MacBook only works with charger plugged in.

Hi everyone.

I picked up a cheap second hand MacBook Pro, 2015.
I was told that it had liquid damage years ago and since then only ever works when plugged in to the charger, but no other symptoms.
battery info and Coconut Battery both showed battery health was good and was charging, but as soon as the charger was unplugged it would shut down instantly.

i tried doing SMC and PRAM resets but nothing changed.
I then thought I would change the battery as I know they can sometimes cause issues after getting liquid damage.
When I opened up the case, I could see evidence of the liquid spill and there was some corrosion on the board which I cleaned with isopropyl alcohol. Unfortunately this still didn’t solve the problem.

The new battery is also showing as good health and charging but still no good.

Can anyone suggest what would be the next thing to look at, or is it a lost cause?!

I forgot to menton, I have also tried a new brand new, genuine Apple charger.

The battery is charging, the MacBook is communicating with the battery to get information from it, but power isn’t getting from the battery to the MacBook.

A few days ago I had some odd behaviour.

It was plugged into the power and I noticed the light on the MagSafe was orange, indicating that it was charging. 
this never happens as it is constantly at 100% because it doesn’t use any battery power!

I opened it up and battery level was down to 35% and it is now telling me the battery has reduced capacity and needs servicing. Although coconut battery is saying everything is fine. 
the battery has charged to 100% and stayed there ever since.