Welcome to Battery Lab – Green Technology Laboratory

Introduction

In the battery lab, we study the behavior lithium-ion batteries of varying chemistries under different conditions. Using this data, we create models, new test procedures, controls, and design systems that take advantage of high energy density storage. Thus, our lab combines mechanical design and analysis, electrical design, thermodynamics, heat transfer, energy systems, and machine shop skills to build “real-world” systems. What we do not do is design or fabricate battery cells. For a detailed view of what we do in the lab, check out the Publications tab and see our research and projects below.

Opportunities for Students

The battery lab welcomes new students to join us, we have plenty to do! While working in the lab, students will be exposed to a variety of tasks/activities relating to energy storage systems, microgrids, electric vehicles, and energy management. We projects include controls communications development, energy management algorithm development, systems design and integration, failure modes analysis, utility grid operator communications for demand response and ancillary services… and that’s just what we thought of off the top of our heads. Working in the GTL battery lab is a great way to learn to use multiple skills in a single context. If any of this sounds interesting to you, send us an email!

An Introduction to Lithium-ion Batteries

Why study Lithium-ion batteries?
Lithium ion batteries are a critical component of modern life, relied on by billions of people to power devices from the smallest health implants to large trucks, and even planes. They can be found in our phones, power tools, laptops, cameras, cars, aircraft, tablets, and the electric grid, to name a few. Despite their prevalence, lithium-ion batteries are a relatively new technology that was first commercialized in 1991. [1] This shows in the repeated incidents of commercial products becoming fire-hazards due to their lithium based batteries. In order to make these products safer, our understanding of these batteries must improve, and there are uses of these batteries that are only recently becoming feasible and study-able on a large scale. No device that stores large amounts of energy can be completely safe, this includes your car’s gas tank, flywheels, dams, or hydrogen fuel cells. However, the risks, with sufficient understanding, engineering, and protocols can be significantly reduced as shown by that last link. Thus the study of lithium ion batteries and their control systems is critical to ensuring consumer and public safety, especially when these devices are placed on/in aircraft and other vehicles.

How Batteries Work?

The inside of a battery is split into two sides, the anode and the cathode, by a separator that allows the passage of ions, but not electrons. On each side of the separator, a Redox half reaction takes place. For one of the reactions to occur, the other reaction much occur, since they ‘feed’ each other. These reactions generate the voltage that the battery provides, and the rate at which the reactions can proceed dictates the maximum amount of current that can be extracted from the battery. By only allowing ions to cross the separator (the name-sake lithium-ions in lithium batteries), the electrons that were stripped from the ions are forced to travel through an external circuit (your phone) to reach the other side of the cell, completing the other half reaction. This reaction looks something like this: [2]

Characterization of Batteries
While many batteries will specify “The” voltage at which they operate, batteries do not work at constant voltage. The figure below shows a typical operating voltage curve for a lithium ion battery. It shows that the voltage decreases as the battery is discharged, and that the useable capacity is reduced if we discharge it faster so that the ‘real’ capacity is not always equal to the nominal advertised capacity, even for a new battery. Also, the starting voltage of these batteries was the same, but the one that was discharged faster had a lower operating voltage.

If you look closely, right at the start of the discharge, the blue and the green curves are closer together in voltage than further from the start of the discharge. The first ‘instantaneous’ voltage drop is due to Ohmic resistance, where the voltage drop is calculated by V=IR. We can use this relation to calculate the internal resistance of the battery due to resistive elements in the battery such as the current collectors and the terminals. The figure below shows the voltage (red) and the current (blue) of a battery. There was a 10 second pulse discharge, and we can see the response of the battery’s voltage. Not that it initially drops ‘instantly’ with the current pulse, then slowly continues to decrease. This second slower decrease in voltage is due to factors relating to the chemical reaction in the battery, notably the concentration of reactants near the current collectors.

Lithium-ion Battery Safety
There’s the obvious one, don’t put a screw driver between the terminals. Doing so will result in two of the other most dangerous things to do with a lithium ion battery. The high current, thanks to P = I^2R will result in significant heating of the battery. Applying a large amount of heat (aka high temperature) is extremely dangrous. When the internal temperature of the battery reaches ~70 degrees C, the solid-electrolyte interface will begin to break down. This exposes the electrolyte to the metals in the anode and cathode, resulting in an exothermic reaction, adding to the heat in the battery. Left alone, this will result in thermal run-away, at which point it is not possible to cool the cell fast enough to counteract the heating caused by the reactions within the cell. The lithium in the battery will begin reacting with the electrolyte, producing flamible gases, this causes cells to expand or ‘ballon’ as shown in the picture below. Eventually, this can result in the cell rupturing, spewing its contents into the environment, and causing fires.

Left short-circuited for long enough, it will also result in over-discharge of the battery, which, while not as dangerous as over-charging, is not exactly desireable. Overcharging has a similar effect to excessive heating, since the cell will get very hot when over-charged, with the higher voltages within the cell promoting reactions to occur faster. Since there is more stored energy in the cell when it is fully charged, this can be very dangerous.

These safety considerations are why lithium batteries require battery management systems (BMS). A BMS’s job is to ensure that the battery is not subjected to excessive temperatures, over charging, over discharging, high current, and to protect against cell failures. It does this by measuring the voltage of every cell in the battery pack individually, and communicating with the chargers/loads to ensure they do not push or draw current when it is not safe. The BMS also has temperature control capabilities, it can be connected to a temperature regulation system to prevent the battery from leaving its safe operating region. A BMS also calculates ‘things’ about the battery such as its state of charge (SoC) aka the percentage bar, the state of health (SoH) of cells and of the pack (measured capacity / nominal capacity), internal resistance of the battery cells, and maximum safe currents for charge and discharge based on the battery’s current state.