In wireless sensor networks (WNSs), sensor nodes are usually battery-powered. Hence, energy efficiency is a critical factor in WSNs. Orthogonal modulations suitable for energy-limited WSNs have been investigated under the assumption that batteries are linear and ideal However, these analyses are not valid when more realistic nonlinear battery models are considered. Based on a general model integrating WSN transmission modules with realistic battery models, we derive two battery power-conserving schemes for M-ary pulse position modulation (PPM) and frequency shift keying (FSK). We analyze and compare their battery power efficiencies in various wireless channels. 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 3dB; whereas in dense WSNs, the power advantage of PPM over FSK can be significant.