Why Does One LiFePO4 Battery Show a Lower Voltage Than the Others in a Parallel Bank?

Introduction

A customer has three 51.2V LiFePO4 batteries connected in parallel.

The monitoring screen shows:

Battery 1: 52.6V
Battery 2: 52.5V
Battery 3: 50.9V

The immediate question is:

“If these batteries are connected in parallel, why is one battery almost 2V lower than the others?”

This is an important question because a genuine parallel electrical connection normally forces the terminal voltages of all connected batteries to be very close.

If one battery shows a substantially different voltage, several possibilities should be investigated:

The battery is not actually connected to the common DC bus
Its branch breaker or fuse is open
Its BMS has disconnected the main power path
The displayed voltage is measured internally rather than at the bus
There is excessive resistance in its cable or connection
The BMS voltage measurement is inaccurate
The battery is switching in and out of protection
The voltage readings were taken under different operating conditions

A large voltage difference should therefore not immediately be interpreted as “one battery has a lower SOC.”

First determine whether the battery is electrically part of the parallel bank at that moment.

1. What Should Happen to Voltage in a Parallel Battery Bank?

In a parallel connection:

Positive terminals share the same positive bus
Negative terminals share the same negative bus

Therefore, the terminal voltage across each active battery branch should be approximately the same.

Consider:

Battery A

52.40V

Battery B

52.37V

Battery C

52.42V

Small differences can occur because of:

Meter accuracy
BMS sensor calibration
Cable voltage drop
Measurement timing

This is generally understandable.

But readings such as:

Battery A: 52.5V
Battery B: 52.4V
Battery C: 49.8V

require further investigation.

2. Important Distinction: BMS Voltage vs Bus Voltage

The number displayed in the battery app is usually measured somewhere inside the battery.

It may represent:

Cell-stack voltage
BMS pack voltage
Voltage before the main contactor
Voltage on the battery side of MOSFETs

It may not necessarily represent the voltage at the external DC bus.

This matters when the BMS main power path is open.

For example:

Inside Battery

50.8V

Common Bus

52.6V

If the battery’s main discharge/charge path is disconnected, these two voltages can remain different.

The battery can still:

Power its BMS
Show SOC
Communicate by CAN
Communicate by RS485
Display Bluetooth data

while being electrically isolated from the parallel bus.

3. Cause #1: The Branch Breaker Is Open

This is one of the simplest explanations.

Imagine three batteries:

Battery 1

Breaker ON

Battery 2

Breaker ON

Battery 3

Breaker OFF

Batteries 1 and 2 are truly paralleled.

Battery 3 is not.

If Battery 3 was previously discharged more deeply, it might show:

50.5V

while the active bank is:

52.4V

The customer may still see all three batteries in the communication system and assume they are electrically connected.

This is why physical breaker position should always be checked.

4. A Breaker Can Look ON but Still Cause a Problem

Possible breaker-related problems include:

Internal contact damage
Tripped mechanism not obvious externally
Loose terminal
Poor cable lug
Incorrectly installed breaker
High contact resistance

Under no load, the voltage may look normal.

Under load, the voltage drop across the damaged breaker may increase dramatically.

This can make one battery appear to operate at a different voltage.

5. Cause #2: The BMS Has Opened Its Main Power Path

A LiFePO4 BMS may stop charging or discharging when it detects:

Cell overvoltage
Cell undervoltage
Overcurrent
Short circuit
High temperature
Low charging temperature
Internal hardware fault

When this happens, the battery may become electrically isolated from the common bus.

Its internal voltage can then move independently from the other batteries.

This can explain a large voltage difference.

6. Example: Low-Voltage Protection

Three batteries are discharging together.

Battery 3 has lower actual capacity.

It reaches:

low cell-voltage protection

first.

The BMS disconnects Battery 3.

Now:

Common bus

approximately 51.5V

Battery 3 internal pack

perhaps 49.5–50.0V

Batteries 1 and 2 continue supporting the inverter.

If the monitoring system still displays Battery 3’s internal voltage, the user sees a clear voltage mismatch.

7. Why Battery 3 May Reconnect Later

After the inverter load decreases:

Battery internal voltage recovers
Low-voltage condition clears
BMS recovery criteria are satisfied

The battery may reconnect.

Once it reconnects, its terminal voltage is again pulled close to the common bus voltage.

The customer may therefore report:

“Sometimes the voltages are the same and sometimes one is much lower.”

That intermittent behaviour can strongly suggest a protection or connection event.

8. Cause #3: High Cable Resistance

In a healthy parallel bank, cable voltage drop creates smaller differences during current flow.

Suppose Battery 1 carries:

60A

through a low-resistance branch.

Battery 2 carries:

20A

through a higher-resistance branch.

The voltage measured:

Directly at the battery terminal
At the common busbar

may differ.

The basic relationship is:

Voltage Drop = Current × Resistance

Even a relatively small resistance becomes important at high DC current.

9. Why a Loose Terminal Can Create a Large Voltage Drop

Assume an abnormal connection resistance of:

0.01Ω

At:

50A

the voltage drop becomes:

0.5V

Power dissipated at the connection becomes:

25W

That heat is concentrated at a small terminal or lug.

Possible symptoms include:

Voltage difference
Hot terminal
Discolored lug
Reduced battery current
BMS low-voltage event under load
Intermittent shutdown

A connection that is only slightly loose can therefore create a significant high-current problem.

10. The Difference May Disappear When the Load Is Removed

This is a useful diagnostic clue.

Under 5kW Load

Battery 3 appears 0.6V lower than the common bus.

With Load Removed

Difference falls to 0.05V.

This strongly suggests an I × R voltage-drop problem.

Possible areas include:

Battery cable
Breaker
Fuse holder
Cable lug
Busbar connection

rather than a large true SOC difference.

11. Cause #4: One Battery Is Not Charging

Suppose Battery 3’s charge path is disabled.

During daytime charging:

Battery 1 charges
Battery 2 charges
Battery 3 accepts no current

Over several hours, Batteries 1 and 2 reach a higher SOC.

Battery 3 remains lower.

If Battery 3 is electrically isolated by its BMS during this period, its internal voltage can remain lower.

Possible reasons include:

High-cell protection
Low-temperature charge protection
Charge MOSFET disabled
Contactor state
BMS setting

Check Charge Enable and Discharge Enable separately where the BMS provides these values.

12. Cause #5: BMS Voltage Calibration Error

Suppose a calibrated multimeter measures:

Battery A: 52.36V
Battery B: 52.35V
Battery C: 52.35V

But BMS displays:

A: 52.4V
B: 52.4V
C: 51.8V

Battery C may not actually have lower terminal voltage.

Its voltage sensing circuit or calibration may be inaccurate.

This is particularly likely if:

Battery current sharing is normal
Bus voltage is normal
Physical voltage measurement is normal
Only the software value is different

Do not attempt voltage calibration unless the manufacturer provides an approved procedure.

13. Check With One Measuring Instrument

When comparing batteries, use the same calibrated meter if possible.

Using:

Meter A on Battery 1
Meter B on Battery 2
BMS display on Battery 3

introduces multiple measurement errors.

For troubleshooting, measure:

Battery 1 terminals
Battery 2 terminals
Battery 3 terminals
Positive/negative busbar

with the same instrument.

14. Cause #6: Different Measurement Timing

LiFePO4 voltage changes immediately when load changes.

For example:

Under Load

51.4V

10 Seconds After Load Removal

52.0V

After Longer Rest

52.2V

If Battery 1 is measured during load while Battery 2 is measured after load removal, the results cannot be compared directly.

Record all battery voltages under the same system condition.

15. Why SOC and Voltage May Seem to Contradict Each Other

A customer might report:

Battery A

SOC 65%
52.3V

Battery B

SOC 65%
52.2V

Battery C

SOC 80%
50.8V

This looks impossible.

But Battery C’s SOC may be:

Incorrectly calibrated
Old/stale SOC data
Recorded before a protection event

while voltage is measured internally after BMS isolation.

SOC should never be used as the sole evidence that a battery is electrically connected.

16. Cause #7: Battery Communication Data Is Delayed

Some monitoring systems update different values at different intervals.

A screen might display:

Current SOC
Previous voltage
Current alarm
Delayed current

temporarily.

If values look electrically impossible, refresh the data and compare with direct measurement before drawing conclusions.

17. What Happens If a Lower-Voltage Battery Suddenly Reconnects?

This deserves caution.

Suppose:

Active bank

53.5V

Isolated Battery 3

50.5V

Difference:

If Battery 3 suddenly reconnects directly to the bus, a potentially high equalization current may flow into it.

The current depends on:

Battery internal resistance
Cable resistance
BMS resistance
Breaker resistance

Possible results include:

BMS overcurrent protection
Breaker trip
Large current spike
Contactor stress

This is why the root cause should be corrected before repeatedly forcing an isolated low-voltage battery back onto the live bus.

18. Do Not Repeatedly Reset the BMS Without Checking Voltage

A common field reaction is:

“Battery is offline. Turn it off and on again.”

If the battery voltage is significantly different from the active bus, repeated manual reconnection may recreate a high equalization current every time.

Instead:

Check battery voltage
Check bus voltage
Read the protection reason
Identify why the battery disconnected

before reconnection.

19. Why One Battery Can Be 0.5V Lower Under Heavy Load but Normal at Rest

This pattern often points to branch resistance.

For example:

No Load

A: 52.30V
B: 52.29V
C: 52.28V

Heavy Load

A: 51.60V
B: 51.55V
C: 51.00V

Battery C’s additional voltage sag may be caused by:

Higher internal resistance
Poor terminal connection
Cable resistance
Weak cells

Now compare its actual current.

If Battery C Current Is High

Internal battery voltage sag may be significant.

If Battery C Current Is Low

External branch resistance may be limiting its contribution.

20. Cell-Level Data Can Identify an Internal Battery Problem

If the external connections are normal, inspect:

Highest cell voltage
Lowest cell voltage
Cell voltage delta

Example under load:

Most cells:

3.20V

One cell:

2.90V

That weak cell can pull down total battery voltage and trigger BMS protection.

The problem is then inside the battery rather than in the parallel wiring.

21. Diagnostic Procedure

Step 1: Confirm the Battery Is Supposed to Be Connected

Check:

Battery switch
Breaker
Fuse
Emergency stop

Step 2: Read BMS Status

Check:

Charge enable
Discharge enable
Contactor/MOSFET status
Alarm history

Step 3: Measure Actual Terminal Voltage

Use a suitable multimeter.

Step 4: Measure Common Bus Voltage

Compare with each battery branch.

Step 5: Apply a Moderate Load

Observe whether the voltage difference increases.

Step 6: Compare Branch Current

Use BMS data or a DC clamp meter.

Step 7: Inspect Connection Temperature

Look for abnormal heating.

Step 8: Check Cell-Level Data

Especially if the physical wiring is normal.

22. Diagnostic Table

Observation	More Likely Cause
BMS voltage low, terminal voltage normal	BMS sensing/calibration
Internal battery voltage low, bus voltage high	Battery isolated
Voltage difference grows with load	Resistance or weak battery
Voltage difference disappears at rest	Load-related voltage drop
Battery current 0A and voltage differs	Open branch/BMS protection
One terminal gets hot	High-resistance connection
One cell voltage collapses	Internal cell problem
Battery reconnects after charging	Protection recovery

23. What Voltage Difference Is Normal?

There is no universal number that applies to every LiFePO4 system.

Measurement tolerance alone can create small differences.

The important diagnostic question is not:

“Is 0.1V acceptable?”

but:

“Are the batteries electrically connected to the same bus, and does the difference change abnormally under current?”

A large persistent difference between supposedly active parallel batteries is a reason to investigate.

24. Why This Is Important for Distributors

When a customer says:

“Battery No. 3 voltage is low.”

there are at least four different potential categories:

Actual lower battery SOC
BMS protection
Wiring problem
Measurement problem

Sending a replacement battery before distinguishing these possibilities can result in the replacement showing exactly the same symptom.

Ask for:

BMS screenshots
Multimeter measurements
Branch-current values
Wiring photos
Breaker status
Cell voltages

before making a warranty conclusion.

Frequently Asked Questions

Should parallel LiFePO4 batteries have the same voltage?

Active batteries connected to the same DC bus should have very similar external terminal voltage.

Why is one battery 2V lower?

It may be electrically isolated by a breaker, BMS, fuse or contactor rather than truly connected in parallel at that moment.

Can cable resistance cause different voltage readings?

Yes, especially under high current.

Why does the voltage become normal when the load is removed?

This often indicates voltage drop caused by current flowing through resistance.

Can a BMS display the wrong voltage?

Sensor tolerance or calibration problems are possible. Confirm using a suitable external meter.

Should I reset a low-voltage battery and immediately reconnect it?

Not before comparing its voltage with the live parallel bus and identifying the protection cause.

Conclusion

A substantial voltage difference between supposedly parallel-connected LiFePO4 batteries should not be treated as a simple SOC difference.

True parallel batteries share the same DC bus.

Therefore, when one battery reports a significantly lower voltage, first investigate whether it is actually connected to that bus.

Possible causes include:

BMS isolation
Open breaker
Fuse failure
High-resistance cable connection
Weak cell
Voltage-sensing error

The most effective diagnostic approach is to compare:

battery internal data + external terminal voltage + bus voltage + branch current

under the same operating condition.

This allows installers and distributors to distinguish a battery problem from a connection or measurement problem before replacing hardware.

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