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As sensor nodes are typically powered by nonrenewable batteries, energy efficiency is a critical factor in wireless sensor networks (WSNs). Orthogonal modulations appropriate for the energy-limited WSN setup have been investigated under the assumption that batteries are linear and ideal, but their effectiveness is not guaranteed when more realistic nonlinear battery models are considered. In this paper, based on a general model that integrates typical WSN transmission and reception modules with realistic battery models, we derive two battery power-conserving schemes for two M-ary orthogonal modulations, namely pulse position modulation (PPM) and frequency shift keying (FSK), both tailored for WSNs. Then we analyze and compare the battery power efficiency of PPM and FSK over various wireless channel models. Our results reveal that FSK is more power-efficient than PPM in sparse WSNs, while PPM may outperform FSK in dense WSNs. We also show that in sparse WSNs, the power advantage of FSK over PPM is no more than 3 dB; whereas in very dense WSNs, the power advantage of PPM over FSK can be much more significant as the constellation size M increases.