by **Sloan Cinelli**

In 1800, Italian physicist Alessandro Volta published results of an experiment he named the Voltaic pile. This stack of zinc and copper is now known as the first electrical battery. Since then, batteries have become increasingly smaller, more powerful, and more efficient. In 1991, the lithium-ion battery was introduced, having the highest energy density and slowest loss of charge in the rechargeable battery market. Today, energy storage and usage rely more heavily on natural energy harvested from their environment. These systems, including solar power, wind energy, or salinity gradients, are becoming more popular due to their renewable nature. However, the power that is generated from using these natural sources fluctuates randomly with time. In the case of solar cells, power attained in one unit could range from 1 μW to 100 mW, a scale 100,000 times different. Hence, charging and discharging these systems are much more variable than in conventional systems.

Bhat *et a.l* (2017) attempt to fundamentally change the way harvested energy is stored and used in batteries with different efficiencies. Whenever naturally harvested power is lower than the power required for system operation, a system cannot run from that source alone. First, energy must be stored in a battery, then simultaneously drawn from the battery and the natural source, enabling the system to run from the combined power.

In order to accomplish this feat, Bhat *et a.l* consider a low-power wireless transmitter, powered entirely by a naturally harvested source, that is equipped with a battery with different capacity constraints. In this paper, they develop novel policies for managing the battery charging and discharging schedules in the transmitter to promote energy efficiency. This dual-path energy harvesting system contains a power splitter, battery, power combiner and transmitter.

First, the power splitter instantaneously divides the harvested power in order to charge the battery, and power the transmitter. The power combiner then combines power directly from the natural source and battery. The transmitter consumes energy to power the circuit, but does not consume energy if it is not transmitting data. Using this system, Bhat *et al. *then input batteries of different resistances. They found that different internal resistances considerably inhibit the energy redistribution, reducing the average communication rate of the transmitter. With the efficiency data they found that using batteries of different resistances, they derived compact expressions for optimal time and power splitting ratios. These ratios determine how much power should be sent to charge the battery, versus how much should directly power the transmitter. Using these models of charge/discharge efficiencies as functions of the internal resistance of the battery, transmission systems of this sort can be run optimally.

Bhat, R.V., Motani, M. and Lim, T.J., 2017. Energy Harvesting Communication Using Finite-Capacity Batteries with Internal Resistance. *arXiv preprint arXiv:1701.02444*.

https://arxiv.org/abs/1701.02444