Desktop Control Panel
System and Security
Power options/Change Battery options
Change Plan Settings
Change advanced power settings
Battery (scroll down to the bottom of the list)
Low battery level (usually 10% by default, but is actually set by the manufacturer's ACPI settings)
Critical battery level (usually 5% by default, but is actually set by the manufacturer's ACPI settings)
You can do two changes to make your computer run longer: Change the Low and Critical battery levels, and change the Low and Critical battery actions. You can set the Low battery action to: Do Nothing, but I would keep the Critical battery action at "hibernate", and leave the "Reserve Battery level" alone.