Losing battery charge while plugged in

What are you using to charge and are you actively using it? The 5W power adapter that comes with all iPhone models is not the fastest. It's rated at 5V at 1A or 5W. More powerful adapters are typically 2.1A or sometimes higher. All current iPhones can charge at up to 2.1A (possibly more with Fast Charge).

Also - I've discovered that sometimes failing Lightning cables start charging slower. Not sure why other than maybe some sort of circuit protection.

Hello and welcome to Apple Support Communities, Maxattach.

If I understand your post correctly, your iPhone loses battery life while it is charging. I’d like to help.

Follow the steps in this article:

If your iPhone, iPad, or iPod touch won't charge

Let us know if that resolves this issue for you. 

Thanks for using the Apple Support Communities. Take care.

I have an Iphone 6 SE And it's doing the same thing too, It was charging properly up until today at about 11:50PM Mountain Standard Time, I've tried all Tricks and tips different power outlets Restarting ETC, but The battery is loosing major power, I send a voice message and it looses 1 Percent Per Second, I would like some help with this issue as I've had this phone for several years with good battery life, but all of a sudden within the last year the battery health has significantly degraded I do have several apps on the device but I do not use them, or have apps open whilst Charging... I am at a loss here If someone can help me It would be greatly appreciated