Embedded Systems and Design & Development - Feb 12, 2026 | Morning | VisionAstraa EV Academy

Understanding NMC Pack Calculations

Introduction to the Session

• The session begins with a greeting and confirmation of audio clarity. The speaker references a previous session focused on calculations related to the NMC pack.

• Participants are reminded that they will have an opportunity to upload their answers via a link provided at the end of the session.

Problem-Solving Approach

• The speaker asks if participants want to solve the problem together or move on, indicating readiness to assist those who may have made mistakes in their calculations.

• A quick overview is given about solving a question using a data sheet, emphasizing its importance for accurate calculations.

Key Data Points for LFP Battery Pack

• Details about an LFP battery pack are shared: it has a voltage of 300 volts and specifications including 100Ah per cell and 50kW power output.

• The objective is to select a Battery Management System (BMS) and charger for this high-voltage car based on given parameters.

Calculation Steps

• To determine BMS and charger requirements, key factors such as power (P), charging current (C), and discharging current (C) must be calculated.

• The number of cells required is calculated as 94 by dividing the pack voltage by nominal voltage.

Discharge Current Calculation

• Maximum discharge capacity from the battery pack is noted as 300 Wh, prompting questions about maximum current extraction capabilities.

• Formula P/V is used to derive current; specifically, calculating total current for the entire battery pack rather than individual cells.

C Rating Insights

• The maximum discharge current determined is 94 amps, leading into discussions about C rating which was found to be approximately 0.94C.

• Previous calculations regarding charging and discharging rates were revisited, highlighting discrepancies in earlier assessments.

Error Identification in Calculations

• An error in calculation methodology was identified; confusion arose between charge power versus discharge power metrics from the data sheet.

• Emphasis placed on careful reading of data sheets to avoid miscalculations that could lead to incorrect conclusions regarding battery performance.

Charging and Discharging Calculations for Battery Packs

Understanding Charging Parameters

• The maximum charge power for the battery pack is specified as 300 W, which is crucial for determining charging capabilities.

• The charging calculation indicates a C rating of approximately 1 C, which is essential to understand the charging limits of the battery.

• With a 1 C rating and a capacity of 200 Ah, the maximum current that can be charged into the battery pack is calculated.

Maximum Charging Current Calculation

• The formula for calculating maximum charging current is given as C (in C rating) multiplied by Ah (capacity), resulting in a maximum of 200 amps.

• While the maximum charge rate is set at 200 amps, lower rates such as 10 or 100 amps are also permissible.

Transitioning to Discharging Parameters

• After establishing charging parameters, attention shifts to discharging characteristics and their calculations based on data sheet specifications.

Discharge Power Specifications

• The data sheet specifies a maximum discharge power of 600 W, which sets limits on how much energy can be drawn from the battery pack.

• By dividing this power by voltage (3.2 V), it’s determined that the maximum discharge current can reach up to approximately 188 amps.

Finalizing Discharge Ratings

• The calculated discharging capability results in a C rating of about 1.88 C when considering total output currents.

• For a complete system with a capacity of 50 kW using a 200 Ah pack, further calculations yield that the maximum discharging current could be around 380 amps.

Selecting BMS and Key Parameters

Criteria for BMS Selection

• Selecting an appropriate Battery Management System (BMS) involves understanding multiple criteria based on previous calculations regarding both charging and discharging capacities.

Essential Considerations

• Key considerations include ensuring cell voltage does not exceed specified limits during operation; this ensures safety and efficiency in managing battery performance.

Battery Management System (BMS) Design Considerations

Key Parameters for BMS Operation

• The microcontroller must monitor cell voltages, ensuring that if any cell exceeds 3.66V, the battery is disconnected to prevent damage.

• The maximum charge voltage for the cells is specified as 3.65V according to the data sheet, which is critical for safe operation.

• It’s essential to check the data sheet for the minimum allowable cell voltage; this ensures that cells do not drop below safe operating levels.

Voltage Calculations and Limits

• The cutoff voltage for discharging a cell is set at 2.5V, as indicated in the data sheet.

• Maximum pack voltage calculation involves multiplying the number of series cells (94) by the full charge voltage (3.65V), resulting in a maximum of approximately 343V.

• Pack voltage should not fall below calculated limits; using minimum cell voltage (2.5V), it results in a lower limit of 235V.

Cell Balancing and Temperature Conditions

• The difference between individual cell voltages must be less than 0.009V to ensure proper balancing across all cells.

• This delta value represents acceptable variations in cell voltages and is crucial for maintaining battery health.

Operating Temperature Specifications

• Recommended operating temperatures are from 0°C to 45°C during charging and -20°C to 60°C during discharging, as per data sheet guidelines.

• Proper programming of the BMS requires feeding these temperature parameters into its operational logic.

Summary of Programming Requirements

• When designing or programming a BMS, it’s vital to incorporate both charging and discharging temperature conditions based on manufacturer specifications found in the data sheet.

Battery Management System Parameters

Charging and Discharging Conditions

• The temperature range for charging is specified as 0° to 45°C, while discharging ranges from -20° to 60°C.

• Maximum overcurrent must be declared under two conditions: charging and discharging, which are critical for battery management system (BMS) safety.

• The discussion highlights that six parameters have been established, with the seventh being overcurrent protection.

Overcurrent Protection Details

• Overcurrent protection is bifurcated into two conditions: charging and discharging, due to differing parameters in each scenario.

• The maximum charging current is set at 200 amps, while the maximum discharging current can reach up to 380 amps.

• If the BMS is set for a maximum discharge of 380 amps, it will not protect against overcurrent during charging if it exceeds 200 amps.

Short Circuit Protection Mechanism

• The eighth condition discussed is short circuit protection; understanding how the BMS detects a short circuit is crucial.

• A fundamental principle of voltage measurement states that zero voltage indicates an infinite current during a short circuit situation.

• Ohm's law (V = I × R) plays a key role in determining voltage across components within the battery pack.

Practical Application of BMS

• The BMS connects in series with both positive and negative terminals of the battery pack to monitor performance effectively.

• In case of a short circuit, if there’s only one measurable point between load connections, it results in zero voltage detection by the BMS.

• This lack of potential difference signifies a short circuit condition, prompting necessary protective measures from the BMS.

Understanding BMS Design and Functionality

Basics of Battery Management System (BMS)

• The BMS is designed to open the circuit when the voltage reaches zero, indicating a critical state for battery safety.

• A microcontroller in the BMS cannot inherently recognize a short circuit; it requires engineers to define conditions for such events.

• The microcontroller acts as a slave, responding to specific conditions set by engineers.

Parameters for BMS Selection

• Eight parameters are essential for selecting a BMS, including maximum current and charge specifications from the data sheet.

• Two common factors can be declared without referring to the data sheet: voltage difference should not exceed 0.9V, and minimum/maximum cell voltages must be defined.

Importance of Practice in Understanding BMS

• Practicing calculations related to charging and discharging currents is crucial for mastering BMS parameters.

• Familiarity with full charge voltages (e.g., LFP at 3.6V and NMC at 4.2V) enhances understanding of battery management.

Safety Features in Battery Systems

• Additional safety features include sensors that detect accidents or falls, prompting the BMS to open circuits immediately.

Microcontroller Selection Criteria

• Different applications require different microcontrollers; some can manage up to 32 connections while others handle more complex systems like Vehicle Control Units (VCUs).

• Multiple microcontrollers may be necessary within a system due to varying control requirements across components.

Cell Configuration and Charger Specifications

• The total number of cells in a configuration is calculated using series (S) and parallel (P): Total Cells = S x P.

• Full charge voltage specifications are critical when selecting chargers; this involves calculating based on the entire battery pack rather than individual cells.

Charging Parameters and Battery Management

Maximum Back Voltage and Charging Current

• The maximum back voltage is identified as 343 volts, leading to a full charge voltage of 340 volts.

• The maximum charging current is approximately 200 amps; however, consistently charging at this rate can reduce battery life. A safer limit for daily charging is recommended.

C-Rating and Practical Application

• Charging at the maximum current daily can lead to decreased battery health, similar to overexerting oneself physically.

• For practical applications, a charging rate of 0.2C (40 amps for a 200 Ah battery pack) is suggested to maintain battery longevity.

Onboard vs Offboard Charger Specifications

• The onboard charger has a recommended maximum charge of 40 amps based on the calculated C-rating.

• In contrast, an offboard charger can handle up to 200 amps for faster charging capabilities.

Example: Mahindra XCV9 Electric Car

• The Mahindra XCV9 features a 50 kWh battery pack that supports a maximum charge capability of around 60 kW when considering rated voltage and current.

• Calculating the power output involves multiplying nominal voltage (300V) by the maximum current (200A), resulting in approximately 60 kW.

Recommended Charging Wattage

• The recommended wattage for charging aligns with the full charge voltage of 343 volts multiplied by the maximum current of 200 amps, yielding about 69 kW.

• This value rounds off to approximately 69 kilowatts as the optimal charging recommendation.

Summary and Future Learning Opportunities

• A comprehensive review of calculations related to charging and discharging will be provided in upcoming notes, including BMS data and formulas.

• Participants are encouraged to practice designing various battery packs using different specifications while understanding fundamental concepts related to electric vehicles or inverters.