*This is a course on batteries and battery management systems* https://www.youtube.com/playlist?list=PLTkAyZQDGrLaIbPapiWv55MTUn8b3nkKY # 1.0 Introduction to the Specialization *Algorithms for Battery Management Systems* Goals of Battery Management Hardware/Software: - Protect the user - Protect the battery - Maximize performance - Maximize service life Via Specialized Electronics and Algorithms Five Sub-courses 1. Intro to BMS's 2. Equivalent Circuit Cell Model Sims 3. State-of-Charge Estimation 4. State-of-Health Estimation 5. Cell Balancing and Power Estimation Math: - Calc, Vectors, Matrices, Linear Algebra, Diffeq Engineering: - Modeling linear circuits Programming: - MATLAB, Octave (FOSS MATLAB) ## 1.1.1 - Intro to Battery Management Systems Another goal of the BMS is to maintain the battery state where it can fulfill the design requirements. Often this means a minimum and a maximum state of charge. Setting hard boundaries on where the battery is allowed to be. *Your battery pack is "cheap enough" if you can't remember the last time you replaced it* The spectrum of Gas to EV: - Gas Car - No battery - HEV - Hybrid Electric Vehicle - Small battery, peak load to keep combustion engine in efficient band - PHEV - Plug-in Hybrid Electric Vehicle - Medium Battery, small all-electric range (~20mi) - E-REV - Extended-Range Electric Vehicle - Larger Battery, some all electric range w/ full load (~50mi) - EV - Electric Vehicle - Battery Only Peak load on this vehicles' battery packs can exceed 1000A These are in the category of: - Large - High Voltage - High Current Grid-Scale Storage - Semi trailers can drive up with a giant battery pack to provide power during substation maintenance ## 1.1.2 - Introducing important battery terminology ### Basic Definitions - **Cell**: Electrochemical energy storage unit. - **Battery**: Collection of cells; batteries contain cells. ### Types of Cells - **Primary vs. Secondary Cells**: Includes categories for differing reusability and application. ### Cell Voltage - **Nominal Voltage**: Defined by active chemical combinations. - **Nickel-Based**: Generally around 1.2V (e.g., NiCd, NiMH). - **Lithium-Based**: Typically over 3V (e.g., 3.7V). - **Nominal**: Represents typical/average voltage, between minimum and maximum values. ### Cell Capacity - **Measurement**: Charge measured in Ah (ampere-hours) or mAh (milliampere-hours), rather than coulombs. - **Lifecycle Performance**: - Initially higher than rated capacity. - Decreases over lifecycle to 70% at end of life. ### C Rate (Charge/Discharge Rate) - **Definition**: Electrical current relative to battery pack size. - **Sustainability**: Rate a cell can sustain for one hour. - Example: - 20Ah cell discharge: 20A for 1h = 1C. - 20Ah cell discharge: 2A for 20h = C/10. - **Calculation**: - Remove "h" from mAh for 1-hour discharge C Rate. - Example: 1900mAh → C Rate = 1900mA. ### Energy Storage - **Form**: Stored in electrochemical form. - **Calculation**: Nominal voltage x nominal capacity in mWh or Wh. - **Energy Release Rate**: Measured as instantaneous power. ### Battery Configurations - **3S1P Configuration**: - Example: 3V, 3x20Ah cells. - Charge Capacity: Still 20Ah. - Energy Capacity: 9V x 20Ah = 180Wh. - **1S5P Configuration**: - Example: 3V, 100Ah cells. - Total Capacity: 300Wh. ![[Pasted image 20250129183933.png]] ## 1.1.3 - What are the parts of an electrochemical cell Electrochemical and LiIon are different The focus is lithium control, but understanding electrochemical is more fundamental ### Electrode Composition ![[Pasted image 20250131075530.png]] Positive Electrode - Cathode Negative Electrode - Anode During Discharge, the negative electrode gives up electrons to an external circuit. Oxidization = Losing Electrons During Charge, the negative electrode accepts electrons from an external circuit Reduction = Gaining Electrons The Negative Electrode is a reservoir of electrons. Fully charged means it has plenty of electrons to hand out. Charging is the process of refilling the electron tank. Current flows `From Anode To Cathode` (FATCAT) However, technically speaking, when the battery is being charged, it is receiving electrons, so it is actually the cathode under charging conditions. Most people will still call it the anode though. Negative Electrode is usually Lead Metal or Paste or Sponge Lead (porous) Positive electrode is usually a Metallic Oxide, Sulfide. Like Lead Oxide. The electrodes are often composed of powders, which are hard to connect a wire to. Instead, we bond the electrode powders to a solid metal foil which acts as the terminal/current collector. Lithium Cells will often use Copper for the Negative Terminal and Aluminum for the Positive Terminal (interesting) ### Electrolyte Function The "equal and opposite" reaction to electrons moving in the battery's external circuit: compensating ions in the electrolyte move inside the cell. Cations - Positive Ions During discharge, they move through the electrolyte towards the positive electrode Anions - Negative Ions During discharge, they move through the electrolyte towards the negative electrode The negative electrode dispenses electrode, anions drift to the electrode terminal to fill the gap The Electrolyte provides the medium for internal ion charge transfer between electrodes. PbA batteries use Sulfuric Acid diluted by water. Aqueous Electrolyte - Plain Water Electrolyte can be Acids, Bases, or Salts The electrolyte must be an **Ionic Conductor** AND an **Electronic Insulator** There's a separator that mechanically isolates the electrodes. It's usually glass, fiber, polyethylene, etc. If it breaks, they will short and discharge. The separator has pores for electrolyte to bridge the two electrodes and allow ion transfer Selecting an Electrolyte with low Ionic Impedance/Ionic Resistance means the battery can provide higher current. Technically Alkaline cells can be recharged a few times (?!) ### Common Electrochemical Cell Chemistries ![[Pasted image 20250131081013.png]] ## How does an electrochemical cell store and release energy? ### Electrochemical Potential Energy and Reversibility ![[Pasted image 20250131082854.png]] In primary cells, the electrochemical reaction is not reversible. The compounds are changed permanently. Secondary cells are designed around reversible chemical processes. Theoretically they should be able to charge/discharge infinitely. However, there are other chemical processes (parasitics) occurring besides the electrochemical energy storage that cause degradation over time. Lithium Cells can be both overcharged and undercharged, causing damage. PbA cells can be slowly recharged by electrolysis of the water electrolyte "Flow Charge" is a small current you can use to keep a battery topped off. When the electrolyte in a Lithium battery decomposes, it turns into expansive flammable gasses (which is why they puff up) Overcharge in Lithium Cells is MUCH more dangerous than overdischarge PbA batteries not maintained at a high SoC will generate lead sulfate crystals, hard crystals on the terminals. These cannot be reconverted. Deep Cycle Cells are designed for long lifespans. ### CC/CV and CP/CV Charging Modes ![[Pasted image 20250131083745.png]] Cells are often first charged with constant-current or constant power. Then, they are topped off by charging via a constant target voltage, which causes the charge rate/current to slowly reduce roughly asymptotically. Because the CV segment is so slow in terms of power transfer, the final few % of charging take about as long as all the rest up to that point. ## 1.1.5 - What are the best materials to use in an electrochemical cell Cell Design & Chemical Selection optimizes for - High Energy Density (per mass or per volume) - High Power Density - Low Cost, Long Life, Low Toxicity, High Recycle, etc. Electrode Potential - the propensity to accept or lose electrons ![[Pasted image 20250131084543.png]] Similar to Electronegativity rating, but apparently not the same thing. Lithium-Air is an upcomer. Valence electrons review. - Lithium: +1 - Carbon: +4 - Cobalt: +2 Lithium Batteries typically use Carbon for the Negative Terminal, and Cobalt for the Positive Terminal. ![[Pasted image 20250131085253.png]] Batteries store energy by changing the energy levels of electrons in their energy shells. Either moving the electrons within their shells, or adding more electrons to the shell. Both affect the potential energy. The *Electrochemical Series* is the list of elements and ions arranged according to a standard half-reaction potential. The Potential (V) is the difference in electrical potential energy before and after the reaction. It's more convenient to list the half reaction potentials, since they represent what happens at one electrode. (e.g. `Li+ + e -> Li`) When you build a battery, you can arbitrarily combine two different half-reactions for each terminal. Listing the potential for every possible combination is not necessary. All voltages need a reference defined at 0V. Seen in `2H + 2e -> H2` Lithium is the most negative - most willing to discard an electron Flourine is the most positive - most willing to accept an electrode. If we made a Lithium-Flourine Battery, it might have 5.91V. We've never made this battery though because we have no known electrolyte capable of handling that much voltage without decomposing. We want high energy density, so lightweight elements that are strong red-ox agents that are abundant in nature and low cost. There are both scientific and economic considerations in electrochemical designs. ## 1.1.6 - Three Example Electrochemical (incl. PbA and NiMH) cells ### Daniell Cell ![[Pasted image 20250131091653.png]] Both Zinc and Copper produce positive ions, negative Sulfate ions Copper receives electrons, so it grows Zinc emits electrons, it shrinks Zinc E = -0.76V Copper E = +0.34V Differencing them: 1.10V Standard Potentials (E) assumes specific electrolyte concentrations and STP ### Lead-Acid Lead - and Lead Oxide + immersed in Sulfuric Acid ![[Pasted image 20250131092032.png]] Lead E = -0.356V Lead Oxide E = +1.685 Fuel Cell Standard Potential: 2.041V Car Batteries are 12V (6 Lead-Acid Cells) #### Lead-Acid Battery Features - Overcharging PbA batteries cause water electrolysis. H2 and O2 are flammable. - Overdischarge causes the formation of Lead-sulphate crystals, causing capacity loss - PbA can be trickle charged to prevent leakage from draining the battery. - Water electrolysis reaction is occurring during trickle charging, but the reverse reaction occurs at the same rate, reaching equilibrium. - Trickle charging can be used to balance the SoC of several cells. It holds the cells at a top point while the other ones catch up. - Lead-Acid batteries don't use solid lead plates. They use spongey lead paste to increase surface area to increase reaction rate. - "Flooded" Cells include a simple spacer separator and a liquid electrolyte - AGM - "Absorptive Glass Mat" - Glass fiber mat soaked in electrolyte to prevent decomposition. Not common anymore - Silica is sometimes added to electrolyte to make it into a gel. - Pure liquid electrolyte tends to stratify and concentration, forming a gradient in space and degrading performance. Gel cells have longer life but higher impedance ### Nickel-Metal-Hydride (NiMH) Cells These work because some metallic alloys (hydrides) can reversibly absorb large amounts of hydrogen. We are taking an electron from the circuit and a hydrogen from the electrolyte and combining them to make an uncharged hydrogen. It's not a redox reaction. Electrolyte is a hydrogen-absorbent aqueous solution (KOH(aq)) - takes no part in the reaction, but can transport hydrogen between electrodes. This hydrogen interpolation process is possible because of hydrogen-absorbing rare-earth alloys at the negative electrode. These alloys can fit hydrogens into their lattice without issue, like a sponge, and then squeeze it out later. This is why NiMH have such long lives. Overcharge causes oxygen gas evolution (and H2) Can slowly discharge it from overcharge to recombine into water. End of module, next module is for Lithium Cells. ## 1.1.7 - Summary of Battery Boot Camp so far Review and Preview Review - Introduce the primary functions of a BMS - When they are needed, applications - Important definitions and terminology - Standard Electrochemical Battery Cell functionality - How Cells are designed and chemistry selected - Review of specific cell types Preview - This course is about Lithium and Computer control over them - Why Lithium? - How do they differ? - What materials are used? - Do we have enough lithium? ## 1.2.1 - Benefits of Lithium-Ion Cells - How Lithium-Ion Cells Work both electrodes in liion cells do the hydrogen sponge thing ![[Pasted image 20250201132324.png]] liion is both smaller and lighter than traditional cell chemistries --- *Stopping here, since I'm second guessing whether I need to know how to roll my own BMS for this project, or if I'm just going down needless rabbit holes*