This chapter provides the design methodology for battery chargers powered by TEGs. To prevent the minimum open-circuit voltage from being limited by the operation of charge pumps, the clock generator is powered by the battery. Even though the clock generator consumes battery power, when the charge pump outputs more power than the consumed power, the net power can be positive. In Sect. 5.1, the system architecture is presented briefly. The design equations used to formulate an optimum number of stages and the required capacitance per stage are reviewed to minimize the open-circuit voltage of the TEG in Sect. 5.2. The impact of the output resistance of the TEG on the minimum open-circuit voltage is discussed. A design demonstration is conducted in Sect. 5.3. The minimum operation voltage is limited by the charge transfer switch (CTS) when a cross-coupled CMOS is used for the CTS. It is also shown that a simple MOS switch can reduce the open-circuit voltage below 100 mV.