One thing you could do to smooth the waveform, since you're building the battery pack anyway, wire it for half or 2x the voltage (with corresponding decrease or

One thing you could do to smooth the waveform, since you're building the battery pack anyway, wire it for half or 2x the voltage (with corresponding decrease or increase in amperage) and run it through a transformer to the correct voltage. You will lose some power in the transformer, but the waveform will be much smoother.