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I have this iPod that starts off with the black background and apple logo, it then gives the sad face icon, then reboots to "please wait, very low battery"

I've replaced the hard drive with a brand new one. Brand new battery, and a brand new hold switch as I tore the cable during dis-assembly. I'm thinking I'm SOL and need a new main board..what are your thoughts?

I'm afraid you're right, the combination of symptoms you describe points towards a defective board.