# Energy Harvesting Communication Using Finite-Capacity Batteries with Internal Resistance

## TL;DR

This paper investigates how internal resistance in finite batteries affects energy harvesting communication systems, proposing optimal and sub-optimal strategies for energy and data transmission under various knowledge assumptions.

## Contribution

It introduces a comprehensive analysis of internal resistance effects and develops optimal and sub-optimal policies for energy and data transmission in energy harvesting systems.

## Key findings

- Internal resistance significantly impacts system performance.
- Optimal splitting ratios depend on battery and channel parameters.
- Proposed policies perform well under different knowledge scenarios.

## Abstract

Modern systems will increasingly rely on energy harvested from their environment. Such systems utilize batteries to smoothen out the random fluctuations in harvested energy. These fluctuations induce highly variable battery charge and discharge rates, which affect the efficiencies of practical batteries that typically have non-zero internal resistances. In this paper, we study an energy harvesting communication system using a finite battery with non-zero internal resistance. We adopt a dual-path architecture, in which harvested energy can be directly used, or stored and then used. In a frame, both time and power can be split between energy storage and data transmission. For a single frame, we derive an analytical expression for the rate optimal time and power splitting ratios between harvesting energy and transmitting data. We then optimize the time and power splitting ratios for a group of frames, assuming non-causal knowledge of harvested power and fading channel gains, by giving an approximate solution. When only the statistics of the energy arrivals and channel gains are known, we derive a dynamic programming based policy and, propose three sub-optimal policies, which are shown to perform competitively. In summary, our study suggests that battery internal resistance significantly impacts the design and performance of energy harvesting communication systems and must be taken into account.