Actually, after watching Sandy Munroe's tear down of the 2nd Gen Volt battery GM could indeed swap out bad modules. There are four modules that can easily be separated from each other. Also, based on that tear down, I suspect that with some care, individual battery cells can be replaced as Sandy took one of the modules apart and showed a battery cell by itself. It's just that GM sold so few Volts that it's not cost effective for them to do this level of work. I can't find this video but there are multiple youtube videos featuring the Chevy Volt's battery and how to disassemble and replace individual cells.