High-leakage transformers play an important role in power conversion. On-chip, silicon-integrated, and printed-wiring-board-integrated transformers are currently being developed for signal and power applications in high-frequency power supplies. High-power transformers have been developed for the inductive charging of electric-vehicle batteries. These transformers can feature relatively high winding resistances, core loss, and leakage inductances compared with conventional transformers. The conventional open-circuit and short-circuit test calculations have limited use for characterizing these relatively high parasitic transformers. In this paper, the differential (series-opposing) and cumulative (series-aiding) series-coupling tests are developed and applied for the accurate characterization of the transformer resistive and inductive elements. In the series-coupling tests, the various resistive and inductive components simply sum together, making transformer characterization more direct and accurate than attempting to interpret the standard open-and short-circuit (SOS) tests. The SOS tests are extended to provide a more accurate three-measurement approach, and improved correlation is demonstrated between experimental results obtained from the series-coupling tests and the extended open-and short-circuit tests. These results are validated by predictions from finite-element analysis. A sensitivity analysis is carried out to determine the sensitivity of the various test methods to measurement error. The cumulative test is highlighted as being particularly sensitive to measurement error compared with the other test methods.