Multi-battery issue

[Gordon Messmer]

I have a Thinkpad T430S with two batteries: primary + ultra-bay.

The laptop will run on one battery only until it is depleted, at which 
point the other battery would normally be used, as I understand it. 
However, when the battery in use nears 0%, the Fedora system will warn 
the user that the battery is critically low, and then attempt to hibernate.

If I pull the ultra-bay battery before the hibernation, the system 
recognizes the other battery as nearly fully charged and continues 
operating as it should.

Does anyone have a different system with multiple batteries?  Is this 
problem common, or unique to this hardware?  How is this supposed to work?