Terminal-voltage equalization is problematic in mid-range SOC because lithium-ion cell voltage changes too little to reveal meaningful SOC differences. A 1% SOC mismatch may produce only a few millivolts of voltage difference in the flat mid-SOC region, making it difficult to distinguish real imbalance from measurement noise. Under load or charging, internal-resistance variation and polarization add further voltage errors, so voltage-based systems may balance the wrong cells or activate too late.
The core issue is that terminal voltage is an indirect and unstable indicator of SOC. SOC-based equalization estimates each cell’s actual charge state, compares it with a pack-level target such as average SOC, and begins balancing earlier—reducing peak equalization current, heat, hardware size, and lost pack capacity.
Why Terminal Voltage Loses Diagnostic Value
Mid-range lithium-ion voltage is relatively flat
The relationship between open-circuit voltage and SOC is nonlinear. In some low-SOC regions, a 1% SOC change can produce a voltage shift of tens of millivolts, but in a mid-to-high SOC range such as approximately 75–85%, the change may be only around 3 mV per 1% SOC.
That means two cells with a real SOC difference can appear nearly identical when evaluated only by terminal voltage.
Small voltage differences are easily obscured
A few millivolts can be comparable to measurement noise, sensor offset, ADC resolution, wiring resistance, and connector voltage drops. As a result, a voltage-based BMS may not detect a meaningful imbalance until the cells approach the steep portion of the charging curve near full SOC.
This is a fundamental observability problem: the cells are different internally, but their measured voltages do not provide enough information to identify the difference reliably.
Terminal voltage is not the same as equilibrium voltage
Terminal voltage includes more than the cell’s equilibrium or open-circuit voltage. A simplified representation is:
[ V_\text{terminal} \approx V_\text{OCV}(SOC) - I R_\text{DC} - V_\text{polarization} ]
The exact behavior depends on the cell chemistry, current direction, temperature, history, and model assumptions. Therefore, a higher terminal voltage does not always mean that a cell has proportionally higher SOC.
Why Voltage-Based Equalization Activates Too Late
Balancing may begin only near full charge
When the voltage-SOC curve is flat, voltage thresholds cannot reliably identify imbalance during much of the charging cycle. The equalizer may remain inactive until one cell reaches the upper-voltage threshold.
By that point, the available balancing time is short, even though the underlying SOC mismatch may have existed throughout the cycle.
Late activation requires high balancing power
If balancing starts only near the end of charging, the equalizer must remove or transfer the accumulated SOC difference quickly. This increases the required peak balancing current and can force the use of larger inductors, switches, thermal paths, and control hardware.
The result is a more expensive and physically larger equalization system than would be required if balancing were distributed across the full charging period.
High current increases thermal stress
A high-power equalizer generates more heat in the balancing circuit and, in dissipative systems, converts more stored battery energy directly into heat. This raises thermal-management demands and can worsen the operating conditions that contribute to cell aging.
Continuous, lower-current correction is generally easier to cool and less disruptive than short-duration, high-current balancing.
Why Load Conditions Make Voltage Decisions Unstable
Internal resistance changes the voltage ranking
Cells do not have identical DC internal resistance. Under charging, a cell with higher resistance can show a higher terminal voltage because of its larger resistive voltage rise, even if its actual SOC is not the highest.
During discharge, the same resistance difference can produce a larger voltage drop. The apparent order of the cells can therefore change with current direction and magnitude.
Polarization introduces additional error
Polarization voltage depends on recent current, charging or discharging history, and relaxation behavior. Two cells with similar SOC can show different terminal voltages because their electrochemical responses differ.
Conversely, cells with different SOC values can temporarily show similar voltages. Equalization based on an instantaneous voltage comparison can therefore target the wrong cell or repeatedly change its decision as operating conditions vary.
Cell rankings can reverse
A cell that appears to have the highest voltage during charging may not be the cell with the highest SOC after the current is removed and the cells relax. This voltage-order reversal is especially problematic when equalization logic assumes that the highest measured voltage always identifies the cell requiring discharge.
How SOC-Based Equalization Solves the Problem
SOC becomes the control variable
SOC-based equalization estimates the charge state of each individual cell rather than treating terminal voltage as a direct measure of stored charge. The controller then compares each cell’s estimated SOC with a target, commonly the average SOC of the series pack.
A typical control relationship is:
[ SOC_\text{target} \approx \frac{1}{N}\sum_{i=1}^{N} SOC_i ]
Cells above the target are discharged or transfer energy away, while cells below the target are charged or receive energy.
Balancing can begin earlier
Because SOC-based control does not depend on a late-stage voltage divergence, it can operate across a much larger portion of the charging cycle. The equalizer gradually reduces the SOC spread before the cells reach the upper-voltage region.
This distributes the balancing work over time and avoids concentrating the entire correction near full charge.
The equalizer can be smaller
Early, continuous balancing reduces the required peak current for a given SOC mismatch. That can reduce the size, cost, and thermal burden of the equalization circuitry.
The benefit applies to both dissipative and nondissipative systems, although the circuit behavior differs: resistive balancing removes excess energy as heat, while charge-shuttling systems transfer energy between cells or back to the pack.
Pack capacity is used more effectively
In a series-connected pack, the usable capacity is constrained by the cell that reaches its charge or discharge limit first. Aligning the cells by SOC helps them reach these limits more closely together.
This improves high-end alignment, reduces premature cutoff caused by a single outlying cell, and increases the practical energy available from the pack.
What Is Required for Reliable SOC-Based Control
Accurate cell characterization
SOC estimation depends on cell-specific information such as discharge capacity, OCV-SOC behavior, internal resistance, temperature response, and aging state. These parameters should be measured under controlled test conditions rather than assumed to be identical across all cells.
Battery testing systems are therefore important during R&D: they provide the data needed to distinguish true cell variation from measurement artifacts.
A suitable state estimator
Because SOC cannot usually be measured directly during operation, the BMS must estimate it. Coulomb counting, OCV correction, equivalent-circuit models, and state estimators such as Extended Kalman Filters can be combined to improve accuracy.
The estimator must account for current, voltage, temperature, model error, and accumulated measurement drift. A poor SOC estimate can simply replace one balancing error with another.
A defined equalization control band
The controller should not attempt to eliminate every tiny estimated SOC difference. Instead, it should use an acceptable SOC band or deadband to prevent unnecessary switching, oscillation, and energy consumption.
The band should reflect estimator accuracy, cell variability, equalizer resolution, thermal limits, and the performance objective of the pack.
Understanding the Trade-offs
SOC estimation is more complex
SOC-based equalization requires current measurement, model development, parameter identification, state-estimation software, and validation across temperature and aging conditions. It is more sophisticated than applying a simple voltage threshold.
That complexity is justified when pack capacity, lifetime, or equalization efficiency matters, but it increases development and verification requirements.
Estimation errors must be managed
SOC estimates can drift because of current-sensor offset, inaccurate capacity values, temperature changes, model mismatch, and cell aging. The system should use appropriate correction mechanisms and validate its estimator against measured cell behavior.
Voltage remains useful as a safety limit and as one input to the estimator, but it should not be treated as the sole equalization target.
Balancing does not repair weak cells
Equalization can reduce differences in SOC, but it cannot restore lost active material, eliminate abnormal self-discharge, or correct a cell with significantly elevated internal resistance. A failing cell may require isolation or replacement rather than increasingly aggressive balancing.
Equalization architecture still matters
A dissipative equalizer is simpler but wastes energy as heat. A nondissipative equalizer improves energy utilization but requires more complex power conversion and accurate control.
SOC-based decision logic improves the choice of which cells to balance; it does not remove the electrical, thermal, and cost trade-offs of the chosen equalizer hardware.
Making the Right Choice for Your Goal
The appropriate strategy depends on whether the priority is simplicity, measurement accuracy, capacity utilization, or long-term pack performance.
- If your primary focus is simple protection: Use terminal voltage as a safety and cutoff signal, but do not rely on it alone to determine mid-SOC equalization targets.
- If your primary focus is accurate pack evaluation: Characterize each cell’s capacity, OCV-SOC curve, resistance, and temperature behavior before designing the equalization algorithm.
- If your primary focus is maximum usable energy: Use SOC- or capacity-based equalization so cells approach charge and discharge limits together.
- If your primary focus is lower hardware cost and heat: Start balancing earlier with lower current rather than waiting for a large voltage difference near full charge.
- If your primary focus is robust BMS operation: Combine SOC estimation with voltage, current, temperature, and resistance information instead of treating any single measurement as definitive.
SOC-based equalization turns balancing from a late reaction to a measured control process, allowing the pack to use its cells more evenly, efficiently, and reliably.
Summary Table:
| Issue with Voltage-Based Equalization | Solution with SOC-Based Equalization |
|---|---|
| Voltage flat in mid-SOC, 1% SOC change yields only ~3 mV | SOC estimated directly, enabling precise imbalance detection |
| Late activation near full charge | Balancing starts earlier, across entire cycle |
| High peak current, larger hardware | Lower current, smaller equalizer |
| Thermal stress from high power | Reduced heat generation |
| Errors from internal resistance and polarization | Uses robust estimation models (e.g., EKF) |
| Voltage ranking can reverse under load | SOC target is consistent regardless of load |
| Reduced usable pack capacity | Aligns cells by SOC, maximizing usable energy |
Optimize your battery pack performance with SOC-based equalization. At KINTEK, we provide advanced battery testing and cell characterization equipment to help you implement robust SOC estimation. Our portfolio supports R&D in lithium-ion batteries and materials science. Contact our experts today to elevate your battery management systems—get in touch.