How to enable display on Dell monitor connected to MacBook Pro when not charging?

I've just purchased a Dell P2415Q monitor which I've connected up to my MacBook Pro (running OS X El Capitan). Everything works great in terms of the display, however I've noticed that it goes into power-saving mode (i.e. shuts off the display) as soon as I disconnect my MacBook Pro from its power source.

Given the battery is already charged up I don't want to keep it plugged in as I suspect it will damage the battery life if I keep it that way for a long time (and I do intend to take my MacBook Pro out and about for other uses).

Does anybody know how I can make the display work irrespective of whether my MacBook Pro is connected to it's power source or not?

I am happy to tell you that keeping it charging will not damage your MacBook in any way! There is also a way to do your requested way of streaming data to the monitor. You can use the amazing Apple TV. This is not only for TV's because you can stream to monitors, and your own TV! (which personally is amazing if you have a large flatscreen!) However I am not 100% sure this is compatible with all types of monitors or TV's so i would recommend just keeping it plugged in because like I said earlier that will not damage your computer. I hope this helped! The reason your having this problem is because monitors aren't computers. They don't have wifi, you can't connect them to anything wirelessly. Monitors don't have any form of computational power except for converting computer code to images which still requires being connected to a computer to work.

As a Single Monitor Workstation Setup (closed-clamshell)

If you are in closed-clamshell mode (your lid is down), one the of the requirements is that you must be on AC power.

As a Dual Monitor Setup

If you are using your MBP's display as well as an external monitor, you should still have your MBP plugged into AC power. Directly from Apple's support site:

You can probably get around this by disabling the power saving features, but that will just cause you to have to recharge your MBP sooner which means hooking it back up to AC power.

Your Battery

As far as your battery goes, it is perfectly fine to leave it plugged in. I have provided an answer specifically about that in the post "How to preserve MacbookPro battery from degradation?" and in this other post, "What's the best way to prolong the lifespan of a Macbook Pro battery?"

Basically, your battery life is dependent on two things...number of cycles and age with age being the biggest factor. A never used 3 year old battery will perform roughly the same and have the same life expectancy as that of a 3 year old battery used every day. So, doing the plug it in, plug it out, do the hokey pokey, and turn yourself about in an attempt to save your battery get's you about the same results as that same dance in a night club.