Force Mac to use Power Adapter as Power Source I just bought a new MacBook Pro 2015 15" and I really want the battery to stay healthy.

Is there a way to force my Mac to use the Power adapter as power source, instead of using the battery as source and charging it at the same time? (The Mac already does this when the battery is at 100% to, well, have the battery stay healthy.)
I still don't understand your question.

You want to plug it in but not charge the battery? Why?

My vague understanding about batteries speaking with hardware repair shops, etc. is that it's no longer truly worth worrying about. You charge when you can, you use the battery when you must. While plugged in, the battery charges until full and then stops charging. Presumably, at that point, it's using the power cord and not the battery.

The other thing that I seem to recall is that it's all about the charge cycles. You want to keep that number down. So, if you work from home one day, it would be better to leave it plugged in all day because that would only be one cycle (assuming you plugged it in first thing in the morning). If you plugged it in until it was charged, then unplugged it and used it until it was low on battery, then repeated, that would be 2, 3, or 4 charges during the day depending on how you use the computer.

So, what I've heard from people much more knowledgeable than I, is that the potential gains you might experience doing "the right thing" are not worth the effort required to realize those gains.