Don't get alarmed but I just read this on a site that uses the GT-11 a lot.

"Do not let the LiIon battery run flat and remain flat for a long period of time - this can permanently damage and reduce the life of the battery.
Always switch off the unit before removing the memory card (or use the REMOVE CARD option from the MEMORY CARD menu).
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the issue with lithium batteries , especially lithium polymer batteries is that if the individual cell voltages drop below around 3 volts ( i think it is ) the cell can die.
and if they are continued to be discharged then they can decompose rapidly resulting in fire and lithium goo everywhere
im pretty sure that the designers of the gps would know about this , and design in some sort of voltage cut out to prevent dangerous cell voltage droppage. however!! it is still not a good idea to let your gps run out of power.