How will make my pc run like it is plugged in when on battery?

Laptops with graphics cards are setup by Intel to automatically change to the integrated HD 620 when on battery and I'm not certain you can even force it to run the GT 940MX on battery, but there are video instructions that you can try.

I know for a while some Lenovo laptops installed a physical switch, but newer ones removed it.
The general settings are about pushing the CPU and display more than the graphics change,

http://www.notebookcheck.net/...849.0.html
940MX is rank 183
HD 620 is rank 270

The 940MX
http://www.game-debate.com/...40MX%202GB
shows a maximum 25 Watts, where higher performance models use even more power.

Between the cpu pushed up(about 15W with graphics, so maybe 8 without), the display brightness, and the graphics card, do expect battery life to be far lower than non-gaming use.

It used to calculate out at about 35 to 60 minutes on typical laptops with higher power needs.
I don't know your model, but if you get a couple of hours, you are getting all that you can expect from fully charged to not working. Some models have external accessible batteries and some internal.
If yours has an external battery pack, you may want to order up a spare and charge it.

There are other youtube videos suggesting how to force the laptop to run the 940mx on battery power.
It may also return back to Intel on re-boot.

Change your power system so that you have a large battery outside of the laptop that duplicates the AC adapter. That way, the power board is fooled into thinking that there's an AC adapter linked to the 110--220V AC utility lines. You won't be operating on the internal battery at all.
External power pack batteries for laptops, including the right plug to go into your laptop (whichever model it is) are widely available. Buy the right size for the time duration that you wish to use it. Should be less than $200 for one that lasts a few hours.

Nvidia optimus. Check its settings for that exe in the database. The nvidia gpu is NOT always used. You could remove your battery as that may help reduce temperatures/load.

That may be difficult to do depending upon what options you have in BIOS or UEFI. I had an old Dell laptop that would announce during boot that it was running at a slower speed than normal because I was using a power supply it did not recognize (even though the generic supply with multiple adapters had enough watts).

Playing high end video games on battery is not a " smart " thing to do…

Go to power options (start menu just type power options), select the "high performance" power plan. This will disable a lot of power saving features and allow your laptop to run at full performance.

One other thing you may have to do is go into the nvidia control panel and in 3d settings, set it to prefer the dedicated or high power gpu. You can set that in global settings tab or on a game by game basis.

Doing both things should make the laptop performance the same as if it was plugged in. But realize this uses a ton of power in comparison. You will go from like 4-6+ hours of batter life down to 2 hours or less. So you really can't do a lot of gaming on battery. It would require a gigantic battery to power full performance gaming for a long period of time. It would increase weight, size and cost substantially, thus why no laptops play games on battery for very long when now making use of heavy power saving features.

The only other thing you could do is carry around multiple batteries. But i would just suggest playing lighter games that can be played without using your dedicated gpu if you are stuck for a period of time not near a power source.