Replaced battery - screen turns on to red battery - not charging

I bought this iPad as 'defective' and am trying to trouble shoot it. I took out the old, very dead, battery, and replaced it with a new one. After this swap, the screen actually shows an image of the battery symbol, but it still fails to charge.

Not looking for a definite answer, but what is another good place to look for a fault? I've tested for shorts around the charging IC, but didn't come up with anything. The only thing that gets hot, and it only gets hot when the battery is connected and the power cable is in, is the charging IC. But doesn't seem that power is making its way to the battery.