Cells optimised for high voltage and capacity have reduced cycle life and higher rates of self discharge. The IB4200 does have a high rate of self discharge and equilsation is a good idea. The reason is this: if you have one cell at 0% residual capacity, two cells at 10% and three cells at 15% in series, and you put them on charge, the three cells with the highest residual capacity will reach top of charge first (the other three guys are still playing catch-up). If you have you -dV set at 5mV per cell these three fully charged cells will have to go into further into overcharge until they reach 10mV for the charge to terminate. These three cells will be hotter than the other three which never reached full charge so there life will be reduced. They will also be more likely to vent electrolyte due to internal pressure increase. When you use the battery, the cell that was furthest behind will discharge first and be driven to a low voltage (even a reverse voltage is possible in very large cell strings such as large powertool batteries) and is more likely to vent due to reversal.
This explanation is a bit simplistic because the cells do have a degree of internal balancing in overcharge due to internal recombination reactions, but high capacity cells dont have the luxuary of good recomination systems due to the designers having to push capacity.. so in effect they are more fragile than commerical NiMH cells made by the likes of Sanyo for powertool applications.
So the best things to do are:
(1) Equalise
(2) Dont charge at high rates (4A, no more)
(3) Store cells with 2000mAh in them
(4) Use low -dV thresholds (0mv -3mV/cell)
(5) Floss
(6) Charge cells which are at room temperature
(7) Accept that lights that burn twice as bright burn half as long
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