Round-Trip Energy Efficiency and Energy-Efficiency Fade Estimation for Battery Passport

TL;DR

This paper presents an algorithm to estimate battery round-trip energy efficiency and its fade over time, using efficiency maps parameterized by temperature and C-rate, demonstrated on electric bus data.

Contribution

The paper introduces a novel algorithm for estimating battery efficiency and fade, utilizing 2D efficiency maps and linear regression for comparison under consistent conditions.

Findings

Efficiency fade of up to 0.86 percentage points over 3.5 years

Algorithm effectively characterizes efficiency based on temperature and C-rate

Demonstrated on data from three electric buses

Abstract

The battery passport is proposed as a method to make the use and remaining value of batteries more transparent. The future EU Battery Directive requests this passport to contain the round-trip energy efficiency and its fade. In this paper, an algorithm is presented and demonstrated that estimates the round-trip energy efficiency of a battery pack. The algorithm identifies round trips based on battery current and SoC and characterizes these round trips based on certain conditions. 2D efficiency maps are created as a function of the conditions `temperature' and `RMS C-rate'. The maps are parameterized using multiple linear regression, which allows comparison of the efficiency under the same conditions. Analyzing data from three battery-electric buses over a period of 3.5 years reveals an efficiency fade of up to 0.86 percent point.