Lithium Battery Cell Balancer Explained charger 18650 and BMS

Lithium Battery Cell Balancer Explained charger 18650 and BMS

Lithium batteries are the workhorses of modern electronics, powering everything from DIY projects and drones to electric vehicles. However, to ensure they deliver long life and safe, reliable performance, the individual cells within a multi-cell battery pack must be kept at the same voltage level. This is where a cell balancer comes into play. This guide, based on a Robojax tutorial, explains the fundamental principles behind lithium battery cell balancers, demonstrates how they work in practice, and shows you how to connect them to your own battery packs.

Understanding and using a cell balancer is essential for anyone building or maintaining multi-cell lithium battery packs. Here are some practical applications:

• Extending the life of power tool batteries: Rebuilding and balancing old drill or saw battery packs to give them a new lease on life.
• Maintaining drone and RC hobby batteries: Ensuring the LiPo packs used in drones, RC cars, and planes are balanced for maximum performance and flight time.
• Building custom power banks: Creating high-capacity portable power supplies from 18650 cells, ensuring all cells charge and discharge evenly.
• Constructing DIY solar energy storage: Building a safe and efficient battery bank for a small off-grid solar setup.
• Upgrading electric bicycles and scooters: Replacing or maintaining the battery packs in personal electric vehicles to ensure safety and longevity.

Hardware Overview

To follow along with this tutorial and build your own cell balancing setup, you will need the following components. Many of these can be found on eBay, AliExpress, or Amazon.

• Cell Balancer Modules: These are small PCBs, often in red or blue, that contain the balancing circuit. They come in various configurations for 1S, 2S, 3S, and up to 8S battery packs. The tutorial specifically uses the HY2213 chip (also known as BB3A).
• Lithium Battery Cells: The tutorial uses 18650 cells, but the principles apply to other lithium-ion and lithium-polymer cells.
• Battery Tester: A simple device that can be plugged into the battery's balance connector to display the voltage of each individual cell. This is invaluable for checking the state of your battery pack.
• DC Power Supply: Used in the demonstration to simulate a battery cell and precisely control the voltage to show exactly when the balancer activates.
• Multimeter: Essential for measuring voltage and current to verify the balancer's operation.
• JST-XH Connectors: The standard connectors used for balance leads on lithium battery packs.

Wiring Guide

Connecting a cell balancer to your battery pack is a straightforward process, but it requires careful attention to the correct sequence. The balancer monitors the voltage of each cell in a series string. It is crucial to connect the wires in the correct order to avoid short circuits or damaging the cells.

For a series battery pack, the cells are connected end-to-end: the positive terminal of one cell connects to the negative terminal of the next. The balancer's wires are then connected to each junction point in this series string. The tutorial demonstrates this for a 3S (three-cell) pack. The first wire (B0) connects to the pack's main negative terminal, the second wire (B1) connects to the junction between cell 1 and cell 2, the third wire (B2) connects to the junction between cell 2 and cell 3, and the final wire (B3) connects to the pack's main positive terminal. This same logic applies to any number of cells, from a 2S to an 8S pack.

For a visual representation of this wiring scheme, please refer to the diagram below.

How the Cell Balancer Works

The core function of a cell balancer is to act as a smart, variable load for individual cells. The balancer continuously measures the voltage of each cell. When a cell's voltage exceeds a specific threshold (typically 4.2V for standard lithium-ion cells), the balancer for that cell activates an internal switch (a MOSFET) that connects a high-power resistor across the cell's terminals. This resistor acts as a load, drawing a small, controlled current from the cell and dissipating the excess energy as heat. This process, known as "passive balancing," slowly brings the higher-voltage cells down to the level of the lower-voltage cells, ensuring the entire pack is balanced.

The HY2213 chip is a popular choice for this task. It has a very precise voltage detection circuit that consumes only microamperes of current, so it doesn't drain the battery when idle. The chip's threshold voltage is set by its internal circuitry, and different versions are available for different battery chemistries. For example, a BB3A version is designed for 4.2V cells, while a BB3B version might be for 4.35V cells. The balancing current is determined by the value of the resistor used. A lower resistance value (e.g., 62 ohms) will result in a higher balancing current (e.g., ~67mA), while a higher resistance (e.g., 100 ohms) will result in a lower current. This allows you to choose a balancer that matches your needs.

Code Explanation

This project is entirely hardware-based and requires no programming. The cell balancer modules are self-contained analog circuits that function without any code.

Live Project Demonstration

The tutorial provides a clear, practical demonstration of the balancing process. Using a DC power supply to simulate a single battery cell, the presenter slowly increases the voltage while monitoring the current with a multimeter. At voltages below the 4.2V threshold, the current remains at zero, showing the balancer is inactive. As soon as the voltage crosses the 4.2V mark (within a 25mV tolerance), the balancer activates, and the current jumps to approximately 66mA. This confirms that the MOSFET has switched on and is now draining current through the 62-ohm resistor. When the voltage is reduced back below the threshold, the balancer switches off, and the current drops back to zero.

The video also demonstrates how to check the state of a battery pack using a battery tester. This device plugs into the balance connector and displays the voltage of each cell, making it easy to see if a pack is balanced or if one or more cells are out of specification. The presenter also shows a brand-new, fully balanced 3S battery pack, where all cells read very close to the same voltage.

Important Considerations

There are a few critical points to keep in mind when working with cell balancers:

• Balancing is a final step: These balancers are designed to work only when the battery is near full charge. If a cell's voltage is below the 4.2V threshold, the balancer will do nothing. This is why most battery chargers incorporate a balancing phase at the end of the charging cycle.
• Do not connect a charger while balancing: When a balancer is actively draining a cell, you must not have a charger or power source connected to the battery. This is why dedicated chargers shut off their charging current before the balancing phase begins. Connecting a charger during balancing can cause damage.
• Prepare your pack correctly: When building a battery pack, it is essential to connect the cells in series (positive to negative) to add up the voltage. The balancer wires are then connected to each of the series connection points. A mistake here can lead to a short circuit.

Chapters

• [00:00] Introduction to the cell balancer tutorial
• [00:53] Why battery balancing is important
• [02:07] How the balancer circuit works
• [03:29] The HY2213 chip and its specifications
• [03:45] Examining the balancer schematic
• [05:47] Main components: MOSFET and resistors
• [06:24] Battery chargers with built-in balancers
• [07:41] Using a battery tester to check cell voltages
• [09:19] Preparing a battery pack for the balancer
• [11:27] Live demonstration of the balancing process
• [15:41] Conclusion and final thoughts