Table of Contents
The battery appears to be working normally, but the app suddenly shows a larger cell voltage difference—sometimes reaching several dozen millivolts (mV). This can quickly raise concerns about whether there is a problem with the battery.
In fact, the voltage difference between cells in a LiFePO₄ battery does not always remain constant during charging. Especially near the end of the charging process, an increase in cell voltage difference is often normal and does not automatically mean that the battery is defective. This behavior is related to the characteristic LiFePO₄ voltage curve, charging current, small differences between individual cells, and the BMS balancing logic.
Whether a cell voltage difference is normal should always be evaluated together with the battery's current operating condition, including the SOC, whether the battery is still connected to the charger, how the values change after a resting period, and whether any other abnormal behavior occurs.
1. What Is Cell Voltage Difference?
Cell voltage difference refers to the voltage deviation between individual cells within a battery pack.
A typical 12V LiFePO₄ battery usually consists of 4 cells connected in series, also known as a 4S configuration. Each cell has its own individual voltage. The BMS monitors these values in real time and displays them through the app.
| Cell | Voltage |
|---|---|
| Cell 1 | 3.55V |
| Cell 2 | 3.50V |
| Cell 3 | 3.46V |
| Cell 4 | 3.42V |
In this example, the highest individual cell voltage is 3.55V and the lowest is 3.42V.
Cell voltage difference: 3.55V − 3.42V = 0.13V, or 130mV.
Small difference: The individual cells are in a relatively similar state.
Large difference: Some cells are beginning to deviate more noticeably from the others.
It is important to understand that cell voltage difference is not a fixed value but a dynamic parameter. It changes depending on charging, discharging, resting time, battery temperature, SOC, and the current balancing status of the BMS.
2. Why Does the Difference Increase During Charging?
In LiFePO₄ batteries, cell voltage differences often become more noticeable during charging, especially near the end of the charging process. This is not accidental but is closely related to the electrical characteristics of LiFePO₄ chemistry.
2.1 The Voltage Curve Is Very Flat in the Middle SOC Range
LiFePO₄ cells have a very distinctive characteristic: the voltage curve remains extremely flat through the middle portion of the state-of-charge range.
When the battery is approximately within the 20% to 80% SOC range, the voltage changes only slightly. Small differences in capacity, internal resistance, or charge acceptance between individual cells are therefore difficult to observe from voltage alone.
Only when the battery approaches a full state of charge does the voltage curve become significantly steeper. This means that even a small amount of additional charge can cause the cell voltage to rise rapidly. If one cell reaches the upper SOC range slightly earlier than the others, its voltage will increase faster than the remaining cells.
This happens because cell voltage becomes highly sensitive to small changes in state near full charge. As a result, minor differences between cells become much more visible during this stage.
2.2 Charging Current Causes Temporary Voltage Differences
The higher the charging current, the more noticeable this effect can become. Due to small differences in internal resistance, temperature, SOC, and cell condition, individual cells may show different instantaneous voltages even when exposed to the same charging current.
Therefore, if one particular cell appears to increase in voltage faster than the others while charging, it does not automatically mean that the cell is abnormal. It may simply reflect temporary behavior under charging conditions.
2.3 Conditions Required for BMS Balancing
Many LiFePO₄ batteries use passive balancing. Passive balancing normally starts only when individual cells enter a higher voltage range. During this process, a small amount of energy from the higher-voltage cells is gradually dissipated so that their voltage can move closer to that of the other cells. This balancing process does not happen immediately. It takes time, and the balancing current is usually relatively low.
In addition, if the battery is rarely charged into a high-voltage range during normal use, the BMS may not have enough opportunities to perform effective top balancing. For example, if charging is regularly stopped at 70% or 80%, the battery rarely reaches the voltage range in which balancing can operate effectively. Over time, small differences between cells may gradually accumulate. During a later full charge, these differences may then become much more noticeable.
Important: Two conditions must be met for the BMS to balance the cells: first, the battery must reach a suitable voltage range; second, the system must remain in that range long enough for balancing to take place.
3. When Is the Difference Normal or Abnormal?
Whether a cell voltage difference is unusual depends mainly on the battery's current operating condition. The same voltage difference can mean very different things depending on the situation.
| Condition | Voltage Difference Behavior | Assessment |
|---|---|---|
| End of charging | Difference temporarily increases | Usually normal |
| 30–60 minutes after disconnecting the charger | Difference decreases significantly | Usually normal |
| Mid-range SOC while resting | Difference remains small | Normal |
| Difference remains above 100mV even after resting | Remains large over time | Further inspection recommended |
| The same cell is consistently much lower | Repeated abnormal readings | Monitor closely |
| One cell repeatedly reaches the high-voltage protection range first during charging | Repeatedly abnormal | Further inspection recommended |
| The battery frequently triggers protection functions | Accompanied by other abnormal behavior | Check the system and battery |
| One cell drops significantly faster during discharge | Difference appears under load | Check cell consistency and battery condition |
A more reliable assessment should consider several factors:
First: Pay attention to when the voltage difference occurs. If the difference mainly appears near the end of charging and becomes noticeably larger only while the charger is connected, this usually points to normal dynamic behavior. However, if the difference remains consistently large in the middle SOC range, such as between 30% and 80%, even while the battery is resting, it deserves closer attention.
Second: Check whether the difference decreases again. If the cell voltage difference clearly becomes smaller after disconnecting the charger and allowing the battery to rest, the difference is likely caused mainly by temporary charging effects. If it remains large even after 1–2 hours of rest and this behavior is repeatedly observed, further inspection is recommended.
Third: Observe whether there are any actual performance problems. Cell voltage difference alone is only a reference value. If the battery operates normally in practical use, there is usually no reason to overreact. However, if symptoms such as incomplete charging, premature charging termination, sudden shutdown during discharge, frequent BMS protection events, or the same individual cell repeatedly showing a large deviation occur at the same time, the issue should no longer be considered a normal voltage fluctuation.
4. How to Interpret APP Data Correctly
A LiFePO₄ battery with a Bluetooth app provides a large amount of internal battery data. This information can be extremely useful—as long as the values are interpreted correctly. When checking the app, pay particular attention to the following points:
Is the battery currently charging? If the battery is charging, especially at a relatively high charging current, the cell voltages shown are dynamic values under charging conditions. In this state, cell voltage difference should only be treated as a reference and should not be used alone for a final assessment.
Is the SOC already close to full? In the upper charging range, the voltage curve of LiFePO₄ becomes steeper. This is exactly the stage at which differences between cell voltages become more visible.
Does the voltage difference decrease after the battery rests? This is one of the most important points to check. If the difference remains large for an extended period after resting, the readings should be recorded and investigated further.
Is the same cell always noticeably higher or lower? If the same cell repeatedly deviates clearly from the others across multiple charging, discharging, and resting cycles, it should be monitored more closely.
5. How Can the Cells Be Balanced Again?
If the cell voltage difference occurs mainly near the end of charging and the battery shows no clear problems during normal operation, an appropriate charging method can help give the BMS enough opportunity to balance the cells.
Use a suitable charger: A compatible LiFePO₄ charger can reliably bring the battery into the correct voltage range where the BMS is able to perform balancing. If the charging voltage is too low, the battery may never reach the upper balancing range. The battery may still function normally, but small differences between the cells can gradually accumulate over time.
Fully charge the battery occasionally: If the battery is operated for long periods only within the middle or lower SOC range, the BMS may not have enough opportunities to balance the cells. This type of usage is particularly common in solar systems and RV systems.
Leave the charger connected for a while after reaching full charge: If the cell voltage difference mainly becomes visible near the end of charging, leaving the battery connected to a suitable charger for some additional time after it reaches full charge can give the BMS more time to gradually balance the cells.
Let the battery rest for 30 minutes after charging and check again: This helps reduce the effects of charging current, surface charge, and temporary polarization. The individual cell voltages can then settle closer to a stable resting condition.
It is important to remember that balancing does not happen immediately. It requires both the correct voltage range and sufficient time.
6. Summary
If the cell voltage difference in a LiFePO₄ battery increases during charging, this is often normal—especially when the battery is close to full charge. The reason is that cell voltage rises much more rapidly in the upper charging range, making small differences in capacity or internal resistance more visible.
For users, the most important steps are to fully charge the battery periodically, give the BMS enough time to perform cell balancing, and correctly interpret the app data. Doing so helps ensure stable and reliable long-term operation of the battery system.
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