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Wow, I have heard of the battbridge before but never actually looked into it. Nice and simple. However, isn't a 2V alarm threshold a bit dangerous, especially when the entire pack is discharged to a point where any cell will quickly drop below the knee? That's when catastrophic cell damage occurs - when one cell is dead and the 17 other cells still have enough jam to force a bunch of current through it.
I believe bottom balancing and an amp-hour counter (EASILY done with simple Arduino code) it the ticket. When you've used enough Ah to be at 80% DOD, STOP! Bottom balancing allows for all cells to kit the knee at precisely the same time, and if your fuel gauge shows "E" long before then, you are good.
After everything is nicely bottom balanced, your first [carefully monitored] charge will show you the weakest cell. It will be the first one to hit cutoff voltage. This cell will always be the weakest, so technically, this cell is really the ONLY cell you need to monitor for charge cutoff, potentially making an Arduino based charger cutoff extremely simple.
Summary. Bottom balance. Use an Ah counter. Maybe use a battbridge to make yourself feel better.
Parts list:
1 - Arduino ($15)
1 - Charger relay ($5)
1 - 20X4 LCD display ($10)
1 - 2N7000 transistor (to power relay, $0.20)
1 - HTFS400 current transducer ($30)
1 - MUX Shield (only if you feel like reading all voltages instead of only the weakest cell, $25)
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