NOVELTY - A fluid container comprises a rigid outer container with an interior space; a closed flexible inner container in the interior space; and fluid in the flexible inner container. The rigid outer container has a port through which the flexible inner container filled with fluid can be transported without disassembling the rigid outer container to disengage and engage the inner container within the outer container.    USE - Useful as a cartridge for storing fuel useful for portable fuel cell, i.e. direct methanol fuel cells, having potential uses in commercial applications and industries for providing power to, e.g. a single family home, prototype cars, and portable electronic devices such as computers, phones, and video projection equipment.    ADVANTAGE - The inventive fluid container has replaceable flexible inner containers.    DETAILED DESCRIPTION - A fluid container comprises a rigid outer container with an interior space (102); a closed flexible inner container (104) within the interior space of the rigid outer container (101); and fluid within the flexible inner container. The rigid outer container has a port through which the flexible inner container filled with fluid can be transported without disassembling the rigid outer container in order to disengage and engage the inner container within the outer container. The inner container is suitable for forming a resealable connection (108). An INDEPENDENT CLAIM is included for a method for replenishing a fuel supply of a fuel cell, comprising removing a fluid container as above, preferably in a form of a cartridge, from the fuel cell; replacing the inner container of the cartridge with an inner container containing a fluid comprising a fuel without disassembling the rigid outer container; and inserting the cartridge back within the fuel cell to provide the fluid from the inner container to the fuel cell.    DESCRIPTION OF DRAWING(S) - The figure is a perspective view of a fuel storage container showing a latch structure.    Outer container (101)    Interior space (102)    Flexible inner container (104)    Resealable connection (108)    Fluid access port (109)    Resealable fastening structure (400)