it'd be interesting to see what the chances actually are.
I did do one statistics module at uni many years ago but it was the most boring subject I took so kinda glazed over for most of it
Using laymans stats though the chance of any given cell being dead in a pack is 1 in 6 (16.6%) (assuming 6S packs of course) The chance of that same cell being dead in 2 packs is 1 in 12 (8.31%), or 1 in 18 (5.5%) for 3 packs.
That is assuming all the packs you're talking about are known to have one faulty cell.
To look at the overall liklihood you'd need to know what the failure rate is across the board and then the probability of getting the same 2 cells go bad is much much less likely.
Eg if you said the overall dud rate from HK was 10% that reduces the chances of getting 2 packs with the same 2 cells being dead less than 1%. 0.83% to be exact, or 0.055% for the cell being bad in 3 packs. So if hypothetically speaking the failure rate out of HK was 10%, you'd have to buy 180 packs before you got 3 packs with the same dead cell...
Clearly this isn't the case, so there must be some external factor affecting these. But as amberwolf said, we just don't know what it is!