Battery not detected, does not charge

Hello, my MacBook Air had water damage and I was able to fix it in a way that it now boots again with the power cord connected, but no battery detected anymore. If the device is switched off and I connect the power supply, I get a flashed image of low battery to see when I press a button on the keyboard.

I had to clean the flat cable connectors between the main board and the I/O board - there was corrosion there. That brought it back to life. The battery does not seem to be damaged.

If I reset the SMC, it does not boot anymore unless I press the power button for 10 seconds and then connect the power supply… then I can keep on using it as long as the supply is connected.

I suppose the problem is related to the SMC - this is located on the main board? Where exactly? Should I replace the I/O board? Could the battery be detected again if I replace the flat cable between the main and I/O board?

Any help and tips are more than welcome !

Danny