Sensor nodes in wireless sensor networks (WSNs) are often expected to operate on batteries for a long period of time. Battery power efficiency (BPE) is therefore a critical factor dictating the lifetime of WSNs. In this paper, we aim to select the appropriate modulation scheme from a battery power efficiency perspective. Pulse position modulation (PPM) and on-off keying (OOK), as low-complexity pulse-based modulation schemes, are used for a case study of our methodology. The analysis is based on a general model that integrates typical WSN transmission and reception modules with a realistic nonlinear battery model. We first present the quantitative comparison results under general system design criteria. Then, we illustrate the comparisons with theoretical and numerical results under the bit error rate (BER) system design criterion.