MacBook air won't power on unless I disconnect battery and plug in

Hi,

in detail this MacBook has been water damaged i have bought it with intent to pull apart for parts but before i did i thought id see if i could get it working.

I managed to get the laptop working and clean up the inside and managed to get the whole thing working once i disconnected the battery and powered on through the power supply, now after i turn it on through that i can turn it off and plug the battery back in and turn on with power connected and the battery charges and works from the power it puts out without charge connected.

now the problem is when i shutdown the computer fully or the battery runs out of charge i have to go through the disconnect the battery and power on with just the power supply again to get it to boot up.

would this mean a faulty battery or the SMC chip is damaged? i have done the hardware test by holding D on the keyboard when i first start the computer but i can only do this when the battery is disconnected because of the reasons above. It only shows a error saying the battery is disconnected and that everything else comes back fine, I'm pulling my hair out trying to figure out what the problem is so any help would be much appreciated.

Thanks for reading,

Rob