I came across this
amazing A123 FAQ. Here's the kicker:
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Q: My Charger can charge A123 and has a cycler, but when I set a 2v per cell cutoff, at the end of the discharge one cell is way below 2 volts! What's up with that?
We've seen this in the Hitec X4, it's also common across several other multichemistry charger/cyclers. What's happening is
the cycler's discharge cutoff trigger is watching the total pack voltage and ignoring the individual cell voltage. This is a source programming error in my opinion, just as the charge routine is individual cell voltage controlled, it's no less important for lithium packs to terminate the discharge routine based on individual cell voltage. Here's the 'issue': A123's have a very,very sharp 'knee' in the discharge curve and actual capacity varies as much as 5-10% between cells. (an artifact of A123's 'nano' plating technology.. negative plate surface area varies a bit from cell to cell) This issue can set up an overdischarge condition on the lower capacity cell, so we've adjusted the discharge target up from 2v per cell to 2.5v to give a cycler that does not have individual terminal voltage control a bit of 'padding' as a safety margin.
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My charger is a HiTec X1; essentially, one quarter of an X4. I think there's a very good chance that the same thing has happened to me. I'll be sure to change the discharge from the default 4.0V to 5.0V in the future.
I was going to try my LHS for a refund on the pack but it seems it's my fault for assuming that the discharge cycle balanced the cells.