Battery Balancing: What, Why, and How

Battery balancing is a vital process for maintaining the efficiency, performance, and safety of battery systems, whether for solar energy storage, electric vehicles (EVs), or other energy applications. Without proper balancing, your batteries can become imbalanced, reducing their lifespan and performance.

This guide explores what battery balancing is, the causes of imbalance, and how to fix it, addressing both individual cells and battery groups.

What does battery imbalance mean?

Battery imbalance refers to a condition where the battery voltage or state of charge (SoC) varies among the cells or groups within a battery pack. Over time, imbalance creates inconsistency—differences in cell performance—worsening the issue and forming a vicious cycle.

This issue is particularly common in multi-cell configurations and can result in several problems, including reduced energy storage capacity, accelerated cell degradation, and heightened risks of overcharging or undercharging, which may pose safety hazards. Importantly, imbalance is not confined to individual cells; it can also occur between battery groups.

What causes batteries to imbalance?

Battery imbalance arises due to inconsistencies in individual cells, stemming from differences in parameters, operating conditions, and environmental factors. Key causes include:

1. Cell Parameter Variations

Even similar batteries can vary in capacity and resistance, causing uneven charging/discharging. Cells with lower capacity or higher resistance degrade faster, as they charge or discharge prematurely, leading to inefficiencies and imbalance over time. These variations accelerate wear, reducing overall battery performance and lifespan.

2. Charge/Discharge Conditions

Improper charging methods, such as overcharging or over-discharging, can damage cells and increase disparities over time.

3. Temperature Effects

Uneven heat distribution within the pack or exposure to extreme temperatures accelerates cell degradation due to increased chemical reactions, reducing overall performance.

4. SOC Estimation Errors

State of Charge (SOC) discrepancies arise from uneven initial capacities or resistance differences, causing uneven current distribution in parallel circuits.

5. Age and Degradation

As batteries age, their capacity and performance vary, causing uneven charging and discharging.

6. External Circuits

Variations in connectors, resistances, and Battery Management System (BMS) input circuits lead to unequal energy distribution, amplifying inconsistencies.

While battery imbalance is unavoidable, proactive measures are essential to minimize inconsistencies and maintain performance.

Why Is Battery Balancing Important?

A balanced battery pack/system ensures that each individual cell operates within its safe voltage range. Here’s why battery balancing is so important:

Preventing Usable Capacity Loss

Variations among battery cells in series and parallel setups reduce the system’s usable capacity.

For example, in a 500 kWh system with 50 series cells, each storing 10 kWh, if one cell reaches only 85% state of charge (SoC) while others are at 100%, the pack’s stored energy drops to 495 kWh. During discharge, it’s limited to 425 kWh (85%), resulting in a 15% capacity loss.

Without balancing, this discrepancy grows, locking away more energy and accelerating cell degradation.

Avoiding Parallel Inconsistencies

In parallel configurations, voltage mismatches cause circulating currents, forcing clusters with lower resistance to charge or discharge faster. This leads to premature stops, overheating, and capacity loss.

For instance, a system recorded a charging current deviation of 60A, with a 40% difference from the average. Such imbalances reduce efficiency, shorten lifespan, and increase safety risks.

Mitigating Temperature Effects

Uneven temperatures among cells exacerbate resistance and capacity differences, accelerating degradation and safety concerns. A 15°C rise can halve system lifespan, making consistent temperature management critical.

By addressing these challenges, battery balancing ensures maximum capacity, efficiency, and reliability, safeguarding your investment and prolonging the system’s life.

How to fix battery imbalance and inconsistency

When battery or cell imbalance occurs, there are several ways to address the issue, either using specialized tools or manual methods. Here are some effective solutions:

Use a BMS (Battery Management System)

A Battery Management System (BMS) is designed to monitor and balance the voltage across individual cells in a battery pack. It automatically balances cells during the charging process, ensuring they all reach the same voltage.

Use a Battery Equalizer

A battery equalizer is a device designed to balance the voltage and charge levels between individual cells or groups of cells in a battery pack. A battery equalizer works by redistributing the charge between cells, ensuring they all maintain the same voltage level.

Balance batteries in parallel

When batteries connected in parallel experience voltage differences, you can often resolve the issue with a straightforward process:

1. Disconnect and Rest in Parallel:

Disconnect the batteries from the charging source and loads. Then connect them in parallel directly. Let the batteries rest for approximately 3 hours. During this time, the higher-voltage batteries will naturally transfer charge to the lower-voltage ones, helping their voltages equalize.

2. Perform Full Charge-Discharge Cycles if Needed:

If the voltage difference persists after resting, you’ll need to disconnect the batteries and perform one or two full charge-discharge cycles for each battery individually. This process resets the Battery Management System (BMS), allowing it to recalibrate and accurately read the voltage levels of each battery.

Fix battery imbalance caused by capacity differences

When using multiple batteries of the same model, capacity differences can lead to imbalances, especially when connected in parallel.

Avoid Parallel Connection for Large Capacity Differences:

If the capacity difference between batteries is significant, it’s not recommended to connect them in parallel. The smaller battery will charge first, leaving the larger one undercharged, which will worsen the imbalance over time.

Test and Replace Low-Capacity Cells:

If you still want to use the batteries in parallel, test the capacity of the lower-performing battery. Replace any underperforming cells to restore balance and ensure consistent charging across the battery pack.

Balance batteries cells

If the voltage of individual battery cells becomes imbalanced, the following steps can be taken to restore balance:

1. Disassemble the Battery Pack

Open the battery casing carefully to access the individual cells. Ensure all safety precautions are followed, such as wearing insulated gloves and working in a well-ventilated area.

2. Measure the Cell Voltages

Use a multimeter to measure the voltage of each cell. Identify cells with voltages lower than the others, as these are the primary cause of imbalance.

3. Charge Low-Voltage Cells

Using a programmable power supply, charge the cells with lower voltages until they match the voltage of the others. This step equalizes the charge levels and helps restore balance.

4. Inspect for Internal Resistance Issues

If voltage imbalance persists even after charging, the issue may be caused by increased internal resistance in one or more cells. This is often a sign of aging or damage.

5. Replace Faulty Cells

Remove any cells with significantly higher internal resistance. Replace them with new cells that have consistent internal resistance values. For this step, use a spot welder or a laser welder to securely connect the replacement cells to the battery pack.

Practical tips to minimize battery imbalance/inconsistency

To minimize battery inconsistency and imbalance during daily use, it’s crucial to follow best practices for maintenance and operation.

• Limit the number of parallel connections: Ensure no more than 8 batteries are connected in parallel. Remove excess batteries if this limit is exceeded.
• Use identical batteries: Confirm that all parallel-connected batteries are identical in brand, model, capacity, and voltage level. Mixing batteries can lead to imbalances.
• Equalize initial voltages: Before parallel connection, charge each battery individually to ensure voltage differences (Voc) are less than 0.1V.
• Standardize wiring: Use the specified wiring method, maintain consistent wire lengths, use the same wire gauge (AWG), and ensure secure connections.
• Adjust charging/discharging currents: Maintain currents above 0.2C for optimal balance. For instance, charge/discharge a 100Ah battery at 20A or more.
• Avoid extreme temperatures: Prevent batteries from operating in high-temperature environments to reduce stress and imbalance.
• Monitor self-discharge rates: Regularly check and balance voltages to account for differences in self-discharge rates.
• Connect to a common busbar: Ensure all parallel-connected batteries share the same DC busbar to minimize inconsistencies.

By implementing these measures, the risk of imbalance is significantly reduced, promoting better battery performance and longevity.