Why Is One LiFePO4 Battery Hotter Than the Others in a Parallel Bank?

Introduction

Four 51.2V LiFePO4 batteries are connected in parallel.

After the inverter has been supplying a heavy load for one hour, the battery temperatures are:

Battery 1: 28°C
Battery 2: 29°C
Battery 3: 30°C
Battery 4: 39°C

All four batteries appear to be working.

There is no obvious BMS alarm.

But one battery is significantly warmer.

The customer asks:

“They are identical batteries connected in parallel. Why is only one battery getting hot?”

A small temperature difference between parallel batteries can be normal.

A large and repeatable difference should be investigated.

Common causes include:

Unequal discharge current
Unequal charging current
Higher internal resistance
Poor cable or terminal connection
Different installation position
Restricted airflow
Battery aging
Internal cell imbalance
Different BMS operating conditions

An important first step is to determine what part of the battery is actually hot.

Is the complete enclosure warmer?

Or is only one terminal, cable lug or breaker hot?

These two situations point to very different problems.

1. Parallel Batteries Do Not Always Carry Equal Current

Suppose four batteries supply a total DC load of:

200A

Perfect current sharing would be:

Battery 1: 50A
Battery 2: 50A
Battery 3: 50A
Battery 4: 50A

But actual BMS readings may show:

Battery 1: 42A
Battery 2: 45A
Battery 3: 48A
Battery 4: 65A

Battery 4 is doing substantially more work.

Higher current generally creates more internal heating.

If this happens every day, Battery 4 may consistently operate several degrees warmer than the others.

2. Why Higher Current Produces More Heat

Electrical losses increase with current.

A simplified relationship is:

Power Loss = I²R

where:

I = current
R = electrical resistance

This relationship is important because current is squared.

If current increases from:

40A to 60A

the heating effect does not increase by only 50%.

For the same resistance:

40² = 1,600
60² = 3,600

The resistive heating can therefore increase significantly.

This is why current imbalance becomes much more important in high-power ESS installations.

3. Cause #1: Shorter or Lower-Resistance Cables

Imagine four battery branches.

Battery 1

1.5m positive + 1.5m negative

Battery 2

1.5m + 1.5m

Battery 3

1.5m + 1.5m

Battery 4

0.5m + 0.5m

All cables use the same conductor size.

Battery 4 has a shorter current path and therefore lower cable resistance.

It may carry more current during:

Charging
Discharging

Over time, this can make Battery 4 warmer.

For parallel banks, similar branch cable resistance helps improve current sharing.

4. Why the Battery Closest to the Inverter May Be Hotter

Some installations use daisy-chain wiring.

The inverter positive and negative cables may effectively favor one battery at one end of the bank.

That battery experiences the lowest resistance path.

Typical symptoms include:

Higher branch current
Faster SOC decrease
Faster SOC increase during charging
Higher enclosure temperature

A properly designed positive and negative busbar arrangement can provide a more symmetrical current path.

5. Cause #2: The Battery Has Lower Internal Resistance

Cable layout is not always the problem.

A newer battery may have lower internal resistance than older batteries.

Suppose a system contains:

Three batteries installed three years ago
One newly added battery

The new battery may naturally carry more current.

During discharge it may supply:

60A

while the old batteries each supply:

35–45A

The new battery may therefore become warmer even though it is perfectly healthy.

This is one reason new and aged battery modules do not always share current equally.

6. A Hotter New Battery Is Not Automatically Defective

This is particularly important for after-sales support.

A customer may say:

“The new battery you supplied is hotter, so it must be worse.”

But if the new battery has:

Lower internal resistance
Better cell condition
Lower-resistance BMS

it may actually be contributing more current than the aged batteries.

Check branch current before making a quality conclusion.

7. Cause #3: Higher Internal Resistance Can Also Create Heat

This may sound contradictory.

A battery with lower resistance may be hotter because it carries more current.

But a battery with higher internal resistance can also generate more heat internally.

Consider two batteries carrying the same 50A current.

If:

Battery A

Internal resistance is relatively low.

Battery B

Internal resistance is higher because of aging or degradation.

Battery B dissipates more energy internally as heat.

Therefore, temperature diagnosis must always consider both:

Current
Resistance

not temperature alone.

8. How to Distinguish the Two Situations

Situation A

Battery is hotter and carries much higher current.

Likely focus:

Wiring resistance
Current sharing
Battery matching

Situation B

Battery is hotter but carries similar or lower current.

Likely focus:

Higher internal resistance
Internal battery condition
Terminal/connection heating
Cooling problem

This comparison is extremely useful.

9. Cause #4: Loose Terminal or Poor Cable Lug

A local hot connection is different from a generally warm battery.

For example:

Battery enclosure:

30°C

Positive terminal:

62°C

This strongly suggests a connection problem.

Possible causes:

Loose bolt
Incorrect terminal torque
Poor crimp
Small cable lug contact area
Oxidized surface
Damaged breaker contact
Multiple poorly stacked lugs

High-resistance connections can become dangerously hot even when the battery itself is healthy.

10. Why a Loose Connection Gets Hot

Suppose abnormal contact resistance is:

0.005Ω

At 80A:

Power loss is:

80² × 0.005 = 32W

Thirty-two watts concentrated around a small terminal connection can generate substantial heat.

This can lead to:

Melted terminal covers
Discoloration
Cable insulation damage
Breaker failure
Voltage drop
BMS shutdown

A hot terminal should never be dismissed as “normal lithium battery heat.”

11. Battery Temperature and Terminal Temperature Should Be Checked Separately

When the customer says:

“Battery No. 2 is hot,”

ask where the temperature was measured.

Useful locations include:

Battery enclosure
Positive terminal
Negative terminal
Branch breaker
Cable lug
Busbar connection

A thermal image can be especially useful in large installations.

If only one connection is hot, inspect the electrical connection first.

If the entire battery enclosure is warmer, investigate current, internal resistance and cooling.

12. Cause #5: Unequal Installation Environment

Sometimes the electrical system is perfectly balanced.

The batteries are simply installed in different environments.

For example:

Battery 1

Near an air inlet

Battery 2

Center of rack

Battery 3

Center of rack

Battery 4

Directly above the inverter exhaust

Battery 4 naturally operates hotter.

Other examples include:

Direct sunlight
Hot exterior wall
Roof heat
Poor cabinet ventilation
Blocked air gap

Battery temperature should therefore be evaluated relative to its physical location.

13. Why the Top Battery in a Rack May Be Warmer

Warm air rises.

In a poorly ventilated battery cabinet, upper modules may experience higher ambient temperature.

Example:

Bottom battery ambient: 27°C
Middle battery: 30°C
Top battery: 35°C

The temperature difference may have little to do with current sharing.

Improving cabinet airflow may solve the problem.

14. Cause #6: One Battery Has High Cell Imbalance

A battery with one weaker cell may experience:

Greater voltage stress
More balancing activity
Earlier protection events

Passive BMS balancing itself normally uses relatively small current, so it is unlikely to make the complete battery dramatically hotter.

However, significant cell imbalance may indicate an underlying battery condition that also affects internal resistance and temperature.

Check:

Maximum cell voltage
Minimum cell voltage
Cell delta
Temperature sensor data

15. Cause #7: Different BMS Current Limits

Suppose four batteries communicate with the inverter.

Three batteries allow:

100A discharge

Battery 4 allows:

150A

depending on model or firmware.

In some system architectures, the larger-current module may take a disproportionate load.

Likewise, one battery may have a lower charge-current restriction and remain cooler during charging.

Identical parallel modules generally provide more predictable thermal behaviour.

16. Why the Problem May Appear Only During Charging

Some systems show:

Discharge

All batteries remain around the same temperature.

Fast Solar Charging

Battery 2 becomes much hotter.

Check:

Charging current per branch
Maximum cell voltage
Charge-current limit
Cable resistance

The battery may be accepting much more current during charging than the others.

17. Why the Problem May Appear Only During High Load

At a 1kW load:

All batteries:

27–29°C

At an 8kW load:

A: 31°C
B: 32°C
C: 32°C
D: 43°C

A high-load-only temperature difference often reveals:

Current imbalance
Voltage drop
Internal resistance
Poor connection

that is not obvious at low current.

18. A Useful Load Test

Apply a stable load within the approved system rating.

Record every 10–15 minutes:

Battery	Current	Voltage	Temperature
A	42A	51.6V	30°C
B	44A	51.6V	31°C
C	41A	51.6V	30°C
D	65A	51.5V	39°C

This clearly suggests Battery D is carrying more current.

Now investigate why.

19. Charging Test

Repeat the comparison during charging.

Example:

Battery	Charge Current	Temperature
A	30A	29°C
B	31A	29°C
C	29A	28°C
D	55A	37°C

If the same battery works harder in both directions, branch resistance or battery internal resistance becomes a strong suspect.

20. Check Whether the Temperature Difference Follows the Battery

For qualified installers, one useful diagnostic concept is:

If the hot condition follows the battery after changing position

Investigate:

Battery internal resistance
BMS
Cell condition

If the new battery placed in the same physical branch becomes hot

Investigate:

Cable
Breaker
Busbar
Installation location

Do not move high-current battery connections without proper isolation procedures.

21. Is a 5°C Difference Normal?

There is no universal temperature-difference limit for every battery design.

A few degrees may occur because of:

Sensor tolerance
Airflow
Installation position
Current variation

22. Why Long-Term Temperature Imbalance Matters

If one battery consistently operates hotter, it may age faster.

Higher operating temperature can contribute to:

Faster capacity degradation
Internal resistance changes
Greater cell imbalance

Then a feedback loop may develop:

Unequal current → higher temperature → unequal aging → different resistance → greater imbalance

Correcting the cause early helps maintain bank consistency.

23. Do Not Deliberately Add Cable Resistance to Cool One Battery

A tempting workaround is:

“Battery 1 carries too much current, so I will use a thinner cable.”

This is not a good design method.

Undersized cable creates:

Heat
Voltage drop
Reduced efficiency
Safety risk

Correct the battery-bank architecture instead.

Use properly sized and reasonably symmetrical branches.

24. Troubleshooting Sequence

If one parallel battery is noticeably hotter:

Record individual branch current.
Determine whether heating occurs during charge, discharge or both.
Measure battery enclosure temperature.
Measure terminal and breaker temperature separately.
Compare cable length and cross-section.
Check terminal torque and lug condition.
Compare battery SOC.
Compare cell voltage delta.
Compare battery age and cycle count.
Check physical airflow and heat sources.

25. Diagnostic Table

Symptom	More Likely Cause
Whole battery hot + current high	Current-sharing imbalance
Whole battery hot + current normal	Internal resistance/cooling
One terminal extremely hot	Poor connection
Battery nearest inverter hottest	Wiring or heat source
New battery hotter than old batteries	Lower resistance/higher current
Top rack battery hotter	Airflow/ambient heat
Heat appears only at high load	Resistance/current issue
High temperature + weak cell	Internal battery investigation

Frequently Asked Questions

Why is one LiFePO4 battery hotter than the other parallel batteries?

It may be carrying more current, have higher internal resistance, have a poor electrical connection or be installed in a hotter location.

Does a hotter battery mean it is defective?

Not necessarily. Check branch current, terminal temperature and installation environment first.

Can unequal cable length make one battery hotter?

Yes. Lower-resistance branches can carry more current.

Why is only the positive terminal hot?

This strongly suggests a local connection-resistance problem rather than normal battery heating.

Can an older battery run hotter?

Yes. Aging can increase internal resistance.

Should all parallel batteries have exactly the same temperature?

No, but a large persistent difference should be investigated.

Conclusion

A temperature difference between parallel LiFePO4 batteries is not a diagnosis by itself.

The key is to determine whether the heat comes from:

Higher current
Higher internal resistance
A poor electrical connection
Installation environment
Internal battery condition

The most useful comparison is:

temperature + branch current + connection temperature

measured under the same load.

If one battery repeatedly runs much hotter than the others, the problem should be corrected before long-term operation accelerates unequal aging.

For distributors and system integrators, thermal behaviour is also an excellent diagnostic tool because it can reveal current-sharing and connection problems that may not be obvious from SOC alone.

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