# EETEngineer - BMS Design Series ## Setting Power Requirements | BMS Design Series Part 01 | Battery Managment System https://youtu.be/6-cEOdpa7dg?list=PLJJ-NKzzRQygLr5ta54anSxbbq5CFP0W2 2025-01-20 - 06:14 ### Overview - Design your own battery management system for the load profile of your devices - Taking a Milwaukee toolset, extracting the stock batteries from the packs, and swapping them with LiPos with a custom BMS for recharging - First, hit the bench and determine the Load Profile of your instrument ### Load Profile - Load Profiling is one of the most important steps in designing the BMS - Startup Current - Steady Operating Current - Stall Current - Overcurrent Protection Trigger - For large DC Motor spikes that your DC Power Supply can't provide, it may be wise to connect a large capacitor bank in parallel - Theory here is, since the DC Supply provides CV, it'll charge up the caps before the inductive load of the power tool turns on. Then, the caps will assist in handling inrush by discharging all that energy. - Consider Current Probes for oscilloscopes, to keep your scope safe - **Load Profiles** - Drill - Startup: 100A - Unloaded: 5A - Loaded: 15A - Sawzall - Startup: 100A - Unloaded: 11.5A - Loaded - 18A - Circular Saw - Startup: 120A - Unloaded: 7A - Loaded: 18A - Setting Protections: - Short Circuit Trip Point: 140-150A (for inrush) - Overcurrent Discharge Trip Point: 30A (for loaded) - 250-300ms Current Spikes - You should always know your load profile so you know what you're designing for. ### Summary of BMS Design Series Part 01: Setting Power Requirements Design BMS for your tool's load profile. Swap Milwaukee toolset batteries with LiPos. Use a custom BMS for recharging. - **Load Profile:** Crucial in BMS design. - **Currents:** Startup, steady, stall. - Use capacitor bank for motor spikes if DC supply can't handle inrush. - Safe oscilloscopes with current probes. - **Example Load Profiles:** - **Drill:** Startup 100A, Unloaded 5A, Loaded 15A - **Sawzall:** Startup 100A, Unloaded 11.5A, Loaded 18A - **Circular Saw:** Startup 120A, Unloaded 7A, Loaded 18A - **Protections:** - Short Circuit: 140-150A for 250-300ms - Overcurrent: 30A Know your load profile for design accuracy! ## # What Battery (Cell) Do I Need? | BMS Design Series Part 02 | Battery Managment System https://youtu.be/aE9rO7SAQ4g?list=PLJJ-NKzzRQygLr5ta54anSxbbq5CFP0W2 2025-01-20 - 06:43 ### Cell Stack Selection - Design Considerations: - Time to pick your - Amp-hours - Watt-hours - Cell internal impedance - Chemistry type - Temperature range - You do not want to design a nice battery pack and realize at the end that the chemistry you selected is simply not compatible with the use case - Comparing LiIon vs. NiH - Nominal Voltage Specs - Lithium Ion - Peak Charge Voltage: 4.2V - Nominal Charge Voltage: 3.6V - Peak Discharge Voltage: 2.5V - Li-Iron Phosphate - Peak Charge Voltage: 3.65V - Nominal Charge Voltage: 3.2V - Peak Discharge Voltage: 2.0-2.5V - Biggest difference is energy density - 26650 vs. 18650 - Have the same Ah rating, but one is twice the size - Why would you want to use a bigger battery if you don't have to? - Li-Iron Phosphate was made as a drop-in replacement for Lead batteries - 300-500 cycles out of a Lithium Ion cell before you see degradation - 1000-2000 cycles out of Lithium Iron Phosphate - Consider the size constraints of your application, as well as the intended lifespan of the cells based on how much they'll get used - Battery Pack Layout & Form Factor - Sometimes you have to find creative ways to stack up the cells in their little container - It's perfectly acceptable to glue together the cells, apparently - REMEMBER: Wires Have Dimensions - There are going to be many wires coming out of your battery core - You have to account for the space they are going to take up - 22-24 AWG wire for each cell back to the BMS ### Summary of BMS Design Series Part 02: What Battery (Cell) Do I Need? Picking the right cell stack is key! Consider amp-hours, watt-hours, cell impedance, chemistry, and temperature. Don't pick an incompatible chemistry last minute. - **Chemistry Comparison - LiIon vs. NiH:** - **Li-Ion:** - Peak: 4.2V, Nominal: 3.6V, Discharge: 2.5V - **Li-Iron Phosphate:** - Peak: 3.65V, Nominal: 3.2V, Discharge: 2.0-2.5V - **Energy Density:** Big difference; 26650 vs. 18650 (same Ah, different size). - **Cycle Life:** Li-Ion about 300-500 cycles; LiFePO4 about 1000-2000 cycles. - Think about size and lifespan for your needs. - **Battery Layout & Form Factor:** - Get creative in stacking cells. - Gluing cells is fine. - Account for wire space; 22-24 AWG for each cell to BMS. Make sure to plan out your battery pack layout, considering cell and wire dimensions! ## What BMS Chipset Do I Need? | BMS Design Series Part 03.1 | Battery Managment System 2025-01-20 - 06:57 ### What is a BMS - BMS monitors voltage, current, charge, and temperature - Goal is to ensure maximum safety during operation - BMS sits between the cell stack and the load so it can isolate one from the other in an event - Four Chipsets to Review - Smart Battery Chipset aka "Fuel Gauge" - COMMS: I2C / SMBus, sometimes SPI - Reads voltage, temperature, etc. as stand-alone devices. - They do not need a microcontroller to function - But you do need a microcontroller to read the registers in them - You can control them as an external device with the microcontroller as well - Programmable - Adjustable Settings for Current, Voltage, Tempearture, etc. - Additional measures - Fuel Gauging - Impedance Gauging - Coulomb Counting - Expensive, but Standalone - Golden File - the firmware for the chipset needs to be made carefully. Then uploaded and calibrated carefully - AFE - Analog Front End - COMMS: I2C / SMBus - Programmable, but reliant on an external microcontroller - System-based, not stand-alone - Some can sense voltage and react to safety events autonomously, others are reliant on the microcontroller for sending trip signals in a safety event - No Fuel-Gauging - you will have to implement this yourself. - Not the best option if you're not good with microcontrollers - If you want CAN bus or flexible system integration options, AFE may be the right choice - Non-Smart BMS Chipset - No COMMS - Hardware Set measures - Voltage, current, temperature - No Fuel Gauging - Fully autonomous, fully stand-alone - Just protects the battery pack, that's all it is designed for - Most of the time, you pick your chipset, and design the battery pack around it - For Non-Smart Chipsets, you pick the battery pack first, and then find a compatible chipset. - You have to pick a SKU from a series - Pick current, voltage, and temperature trip points that are appropriate for your application - TI will make you a custom SKU for your application if you pledge a MOQ - BQ77915 is the chip for this application ### Summary of BMS Design Series Part 03.1: What BMS Chipset Do I Need? BMS ensures safety by monitoring voltage, current, charge, temperature. Positioned between cell stack and load. - **Smart Battery Chipset (Fuel Gauge):** - COMMS: I2C/SMBus, sometimes SPI - Stand-alone, no microcontroller needed to function - Requires microcontroller for register reading - Features: Fuel Gauging, Impedance Gauging, Coulomb Counting - Programmable, expensive, needs precise Golden File firmware - **AFE (Analog Front End):** - COMMS: I2C/SMBus - Not stand-alone, relies on external microcontroller - Some autonomous safety features, no Fuel-Gauging - Suitable for CAN bus or system integration - **Non-Smart BMS Chipset:** - No COMMS - Standalone, measures voltage, current, temperature - Fully protects battery pack, design pack around chipset - Example: BQ77915, offers custom SKU with MOQ Selection depends on specific application requirements. ## What Safety Features? | BMS Design Series Part 03.2 | Battery Management System ### Features - Now that the chipset is selected, what features of that chipset are we using? - Why did we pick this chipset over the others? - Important things to look for: - OCD - Overcurrent In Discharge - This sets the Max Load Current, ensures the BMS only delivers the design current - Too much discharge causes "Fire and Disassembly" (the battery pack explodes) - There can be multiple OCD's - OCD1 - 15A, 5 seconds - OCD2 - 30A, 2 seconds - Gives you more granular control over your system - OCC - Overcurrent In Charge - You want your BMS to have protection for current flowing in both directions - Not offered by all chipsets - Say your charger goes rogue and starts blasting the battery pack - Cell might have a current limit of 3A, charger provides 6A, - cells heat up, venting, fire, disassembly, etc. - Protection in both directions for maximum safety - Temperature - Some BMS chips will cutoff at a certain temp no matter what's going on - Some have up to 4 temp settings - OTD - Over Temp in Discharge - Discharge current is usually higher than charge current - Cells will heat up during discharge, don't want BMS to trip prematurely - OTC - Over Temp in Charge - Lower than OTD, usually - UTD - Under Temp in Discharge - If the pack is too cold, the BMS won't let the cells discharge. - Or just limit current during undertemperature - UTC - Under Temp Charge - Won't let the pack charge if it's too cold, or - Let in a limited current in order to warm up the battery - No matter what, you want SOME kind of temperature setting on your batteries. - This is a crucial safety matter - SCD - Short Circuit in Discharge - If your battery is shorting, it will heat rapidly and the cells will vent and catch fire - Short Circuit Arcs can be 100s of amps - Dead short creates fires extremely quickly ### Summary of BMS Design Series Part 03.2: What Safety Features? Focus on critical BMS safety features and why certain chipsets are chosen: - **Overcurrent in Discharge (OCD):** - Controls Max Load Current; prevents excessive discharge that can cause explosions. - Multiple OCD settings: e.g., OCD1 - 15A for 5s, OCD2 - 30A for 2s. - Provides granular control. - **Overcurrent in Charge (OCC):** - Ensures protection against high charging currents. - Not all chipsets offer this; crucial for rogue charger protection. - **Temperature Control:** - Essential for safety, varying by scenario: - **OTD (Over Temp Discharge)**: Prevents premature trips during high discharge. - **OTC (Over Temp Charge)**: Lower threshold than OTD. - **UTD (Under Temp Discharge)**: Limits or prevents discharge when too cold. - **UTC (Under Temp Charge)**: Limits charge to pre-warm battery when cold. - Temperature settings are a crucial safety feature. - **Short Circuit in Discharge (SCD):** - Protects against rapid heating and fire from shorts. - Addresses short-circuit currents reaching hundreds of amps.