I would have thought with checked bag screening bags that have batteries in them would be reasonably easy to identify.
We pull laptops out and screen them separately as hand luggage because the batteries and hard drives are too dense to get a decent image on the scanners, surely a bunch of rectangular black objects on the X-ray would be easy to see?
I have never seen the images produced by the checked bag screening machines, so perhaps I am over stretching on the capability of the devices.