Here goes got a c max1.8 2004

I've changed the battery,
checked voltage from alternator its charging at 14.1 volts running with everything on so i presume alternators ok

Changed aux belt so its not slipping

checked smart charge fuse F31 10a and replaced with a new one

taken off alternator 3 pin smart charge wire checked red one and getting 12.7 volts from batterty when not started

undone the 3 pin loom and checked for any breaks at plugetc got it right back to injector loom alls ok full continuity beeping on meter

checked and cleaned all earth wires and alternator main live all ok

Still the battery light comes up 20 seconds after starting up

Done the dash diagnostics and it seems code d900 is stored

it hasnt erased by hand held code reader

Has anyone any ideas where next to try

And where is the ecu on this car as i'll try and trace the blue and grey wires right back and check for continuity straight from ECU

I'm baffled now

cheers