Dell XPS 13 (9343) best setting in BIOS to extend battery lifespan?

Hi.

I basically want have my XPS 13 (2015) plugged in and want to extend the battery llifespan which isn't user replaceable.

In the BIOS there are two options which are making me doubt.
On older laptops I there was the battery saver option which stopped charging at 80%. The Bios offers the "Custom" settings which will stop charging at a certain %. When it reaches for example 80% it will say: plugged in, not charging.

There's also the "Primarliy AC Use" settings which according to the descriptiom "Extendsbattery lifespan for users who primarily operate their system while plugged in to an external power source." This seems like the logical choice, but when I choose this option, the battery is always at 100%. With this option, Windows will say Fully charged, but it doesn't say "not charging" which implies it is charging? And I know overcharging is bad for the battery.

What option did you guys choose in the BIOS? Some help is appreciated. (Please see the image included)

 

Hi blue13x, I use the Dell Command Power Manager to manage my charging patterns. I use the custom settings and have my laptop charge at 40% until 80% and stop charging then. I then use my laptop unplugged until it discharges to 40%.