Dell E6530 Motherboard/ Slice Battery

I have a Dell Latitude E6530 and having a problem I have never seen before. I have two 9 cell batteries in it, one as the primary and other as a slice for the bottom. When the slice battery reaches 1% the laptop abruptly cuts off instead of switching to the primary battery. However, I can unplug the slice battery while the laptop is on with no problems occurring. I have ran all diagnostics tests I could find and each time everything passes. I have been told that the problem could be either the BIOS outdated or the motherboard. All drivers have been updated and the most current BIOS is installed. I would like to know if the motherboard is indeed the culprit or to see has anyone else had this problem before I buy a new motherboard. Thanks