Provided are animal containment systems and components, including single-use animal containment cages and modular rack units. Reference clocks clk are supplied to the SDRAM as the operating clock when control signals are output and data is written to the SDRAM, that is during period between time t1 and t3 and between time t4 to t9. Then, the battery charger performs a first attachment detection process and when determining whether a battery is connected performs a third temperature estimation process. In some implementations, a transaction initiated earlier is completed when the mobile device arrives at the merchant. Lists are stored and examined to determine the hierarchical order of the identifiers relative to the other identifiers, and a unit of information is linked to at least one other unit of information based on a relative hierarchical order between an identifier identifying the unit of information and another identifier identifying at least one other unit of information.