The reason they are stored based on capacity is because a pack can reach just about full voltage way before the pack is full. Your 4000 pack at 2000 mAh is 50% not 60%. 60% of a 4000 mAh pack would be 2400 mAh roughly. Most newer chargers such as the Hyperion line we sell offer store mode which will either charge to 60% or will discharge to 60% based on the packs current charge status.
Jason
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