Introduction
Four 12.8V 100Ah LiFePO4 batteries are connected in series to create a nominal 51.2V battery bank.
During charging, the customer measures:
- Battery 1: 13.7V
- Battery 2: 13.8V
- Battery 3: 13.6V
- Battery 4: 14.4V
The total battery-bank voltage may still appear reasonable.
However, one battery is clearly reaching the upper end of its charging range much earlier than the others.
A common question is:
“They are connected in series, so the same current flows through every battery. Why doesn’t every battery charge at the same rate?”
The answer is important.
Series-connected batteries carry the same current, but they do not necessarily have the same usable capacity, state of charge, internal resistance or BMS behaviour.
As a result, one battery can reach its upper voltage limit long before the others.
If this repeatedly happens, the usable capacity of the entire series string can be reduced.
1. What Happens Electrically in a Series Connection?
When batteries are connected in series:
- Battery voltages add together
- Ah capacity remains approximately the same
- The same current passes through every battery
For example:
4 × 12.8V 100Ah
creates approximately:
51.2V 100Ah
Nominal energy:
5.12kWh
If the charger supplies 30A:
- Battery 1 receives 30A
- Battery 2 receives 30A
- Battery 3 receives 30A
- Battery 4 receives 30A
The current is identical.
However, identical current does not guarantee identical SOC movement.
2. Why the Same Current Can Produce Different Battery Voltages
Imagine two 100Ah batteries.
Battery A
Actual usable capacity: 100Ah
Battery B
Actual usable capacity: 80Ah
Both are charged at:
20A
Battery B requires less energy to move from low SOC to its upper charging condition.
It can therefore reach its upper voltage first.
In a series string, the charger cannot independently reduce current only for Battery B.
The same charging current continues flowing through the complete string until:
- The charger reaches its voltage target, or
- One BMS limits or interrupts charging
This makes battery matching particularly important in series systems.
3. Cause #1: Different Starting SOC
This is one of the most common causes.
Suppose four batteries are connected in series at:
- Battery 1: 50%
- Battery 2: 53%
- Battery 3: 49%
- Battery 4: 80%
Battery 4 begins with significantly more stored energy.
During charging, every battery receives the same amp-hours.
Battery 4 therefore reaches full charge much earlier.
Its voltage begins rising sharply while the others still require charging.
The total string voltage may hide this problem.
4. Why Total String Voltage Can Be Misleading
Consider a nominal 51.2V series bank.
The charger sees only the total voltage.
Suppose:
- Battery 1: 13.4V
- Battery 2: 13.5V
- Battery 3: 13.4V
- Battery 4: 14.5V
Total:
54.8V
From the charger side, 54.8V may not look unusual.
But Battery 4 is operating much closer to its upper limit than the other three.
This is why series-connected modular batteries should sometimes be checked individually rather than evaluating only total pack voltage.
5. Cause #2: Different Actual Capacity
Two batteries can have the same 100Ah label but different real-world capacity.
Possible reasons include:
- Different age
- Different cycle history
- Storage conditions
- Cell variation
- Previous over-discharge
- Temperature history
For example:
Battery 1
Actual capacity: 98Ah
Battery 2
Actual capacity: 97Ah
Battery 3
Actual capacity: 94Ah
Battery 4
Actual capacity: 78Ah
Battery 4 will typically move through its usable SOC range faster.
During charging it may become full first.
During discharge it may also reach low-voltage protection first.
This creates a characteristic pattern:
First battery to become full → also first battery to become empty.
6. Cause #3: One Battery Has Internal Cell Imbalance
Each 12.8V LiFePO4 battery normally contains multiple cells in series internally.
The battery may show:
13.6V total
while its individual cell voltages are not equal.
For example:
- Cell 1: 3.37V
- Cell 2: 3.38V
- Cell 3: 3.36V
- Cell 4: 3.49V
During further charging, Cell 4 may reach the BMS upper-voltage threshold before the other cells.
The BMS then reduces or interrupts charging for the entire 12.8V battery.
Because that battery is itself part of a larger series string, the behaviour can affect the entire battery bank.
7. Battery-Level Imbalance and Cell-Level Imbalance Are Different
This distinction is extremely important.
Cell-Level Imbalance
Occurs between cells inside one battery.
Example:
A 12.8V battery contains four internal LiFePO4 cells with different voltages.
The internal BMS may provide cell balancing.
Battery-Level Imbalance
Occurs between complete 12.8V battery modules connected in series.
Example:
- Battery A: 100% SOC
- Battery B: 80%
- Battery C: 75%
- Battery D: 78%
The BMS inside Battery A generally cannot transfer energy into Batteries B, C or D.
Therefore:
Internal BMS cell balancing does not automatically balance complete batteries connected externally in series.
This is a common misunderstanding.
8. Why a BMS Cannot Automatically Fix Every Series Imbalance
A battery’s internal BMS usually monitors:
- Internal cell voltage
- Battery current
- Temperature
- Overcharge
- Over-discharge
- Overcurrent
It may also balance the internal cells.
But when four independent 12.8V batteries are connected externally in series, each battery has its own separate BMS.
There may be no centralized controller comparing the SOC of all four complete modules.
As a result, the batteries can gradually move apart in SOC even though every individual battery remains internally protected.
9. Cause #4: Different Internal Resistance
Battery resistance affects how voltage behaves under current.
During charging, a battery with higher internal resistance can show a larger voltage rise.
A simplified relationship is:
Terminal voltage = internal battery voltage + current-related voltage rise
Therefore, under the same charging current, one higher-resistance battery may appear to reach a higher voltage sooner.
This becomes more noticeable at high charging current.
If the charging current is reduced and the voltage difference becomes much smaller, internal resistance may be contributing to the problem.
10. Why the Problem May Only Appear at High Charging Current
Suppose the bank charges at:
10A
Individual battery voltages remain relatively close.
At:
50A
one battery suddenly rises much faster.
Higher current increases:
- Voltage polarization
- Internal voltage drop/rise
- Connection losses
- Heating
A marginal battery can therefore look acceptable at low current but show significant imbalance at higher current.
11. Cause #5: Temperature Difference
Battery temperature affects:
- Internal resistance
- Charge acceptance
- BMS protection
- Cell behaviour
Imagine four batteries installed vertically.
The upper battery is close to:
- An inverter
- A hot roof
- A ventilation outlet
Its temperature may differ significantly from the lower batteries.
Temperature variation can contribute to different charging behaviour.
In a series bank, batteries should ideally operate in a similar thermal environment.
12. What Happens When One Battery Reaches BMS Overvoltage Protection?
Suppose Battery 4 reaches its internal high-cell voltage threshold.
Its BMS may interrupt charging.
Because all four batteries are connected in series, the charging current path through the complete string is interrupted.
The charger may then report:
- Battery disconnected
- Charging stopped
- Battery overvoltage
- DC voltage abnormal
After resting, Battery 4 voltage falls slightly.
Its BMS recovers.
Charging begins again.
Battery 4 quickly reaches high voltage again.
The system may cycle:
charge → protection → recover → charge → protection
This repeated behaviour is a strong sign that the series bank requires investigation.
13. Why the Charger May Never Fully Charge the Other Batteries
Assume:
- Battery A reaches full first
- Batteries B, C and D remain partially charged
Every time Battery A reaches protection, charging stops.
The other batteries therefore cannot receive enough additional energy.
Over time:
Battery A:
100%
Other batteries:
70–85%
The usable energy of the complete string becomes limited.
14. The Same Problem Appears During Discharge
Now the system begins discharging.
If Battery C has the smallest real capacity, it may reach its lower cell-voltage threshold first.
Its BMS disconnects.
Because the batteries are in series, the complete system stops providing current.
The other batteries might still show:
- 30%
- 40%
- 45%
SOC.
Yet the inverter shuts down.
This is why one weak or imbalanced module can determine the usable capacity of an entire series string.
15. Why a Series Bank Can Have Less Usable Capacity Than Expected
Four 12.8V 100Ah batteries theoretically provide:
51.2V 100Ah
But suppose one battery has only:
75Ah actual capacity
The complete series bank may behave more like a bank limited by that 75Ah module.
The other batteries cannot contribute their remaining capacity once the weakest battery reaches its BMS limit and interrupts the series path.
16. How to Diagnose Which Battery Is Causing the Problem
During charging, record each individual battery voltage.
For example:
| Battery | Start | Mid-Charge | Near Full |
|---|---|---|---|
| Battery 1 | 13.1V | 13.3V | 13.6V |
| Battery 2 | 13.1V | 13.3V | 13.6V |
| Battery 3 | 13.0V | 13.3V | 13.5V |
| Battery 4 | 13.2V | 13.6V | 14.4V |
Battery 4 clearly deserves further investigation.
Also check its:
- Maximum cell voltage
- Minimum cell voltage
- Cell delta
- Temperature
- BMS alarms
- Actual capacity
17. Resting Voltage Is Also Useful
Immediately after charging, voltage includes dynamic effects.
Allow the system to rest according to the manufacturer’s procedure and compare battery voltages again.
If the difference becomes much smaller after resting:
Internal resistance or charging polarization may be significant.
If the difference remains large:
SOC or actual capacity mismatch may be more important.
18. Can You Charge Each Battery Separately to Rebalance the Series Bank?
In some manufacturer-approved systems, individual batteries may be brought to a similar SOC before reconnecting the series bank.
However, this should be performed using:
- Correct LiFePO4 charger
- Correct battery voltage
- Approved charging procedure
- Correct isolation procedure
Do not connect an individual charger to one battery while the complete live series bank remains connected unless the system is explicitly designed for that procedure.
Incorrect charging arrangements can create dangerous electrical conditions.
19. Do Not Simply Increase the Total Charge Voltage
A common incorrect response is:
“The other three batteries are not full, so I will increase the charger voltage.”
This can make the problem worse.
The high battery reaches an even higher voltage while the lower batteries still lag behind.
The correct solution is to identify why the individual battery voltages differ.
20. Do Not Bypass One Battery’s BMS
Another dangerous approach is attempting to prevent the high battery from disconnecting by bypassing its BMS.
The BMS may be stopping charging because:
- One cell is already too high
- Battery temperature is excessive
- Current exceeds limits
Bypassing protection can expose the battery to unsafe operating conditions.
Never defeat BMS protection to force a series bank to continue charging.
21. Why Native 51.2V Batteries Can Simplify the System
Instead of using:
4 × separate 12.8V batteries in series
many energy-storage systems use:
1 × native 51.2V battery
A native 51.2V battery commonly has one integrated BMS monitoring the complete internal series cell string.
Advantages may include:
- Centralized cell monitoring
- Simpler SOC calculation
- Fewer external series connections
- Easier inverter communication
- Less battery-level imbalance between independent modules
For larger ESS installations, this architecture is often easier to manage.
22. When External Series Connection Still Makes Sense
Series-connected modular batteries remain useful in applications such as:
- Small off-grid systems
- Marine systems
- RV systems
- Portable systems
- Legacy 24V or 48V equipment
provided the battery model explicitly supports series connection.
The important point is to use the system within the manufacturer’s approved series quantity and operating limits.
23. Troubleshooting Checklist
If one battery repeatedly reaches high voltage first:
- Confirm the battery model supports series operation.
- Compare starting SOC.
- Measure each battery voltage while charging.
- Check individual cell voltages.
- Compare battery temperatures.
- Review BMS alarm history.
- Reduce charging current for diagnostic comparison.
- Check actual usable capacity.
- Inspect cable and terminal resistance.
- Rebalance only according to the manufacturer’s approved procedure.
Frequently Asked Questions
Why does one LiFePO4 battery in series reach full charge first?
It may have a higher starting SOC, lower usable capacity, higher internal resistance or internal cell imbalance.
Do batteries connected in series receive the same charging current?
Yes. The same current flows through all batteries in a series string.
Why do they have different voltages if current is the same?
Because their SOC, internal resistance, actual capacity and cell condition can differ.
Can the BMS automatically balance separate batteries connected in series?
Usually not. A battery’s BMS primarily manages cells inside that battery.
Can one battery shut down the entire series string?
Yes. If one BMS disconnects, the current path through the complete series string can be interrupted.
Should I increase the charger voltage to make the low batteries full?
No. This can push the already-high battery into overvoltage protection.
Conclusion
In a series-connected LiFePO4 battery bank, the same current flows through every battery, but the batteries do not necessarily remain at the same SOC or voltage.
One module may reach its upper voltage limit first because of:
- Different initial SOC
- Reduced capacity
- Higher internal resistance
- Internal cell imbalance
- Temperature difference
- Battery aging
When this happens repeatedly, that single battery can limit the usable capacity of the entire series string.
The correct approach is to monitor individual battery voltage and cell-level data, not only total system voltage.
For applications that require larger energy-storage systems, native 48V/51.2V LiFePO4 batteries can also simplify BMS management and reduce the complexity associated with externally series-connected low-voltage battery modules.
HIZN Lithium supplies LiFePO4 batteries for 12V, 24V, 48V and 51.2V energy-storage applications, with configurable BMS, communication and OEM solutions for solar, UPS, telecom and off-grid systems.