Force Power Manager into Maximum lifespan mode

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On my W520 (Windows 7, Power Manager 6.61.1 I have three options in the Power Manager >> Battery >> Battery Maintenance:

*)Always full charge
*)Automatically optimize for battery lifespan
*)Custom charge threshold

In the 2nd option it will "automatically" optimize. And when I enabled this option, some time later it went into "Maximum lifespan mode" and the gauge got a green lining. However, a few months ago when I accidentally discharged it to empty a few times, it went automatically to "maximum runtime" mode. I don't want this. How can I force it back to "Maximum lifespan mode"?