Battery Balancing: Techniques, Benefits, and How It Works
This article examines the concept of battery balancing, its significance, and methods for achieving effective battery balance.
What Is Battery Balancing?
Battery balancing is the process of equalizing the charge across individual cells in a battery or individual batteries in battery groups to ensure uniform voltage levels, or state of charge (SOC). This process helps prevent overcharging or undercharging of cells, which can lead to performance degradation, reduced capacity, and shortened battery lifespan. By balancing the cells, the battery system operates more efficiently, delivering optimal performance and extending the overall lifespan of the battery pack.
Why Do We Need Battery Balancing?
When cells in a battery have different SOCs, the overall battery capacity is equal to the weakest cell. When discharging a set of batteries, the lowest charged battery will empty first, cutting off the system. When charging, the highest charged battery will cut off the charging system before the lower charged batteries are fully charged.
Overall, battery balancing helps address the following issues:
- Avoiding overcharging or over-discharging of individual cells
- Equalizing the charge levels across all cells
- Maximizing the usable capacity of the battery
- Prolonging the overall lifespan of the battery
This section discusses the significance of battery balancing for both cells in a battery and individual batteries in a paralleled battery group.
Battery Cell Balancing
Typically, the individual cell group in a battery have somewhat different capacities and may be at different levels of SOC due to the manufacturing variances, assembly variances, and different charge/discharge histories experienced. When cell groups are connected in series, these differences may limit the energy that can be taken from or return to the battery and result in overcharge or over-discharge without effective and appropriate balancing circuit.
Paralleled Battery Balancing
Battery balancing is also essential for battery groups connected in parallel.
Typically, the voltage difference between individual batteries is larger than that between individual cell groups. When batteries are connected in parallel, the balancing will start automatically between batteries as the current flows from the higher-voltage batteries to the lower-voltage batteries. However, due to the small internal resistance of the battery, the balancing current will be so large that trigger the over-current protection of the battery when the voltage difference is too large. As the number of paralleled batteries increases, the voltage difference will become more restrictive. As a result, few or no restrictions are imposed on the voltage difference between batteries when the number of paralleled batteries is small, while a voltage difference smaller than 0.1V between the highest-voltage battery and the lowest-voltage battery is recommended when the number of paralleled batteries is large.
How to Balance Batteries?
An imbalance is most commonly indicated by varied cell voltages, which can be addressed either instantly or progressively by bypassing the cells that have higher voltage levels.
In general, battery balancing methods can be categorized into the following types:
- Active balancing
- Passive balancing
- Software-based balancing
Passive balancing dissipates excess energy from higher-charged cells as heat, while active balancing employs a switch matrix and transformer to transfer energy between individual cells. In software-based balancing, each cell’s SOC is monitored, and algorithms are implemented to manage charge distribution.
Active Balancing Methods
- Capacitive Balancing: This method uses capacitors to transfer charge between cells. Capacitors temporarily store energy from higher-voltage cells and then discharge it into lower-voltage cells, helping to equalize their voltages.
- Inductive Balancing: This method employs inductors to transfer energy from higher-voltage cells to lower-voltage ones. Inductive balancing can be more efficient than resistive methods as it minimizes energy loss.
- Energy Redistribution: In this approach, energy is transferred between cells using power electronics. This can involve more complex circuitry to manage energy flow and maintain balance effectively.
Passive Balancing Methods
- Resistive Balancing: This method involves connecting resistors in parallel with higher-voltage cells to dissipate excess energy as heat, allowing the voltages of all cells to equalize over time.
- Shunt Balancing: Similar to resistive balancing, this method uses shunt resistors to divert current away from cells with higher voltages, allowing them to discharge more quickly.
- Discharge Balancing: In this approach, the cells with higher voltages are selectively discharged to bring their levels in line with lower-voltage cells, usually done during charging cycles.
Software-Based Balancing
- State of Charge (SOC) Monitoring: Advanced battery management systems (BMS) monitor the state of charge of individual cells and adjust charging/discharging protocols accordingly to ensure balance.
- Algorithmic Balancing: Some systems use algorithms to predict the state of each cell and actively manage charging and discharging cycles to maintain balance.
For end users, we recommend the following battery balancing methods to expand your solar battery lifecycle.
Method #1: Built-in Bypass Circuit
Many batteries employ built-in bypass circuit to maintain the balance between each cell group in the battery. Choose such batteries can effectively prevent unbalanced issue.
Each cell group is connected in parallel with a bypass resistor and a switch. During the charging process, when the voltage of the highest voltage cell group reaches the predetermined balancing threshold and the voltage difference between the highest and lowest voltage cell groups exceeds a specified limit, the switch for the highest voltage cell group closes. This action diverts the charging current around the highest voltage cell group through the bypass resistor until the voltage difference falls below the set limit. To minimize energy loss, battery cell balancing is conducted solely during the charging process.
Examples include:
- Renogy Smart Lithium Iron Phosphate battery
- 12V 100Ah Pro Smart Lithium Iron Phosphate Battery w/Bluetooth & Self-heating Function
Method #2: Manual Balancing
This method applies to scenarios where multiple batteries are connected in series or parallel. Before connecting batteries in series or parallel, it is important to balance them to reduce voltage differences and optimize their performance. Take 12V 100Ah Pro Smart Lithium Iron Phosphate Battery w/Bluetooth & Self-heating Function as an example. You can follow these three steps:
Step 1: Charge each battery individually to its full capacity using a suitable charger.
Step 2: Use a voltmeter to measure the voltage of each battery. It is best to keep the voltage difference of each battery less than 0.1V.
Step 3: Connect all the batteries in parallel and allow them to rest together for 12 to 24 hours.
FAQ
Q1: Do All Battery Types Need Balancing?
Not all battery chemistries require balancing, but balancing is essential for lithium-ion batteries and other multi-cell systems where consistent charge across cells is crucial for performance and safety.
Q2: How Often Should I Perform Battery Balancing?
The frequency depends on the battery type, usage, and the balancing system itself. Some systems perform balancing continuously or periodically based on thresholds, while others balance only when needed. It is recommended to periodically rebalance the battery voltages every six months when connecting multiple batteries as a battery system. Slight voltage differences can occur among batteries over time due to factors like battery chemistry, capacity, temperature, and usage patterns.
Q3: How Can I Tell If My Battery Pack Needs Balancing?
Signs include irregular voltage readings between cells, reduced overall battery performance, and shorter runtimes. Many battery management systems (BMS) offer built-in diagnostics to detect and alert for imbalance issues.
