how do I stop lg monitor charging my MacBook Pro

This is completely wrong. A simple google search will tell you that constantly keeping your battery at 100% charge by leaving it plugged in is the absolute worst thing you can do to your battery. My battery capacity has been destroyed by prolonged use with my LG Ultrafine 24MD4KLB-B display. The battery was replaced one year ago, and in only 114 charge cycles has gone from 100% of capacity to 88.9%. 88.9% sounds like a lot, but not when I tell you that at below 80% even Apple recommends replacing the battery. I have seen 7 year old MacBook Pro's with higher battery capacity even at over 1000 charge cycles, which is the maximum number of charge cycles these batteries are designed for. The owner of the specific MacBook Pro (2012 Retina 13") I have in mind told me specifically that he always used the battery up completely and then charged it up completely. (ideal charging recommended by experts is to keep between roughly 20 and 80 percent) One year of extremely light use with my LG Ultrafine display has done more damage to my brand new 2018 15" MacBook Pro battery than 7 years of the kind of usage that 2012 MacBook Pro had. Because the display appears to have no way to turn off charging/power delivery, I have no way to protect myself from this.

yorkshiregray wrote:

I have a LG 27UK850 monitor which I run off my MacBook Pro. I know that it power down charges the MacBook, resulting in 100% charge always being indicated. Does anybody know how to run this type of monitor without power down? thanks

What is the issue? Your Mac manages its power/charging system very well. Neither your computer or battery will be damaged by leaving it plugged in.

About battery health management in Mac laptops - Apple Support

Change Battery settings on a Mac laptop - Apple Support

Battery cycles are only one of multiple factors that affect battery life and capacity. Keeping your MacBook plugged in continuously will significantly shorten your battery's life and ensure that you never come even close to the 1000 charge cycles your battery is capable of. The 2012 MacBook Pro I wrote about above had over 91% of original capacity even when it was over 1000 charge cycles. My 2018 MacBook Pro has lost over half of it's useful battery life in just one year with just 114 charge cycles from being plugged into the continuously charging LG Ultrafine 24MD4KLB-B display.

As BobTheFisherman says, that is simply NOT a problem.

Your MacBook Pro  uses ‘smart charging’ to charge in the optimum way, and only when necessary. Plugged in is Not necessarily actively charging. There is substantial hardware and software cooperating on battery and charging issues. Simply asserting a charging voltage against one of the interface(s) will NOT successfully charge your MacBook Pro.

Any external power supply that provides "USB Power Delivery" (including certain Docks and Displays) must negotiate over the USB cable using USB Power Delivery Protocol, and can not 'force itself' on your Mac. The Voltage and Current are delivered only after your Mac requests and the charger agrees to supply power under certain controlled conditions. The computer is in control of the entire process.

On a MagSafe power adapters, the little green and amber LEDs are set by commands from the Computer.

In general, you should ALWAYS connect AC power when it is possible to do so, and only run on batteries (which could be somewhat slower) when no AC sources are at hand. Your Mac will NEVER over-charge. 

jasonfromcentennial wrote:

This is completely wrong. A simple google search will tell you that constantly keeping your battery at 100% charge by leaving it plugged in is the absolute worst thing you can do to your battery. My battery capacity has been destroyed by prolonged use with my LG Ultrafine 24MD4KLB-B display. The battery was replaced one year ago, and in only 114 charge cycles has gone from 100% of capacity to 88.9%. 88.9% sounds like a lot, but not when I tell you that at below 80% even Apple recommends replacing the battery. I have seen 7 year old MacBook Pro's with higher battery capacity even at over 1000 charge cycles, which is the maximum number of charge cycles these batteries are designed for. The owner of the specific MacBook Pro (2012 Retina 13") I have in mind told me specifically that he always used the battery up completely and then charged it up completely. (ideal charging recommended by experts is to keep between roughly 20 and 80 percent) One year of extremely light use with my LG Ultrafine display has done more damage to my brand new 2018 15" MacBook Pro battery than 7 years of the kind of usage that 2012 MacBook Pro had. Because the display appears to have no way to turn off charging/power delivery, I have no way to protect myself from this.

You should not always believe Internet search results.

Neither your Mac's battery or computer will be damaged by leaving it plugged in. The MacOS manages the computer's power/charging system very well. You do not need to micromanage the system. In fact, leaving the computer plugged in reduces the rate of accumulation of battery cycles.

Update: I just found out that there is an app that will allow you to control your charging from the MacBook side of things. It's called AlDente. There are free and paid versions.

https://apphousekitchen.com/

Third party apps that claim to manage, etc. your Mac are not needed. The MacOS does a great job of managing the system.