A hydrogen generator and a fuel cell system including a fuel cell battery and the hydrogen generator. The hydrogen generator includes a cartridge, a housing with a cavity to removably contain the cartridge, and an initiation system. The cartridge includes a casing; a plurality of pellets including a hydrogen containing material; a plurality of solid heat transfer members in contact with but not penetrating the casing; a hydrogen outlet in the casing; and a hydrogen flow path from each pellet to the hydrogen outlet. A plurality of heating elements is disposed inside the housing. When the cartridge is in the cavity, each heating element is disposed so heat can be conducted from the heating element and through the casing and corresponding heat transfer member to initiate the release of hydrogen gas. The initiation system can selectively heat one or more pellets to release hydrogen gas as needed.