I have doubts about the effectivess and reliability of the vents on these cells. Most Cs cells now have a rubber slug type vent backed with a metal disc. When the cell is in overcharge, gas is developed which is combined internally, however higher charge rates produce more gas, more than be combined. When this happens the pressure builds up inside the cell and the rubber disc should move to release the gas. Soldering to cells can damage this vent mechanism, or sometimes the vent holes can become blocked by the internal cell components (electrode or seperator).
I should imagine that the cell in question had 'issues' (low capacity) and this caused it to be in overcharge while the others were still charging.. so the charger was not aware... its all not very good. I doubt these cells could pass the safety tests insisted upon by the power tool manufacturers.
As with all batteries, they should be treated with extreme caution whilst being charged. they shoudl be soldered very carefully and not charged >4A. They shoudl be matched as often as possible and dead cells should be substituted.
Neil
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