Why Do Only One or Two LiFePO4 Batteries Take Most of the Charging Current in a Parallel Bank?

Introduction

Four LiFePO4 batteries are connected in parallel.

The inverter is charging the bank at:

100A

The customer expects approximately:

25A per battery

Instead, the BMS screens show:

Battery 1: +52A
Battery 2: +39A
Battery 3: +8A
Battery 4: +1A

The total current is approximately correct.

But why are only one or two batteries accepting most of the charge?

This is an increasingly common question in residential and commercial solar-storage systems.

Parallel batteries share the same DC bus voltage, but they do not automatically divide charging current equally.

Charging current is influenced by:

Battery SOC
Battery voltage
Internal resistance
Cable resistance
BMS charge-current limit
Cell voltage
Temperature
Battery age
Communication control

Understanding the cause is important because uneven charging can be either:

Completely normal during certain stages of charging, or
Evidence of an installation or battery problem.

1. Charging Current Does Not Have to Be Perfectly Equal

Suppose four identical batteries are at similar SOC.

Total charging current:

80A

Possible normal values might be:

Perfect 20A / 20A / 20A / 20A sharing is not required.

Small differences occur because no two batteries and branches have exactly the same resistance.

The concern begins when the imbalance is large and persistent.

For example:

through most of the charging cycle deserves investigation.

2. Cause #1: Different State of Charge

This is often completely normal.

Suppose:

Battery A: 45% SOC
Battery B: 48%
Battery C: 92%
Battery D: 100%

When charging begins, Batteries A and B may accept much more current.

Battery D may accept almost none.

Therefore:

A: +38A
B: +35A
C: +7A
D: 0A

does not automatically indicate a fault.

The batteries simply do not need the same amount of energy at that moment.

3. Why Equal Voltage Does Not Mean Equal SOC

Parallel batteries have similar bus voltage.

But LiFePO4 chemistry has a relatively flat voltage curve.

Two batteries can therefore operate at almost the same terminal voltage while their BMS SOC estimates differ.

In addition, each BMS independently calculates SOC.

Charging-current behaviour should therefore be evaluated together with:

SOC
Cell voltage
Battery temperature
Previous charge/discharge history

4. Cause #2: One Battery Has Lower Branch Resistance

If all batteries are at similar SOC but one takes much more charging current, branch resistance becomes an important suspect.

Current tends to favor the lower-resistance path.

Branch resistance includes:

Positive cable
Negative cable
Cable lugs
Breaker
Fuse
Battery terminal
Busbar connection
BMS resistance

One battery with shorter cables may accept more charging current.

5. Example of Cable-Length Imbalance

Assume all batteries use 35mm² copper cable.

Battery 1

0.5m positive + 0.5m negative

Battery 2

0.8m + 0.8m

Battery 3

1.5m + 1.5m

Battery 4

2.0m + 2.0m

Battery 1 has the lowest external branch resistance.

When the charger applies bus voltage, Battery 1 can accept more current.

This may cause it to:

Charge faster
Reach 100% sooner
Experience more cycle throughput

6. Why the Problem May Be Worse With High Charging Current

Suppose the charger supplies only:

20A total

Current imbalance may be small.

At:

200A total

the electrical differences become much more visible.

Higher current creates greater:

Cable voltage drop
Terminal voltage drop
Heating
BMS voltage difference

A bank that appears well balanced at low current may become strongly unbalanced during fast charging.

7. Cause #3: One Battery Has a Lower Charge-Current Limit

Consider four batteries.

Three BMS units allow:

100A charge current

Battery 4 currently allows only:

10A

Why?

Possible reasons include:

High SOC
High cell voltage
Low temperature
High temperature
BMS protection
Firmware setting

The inverter may still send a high total charging current to the bank.

The other batteries absorb most of it.

This produces an apparently severe charging imbalance even though the BMS is intentionally limiting Battery 4.

8. Check Allowable Charge Current, Not Only Actual Current

Some BMS systems report values such as:

CCL — Charge Current Limit
DCL — Discharge Current Limit

Example:

Battery	Actual Charge	Allowed Charge
A	42A	100A
B	39A	100A
C	17A	20A
D	2A	5A

Now the current distribution makes much more sense.

C and D are being restricted.

The question becomes:

Why is their allowed charge current low?

9. Cause #4: One Cell Is Reaching High Voltage Early

A battery may have 16 LiFePO4 cells internally.

Suppose:

15 cells:

approximately 3.42V

one cell:

3.57V

The BMS sees the high cell approaching its upper limit.

It may:

Reduce charge-current request
Disable charging
Begin balancing
Temporarily disconnect

The battery therefore accepts much less current than its parallel neighbors.

This can happen even if its total battery voltage looks normal.

10. Why This Often Happens Near 100% SOC

Cell-voltage differences become more visible near the top of charge.

From approximately middle SOC, all batteries may share charging current reasonably well.

Near full charge:

Battery A: 20A
Battery B: 18A
Battery C: 5A
Battery D: 0A

This may simply mean C and D reached their upper charge region earlier.

Uneven current near the final charging stage is therefore less concerning than large imbalance throughout the entire cycle.

11. Charge-Current Imbalance at 20% SOC Is More Interesting

Suppose all four batteries are:

20–30% SOC

and charging current is:

A: 60A
B: 25A
C: 10A
D: 5A

There is no high-cell warning.

Temperatures are similar.

Now the imbalance deserves more attention.

Likely areas include:

Branch resistance
BMS state
Battery connection
Capacity mismatch
Communication configuration

12. Cause #5: One Battery Is Colder Than the Others

LiFePO4 charging can be limited at low temperature.

Suppose:

Battery A: 22°C
Battery B: 23°C
Battery C: 21°C
Battery D: 2°C

Battery D’s BMS may significantly reduce or completely block charging.

The other batteries then receive nearly all of the charger output.

This situation is common in outdoor installations where batteries experience uneven temperatures.

13. Different Temperature Can Also Occur Indoors

Possible reasons include:

One battery beside an inverter
One close to an exterior wall
Uneven cabinet airflow
Cooling fan location
Sun exposure

For multi-module ESS installations, try to maintain a similar thermal environment for every battery.

14. Cause #6: Different Actual Capacity

Consider:

Battery A

100Ah actual capacity

Battery B

100Ah

Battery C

80Ah

Battery D

75Ah

During a complete charging cycle, C and D may reach full earlier because they require less energy.

Once they approach their upper limit, charge current shifts toward A and B.

The customer then observes that:

“Only two batteries are charging.”

In reality, the other two may already be close to full.

15. Why Aging Can Change Charging Distribution

Older batteries may have:

Lower actual capacity
Higher internal resistance
Different SOC calibration

A mixed-age battery bank can therefore show changing charging-current distribution throughout the cycle.

This is another reason why adding new batteries to a heavily aged bank should be evaluated carefully.

16. Cause #7: One Battery Has High Connection Resistance

This creates the opposite effect from the lower-resistance battery.

A battery with:

Loose terminal
Poor crimp
Bad breaker
Corroded connection

may take very little charging current.

Example:

Total charge:

120A

Battery A: 39A
B: 38A
C: 40A
D: 3A

Battery D has similar SOC to the others.

Its branch should be inspected.

17. High-Resistance Connections Can Become Hot During Charging

Even though Battery D takes less current, a poor terminal can still heat locally.

Use appropriate inspection methods to check:

Battery terminal
Breaker terminal
Cable lug
Fuse holder
Busbar bolt

A warm or hot connection compared with equivalent branches is a valuable diagnostic clue.

18. Cause #8: The BMS Charge Path Is Disabled

Some BMS units independently control charging and discharging.

Battery D may:

Discharge normally at night
Show 0A during daytime charging

Possible reason:

Charge path disabled

while:

Discharge path enabled

This may result from:

Cell overvoltage
Temperature
Charge-current protection
BMS configuration

Do not conclude that the battery is disconnected completely.

Check charge and discharge status separately.

19. Cause #9: Master BMS or Inverter Communication

In a closed-loop ESS, the battery bank may communicate with the inverter through CAN or RS485.

The master BMS can report a total allowable charging current.

If one battery:

Is not recognized
Has duplicate address
Has different firmware
Is missing from the parallel communication chain

the system may not control current as intended.

Possible symptoms include:

Incorrect total capacity
Incorrect charge-current limit
One battery behaving differently
Communication alarms

Power cables alone are not enough for a communication-controlled battery system.

20. Why Total Charging Current May Be Correct Even When Branch Current Is Wrong

The inverter might show:

100A charge current

and therefore appear to be working perfectly.

But individual branches could be:

The inverter normally sees the total battery-bank current.

It may not know how that current divides internally unless the battery communication architecture provides individual-module information.

This is why commissioning should include branch-current checks.

21. Multiple Charging Sources Make Diagnosis More Difficult

A battery bank may simultaneously receive charging current from:

Solar MPPT
Grid charger
Generator charger
Separate DC charger

The total charging current is the sum of these sources.

If the customer says:

“My inverter only charges at 80A,”

another MPPT may simultaneously contribute 60A.

The battery bank is actually receiving:

140A

The distribution between modules may therefore be more demanding than expected.

22. Why One Battery Can Reach 100% and Then Show 0A

Example:

At 90% SOC:

A: 28A
B: 27A
C: 25A
D: 20A

Later:

Battery D reaches full condition.

Now:

A: 35A
B: 34A
C: 31A
D: 0A

The charging source is still delivering approximately 100A.

Current simply redistributes to the batteries that continue accepting charge.

This can be normal.

23. Why the Remaining Batteries Can Suddenly Receive Too Much Current

Current redistribution can also create a problem.

Suppose:

Four batteries initially share:

160A

approximately 40A each.

Two batteries reach their charge limit and stop charging.

The remaining two may now receive:

approximately 80A each.

If their recommended charge current is only:

50A

the charging source should reduce total current.

A properly integrated closed-loop system may do this automatically.

In an open-loop voltage-controlled system, inverter charge-current settings become particularly important.

24. Do Not Size Charge Current Only by Number of Batteries

Example:

Four batteries × 100A maximum charge rating

does not automatically mean:

400A should always be configured.

Why?

At high SOC, only one or two batteries may continue accepting significant current.

The system must manage current safely when battery availability changes.

Follow the approved bank-level charge-current recommendation.

25. How to Diagnose Uneven Charging

Step 1: Record SOC

Is one battery already much fuller?

Step 2: Record Actual Charge Current

Check each battery.

Step 3: Check BMS Charge Limit

If available.

Step 4: Compare Highest Cell Voltage

One high cell can explain reduced charging.

Step 5: Compare Temperature

Look for charge derating.

Step 6: Inspect Cable Length and Size

Confirm consistent branch design.

Step 7: Inspect Breakers and Terminals

Check for resistance or heating.

Step 8: Verify Communication

Battery quantity and addresses should be correct.

26. Perform the Test at Different SOC Levels

This is important.

Record branch current at:

Low SOC

approximately 20–30%

Mid SOC

approximately 50–60%

High SOC

approximately 90%+

If current is fairly balanced at low and mid SOC but becomes uneven near full:

This may be normal charging behaviour or cell balancing.

If one battery carries almost no current throughout the entire charge cycle:

Further investigation is justified.

27. Diagnostic Table

Symptom	Possible Cause
Full battery takes 0A	Normal
Low-SOC battery takes little current	Branch/BMS issue
Imbalance only near 100%	Cell voltage/BMS charging limit
One battery gets most current at all SOC levels	Lower branch resistance
One battery always takes very little current	High branch resistance
Battery charges little at low temperature	Temperature protection
Charge current changes after communication reset	BMS/inverter communication
New battery takes more current	Lower internal resistance or lower SOC

28. Should You Try to Force Equal Charging Current?

No.

Do not attempt to artificially balance current by:

Installing intentionally undersized cables
Adding random resistance
Changing BMS settings without approval
Bypassing protection

The goal is not to force identical numbers.

The goal is to ensure:

Every battery participates
No battery is overloaded
SOC eventually converges appropriately
No branch has abnormal resistance
BMS limits are respected

29. What Installers Should Record at Commissioning

During the first charging cycle, record:

Battery SOC
Battery voltage
Branch current
Maximum cell voltage
Minimum cell voltage
Charge-current limit
Temperature
Alarm status

Do this at several SOC levels.

This provides valuable baseline data for future troubleshooting.

Frequently Asked Questions

Why does one parallel LiFePO4 battery take most of the charging current?

It may have lower SOC, lower branch resistance, lower internal resistance or different BMS charging limits.

Is uneven charging current normal?

Some difference is normal, especially near full charge. Large persistent imbalance throughout the cycle should be investigated.

Why does one battery show 0A while the others are charging?

It may already be full, have charge protection active or have an open/high-resistance branch.

Can cable length affect charging current?

Yes. Branch resistance affects both charging and discharging current sharing.

Why does charging become uneven only above 90% SOC?

Some batteries reach their high-cell or balancing region earlier and begin limiting charge current.

Should every battery receive exactly 25A if total current is 100A and there are four batteries?

No. Perfect equal sharing is not required.

Conclusion

Parallel LiFePO4 batteries do not automatically divide charging current equally.

Charging-current distribution changes according to:

SOC
Branch resistance
Cell voltage
BMS limits
Temperature
Battery capacity
Communication status

A major diagnostic distinction is:

Does the imbalance happen only near full charge, or throughout the complete charging cycle?

If batteries share current reasonably at low and middle SOC but diverge near 100%, the behaviour may be related to normal charge limiting or cell balancing.

If one battery takes little or no current from low SOC onward, investigate its electrical branch, BMS and internal condition.

For distributors and installers, recording branch charging current during commissioning can prevent many later disputes about whether a battery is “not charging.”

HIZN Lithium supplies modular LiFePO4 energy-storage batteries with scalable parallel configurations, BMS protection and CAN/RS485 communication for solar, UPS, telecom and commercial energy-storage systems.