T520 with battery slice "dies" with 3-4 hours remaining

[baertracks]

I'm using a T520 with an extended main battery and a 27++ battery slice added. If I keep the display dimmed slightly I should be able to get 12+ hours of operation (according to the battery meter). That's operating at a fairly intensive level with multiple programs running.

However, twice now the laptop has "died" (black screen) without warning when there was still around 3-4 hrs of charge supposedly remaining on the power meter. There was no warning of low battery, and no transition to hibernation provided.

Has anyone else experienced this problem? Am I doing something wrong in using the battery slice? I was hoping to be able to get a full day of the laptop use without having to lug around the "brick" power supply adapter.