---
title: A quick guide to cell balancing
description: "Is cell balancing necessary and what does such a process look like? You can find out in our JMBS company blog. Check it out now!"
url: "https://embatterysystems.com/blog/a-quick-guide-to-cell-balancing/"
category: Blog
date: "2023-10-12 06:55:31"
author: Anna Cembrzynska
ai_policy: custom
x-markdown-tokens: 3395
---

# A quick guide to cell balancing

Even with cell batch management and cell sorting processes in place, there will always be some minor differences between cells after manufacture. Over time, these differences can become exacerbated, especially as not all cells in a battery will be subjected to the same temperature conditions. 
Cell balancing is a key function of a Battery Management System that helps keep all cells in a battery pack at similar voltage levels. It improves battery safety, extends service life, increases usable capacity, and supports stable performance during charging and discharging. Without proper cell balancing, the weakest cell can limit the performance of the entire battery pack.

Key Takeaways:

Cell balancing ensures uniform charging across all battery cells, which improves battery performance and longevity.

Passive cell balancing is simpler and more common but results in some energy loss.

Active cell balancing is more efficient but requires more complex circuitry and is ideal for applications needing high energy efficiency.

Cell balancing is a critical feature to look for in battery management systems, especially for long-term battery usage.

Why Is Cell Balancing Important in a Battery Pack?

Cell balancing is important because it keeps all cells in a battery pack at similar voltage levels, improving safety, performance, usable capacity, and battery lifespan. This is why it is essential to implement cell balancing functionality in a battery’s BMS. Cell balancing uses the same wires as SCM, or single cell monitoring, and is usually performed during the final phase of charging.

Individual cells naturally age at different rates and can have slight differences in capacity, internal resistance, and voltage behaviour. Over time, these differences become more noticeable and may lead to uneven charging and discharging across the battery pack.

Without cell balancing, the weakest cell limits the usable capacity of the entire battery pack. This means the battery may stop charging or discharging earlier than expected, even when other cells still have available capacity. As a result, runtime is reduced, energy storage is not fully used, and individual cells can become overstressed.

Cell balancing also helps protect cells from operating outside safe voltage limits. This is especially important for lithium-ion batteries, which are sensitive to overvoltage, overcharge, and deep discharge conditions. Keeping cells balanced reduces safety risks and helps prevent premature battery degradation.

What Does the Battery Cell Balancing Process Look Like?

With passive cell balancing, battery charging stops when the voltage of one cell exceeds the defined limit, often around 4.2V for many lithium-ion cells. The affected cell is then discharged through a small resistor, and charging resumes once its voltage has been reduced. This process repeats until all cells are balanced within a defined mV threshold. Balancing to +/- 1mV is usually not practical in real applications.

This step-by-step regulation allows the remaining cells to continue charging until they reach a similar voltage level. As a result, the entire battery pack reaches a more uniform state of charge, which improves performance and helps maintain consistent operation over time.

By using passive cell balancing, both service life and available capacity can be prolonged. However, there are some limitations. The charging process may take longer, and a small amount of energy is lost as heat because resistors are used to discharge higher-voltage cells.

This energy loss is generally acceptable in most battery systems, especially where simplicity, reliability, and cost efficiency are more important than maximum energy optimisation. Passive balancing is widely used because it is proven, relatively simple, and suitable for many everyday and industrial applications.

What Is the Difference Between Passive and Active Cell Balancing?

The problem of energy loss can be reduced by using active cell balancing. This process works differently from passive balancing. Instead of simply discharging cells with higher voltage, active balancing transfers energy from cells with a higher state of charge to cells with a lower state of charge, or charges lower-voltage cells individually.

Active cell balancing is more efficient because it redistributes energy within the battery pack instead of wasting it as heat. This can maximise usable capacity and improve system efficiency, particularly in large, expensive, or high-performance battery systems.

However, active balancing requires more complex circuitry, advanced control logic, and additional components. This makes it more expensive and more difficult to implement than passive balancing. For this reason, active balancing is mainly used in specific applications where every bit of energy matters.

Such applications may include electric vehicles, stationary energy storage systems, high-capacity industrial batteries, and other demanding systems where efficiency, long-term performance, and energy optimisation justify the higher complexity and cost.

Which Cell Balancing Method Should You Choose?

In most cases, passive balancing is sufficient and provides the right balance between safety, reliability, cost, and performance. The most important factor is to ensure that the battery system includes some form of cell balancing if the battery is expected to operate reliably for many years.

Even basic balancing functionality can significantly improve battery reliability, consistency, and safety over extended operating periods. It helps reduce the risk of cell mismatch, protects against excessive voltage differences, and supports more predictable battery behaviour during charging and discharging.

Low-quality batteries may not include cell balancing, which can lead to reduced capacity, faster degradation, and higher safety risks. At EMBS, we offer cell balancing in the Battery Management System as standard. We can advise when cell balancing is beneficial and determine the optimal settings for the best results in your application.

This ensures that the balancing strategy is tailored to the specific battery chemistry, capacity, voltage range, operating conditions, and usage profile. A properly selected cell balancing strategy helps deliver stable performance throughout the battery’s lifecycle.

FAQ

What is cell balancing in battery systems?

Cell balancing is a process used in battery management systems (BMS) to ensure that all individual cells within a battery pack maintain equal voltage levels. This is crucial because even minor differences in cell voltages can lead to reduced performance, capacity loss, or premature failure of the battery.

What are the two main types of cell balancing?

There are two primary methods of cell balancing:
Passive Balancing: When a cell's voltage exceeds a set threshold (e.g., 4.2V), the excess energy is dissipated as heat through a resistor. While effective, this method results in energy loss and longer charging times.
Active Balancing: Instead of dissipating excess energy, active balancing redistributes it from higher-voltage cells to lower-voltage ones, maintaining overall energy efficiency. However, this method requires more complex circuitry and is typically used in applications where energy conservation is critical.

Which balancing method is more commonly used?

Passive balancing is more commonly implemented due to its simpler design and cost-effectiveness. It's suitable for most applications where minor energy losses are acceptable. Active balancing, while more efficient, is reserved for specialized applications where every bit of energy counts.

How does cell balancing affect battery charging time?

Cell balancing can extend charging times, especially when passive balancing is employed. This is because the BMS may need to pause charging to allow for balancing, ensuring that all cells reach the desired voltage levels.

Is cell balancing necessary for all battery systems?

While not all battery systems incorporate cell balancing, it's highly recommended for multi-cell configurations. Implementing cell balancing ensures uniform cell performance, enhancing the battery's longevity and safety.

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Sources

Qi, J., & Lu, D. D. C. (2014, September). Review of battery cell balancing techniques. In 2014 Australasian Universities Power Engineering Conference (AUPEC) (pp. 1-6). IEEE.

Barsukov, Y. (2009). Battery cell balancing: What to balance and how. Texas Instruments, 2-1. https://www.ti.com/download/trng/docs/seminar/Topic%202%20-%20Battery%20Cell%20Balancing%20-%20What%20to%20Balance%20and%20How.pdf

Lee, W. C., Drury, D., & Mellor, P. (2011, September). Comparison of passive cell balancing and active cell balancing for automotive batteries. In 2011 IEEE Vehicle Power and Propulsion Conference (pp. 1-7). IEEE. 

Koraddi, S., Samprita, K. V., Yadgir, K. S., Biradarpatil, L. M., & Nayak, S. V. (2022, January). Analysis of different cell balancing techniques. In 2022 International Conference for Advancement in Technology (ICONAT) (pp. 1-4). IEEE.

Drori, Y., & Martinez, C. (2005). The benefits of cell balancing. Application Note, Xicor Incorporated, 1511, 95035-7493. https://hallaweb.jlab.org/tech/Detectors/public_html/manuals/chip_specs/A-L/intersil/The_Benefits_of_Cell_Balancing_an141.pdf

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