When I disconnect the power cord the unit goes dead. what's wrong?

There is not enough charge remaining in the battery at that moment to continue to operate.

The underlying reasons for that are likely to be far more complex.

Connect it to the power adapter and leave it connected for a long time -- at least overnight. Your Mac manages the state of battery charge automatically. Your Mac will NEVER over-charge.

what model-year MacBook Pro?

what version MacOS?

If it’s on your Mac Mini …

… NOTHING … it’s because you interrupted the power supply circuit.

(A mini doesn’t run on a battery like a MacBook)

If it’s about a MacBook; while a defective battery is the leading culprit, it might also be another “power” component inside the device’s case.

Have the device evaluated:

Genius Bar Reservation and Apple Support Options - Apple

dreammaui wrote:

It's a Mac Book Pro. Unit doesn't hold power. when io disconnect power cord, it stops. what's broken??? not nothing, I'km sure

What you are describing is a classic symptom of a battery that has failed.

In the Apple Menu, open About This Mac. Scroll down and select More Info, then System Report. Look under Power, what does it say about the battery condition, cycle count, does it say "normal," what is the level of charge, and is it charging?

You can also get more info about your battery by downloading and running Coconut Battery.

dreammaui wrote:

It's a Mac Book Pro. Unit doesn't hold power. when io disconnect power cord, it stops. what's broken??? not nothing, I'km sure

Get help with your Mac notebook battery

Learn how to optimize the life of the battery in your Mac notebook, fix battery issues, and get service.

If you see Service Recommended on your MacBook Air or MacBook Pro - Apple Support

Re: “…not nothing, I'km sure …”

It might appear that you didn’t have the time to read all of a short, 2-part reply.

The 2nd part discussed “if it’s a MacBook”

New members here frequently post in the wrong community; hence the 2-part answer based on 1) your “signature” AND another based on 2) the “community” in which you posted.

Best of luck discovering the true “culprit.”