Introduction
Four 12.8V 100Ah LiFePO4 batteries are configured as: 2S2P
Two batteries in series form String A.
Two more form String B.
The two 25.6V strings are then connected in parallel.
Theoretical system: 25.6V 200Ah
Under a 100A load, the installer expects:
- String A: approximately 50A
- String B: approximately 50A
Actual measurement:
- String A: 78A
- String B: 22A
All four individual batteries appear to be online.
No obvious BMS alarm is visible.
What is happening?
Series-parallel battery systems can experience two different types of imbalance simultaneously:
Inside each series string
Individual battery voltage/SOC imbalance.
Between parallel strings
Unequal current sharing.
This makes troubleshooting more complex than a simple parallel bank.
1. Understand Current Flow in 2S2P
String A: Battery A1 → Battery A2
String B: Battery B1 → Battery B2
Inside String A: the same current passes through A1 and A2.
Inside String B: the same current passes through B1 and B2.
But String A current does not have to equal String B current.
The two complete strings share current according to their total electrical characteristics.
2. String Resistance Determines Current Sharing
Each string includes:
- Battery internal resistance
- Two BMS units
- Series jumper
- Positive cable
- Negative cable
- String breaker/fuse
- Busbar connections
If String A has lower total resistance, it can carry more current.
Therefore, current imbalance should be diagnosed at the complete string level, not only at individual battery level.
3. Cause #1: Different String Cable Resistance
Suppose:
String A
0.5m positive + 0.5m negative
String B
2m positive + 2m negative
String B has more conductor resistance.
Under high load, String A may carry much more current.
The series jumpers inside each string should also be included in the comparison.
4. The Series Jumper Is Easy to Forget
Installer matches:
- Main positive cables
- Main negative cables
but String B has a long or undersized jumper between its two batteries.
That jumper is part of the current path.
A poor series jumper can make the complete string high resistance.
Possible symptoms:
- Lower string current
- Jumper heating
- Higher voltage drop
- Unequal SOC between strings
5. Cause #2: One Battery in String B Has High Internal Resistance
Because both batteries in a series string carry the same current, one weak battery increases the effective resistance of the entire string.
Example:
String A batteries: healthy.
String B:
B1 healthy
B2 aged/high resistance
Now the complete String B carries less parallel current.
One battery can therefore reduce participation of an entire string.
6. Resting Voltage May Still Look Normal
At idle:
String A: 26.4V
String B: 26.4V
Everything appears matched.
Under load:
String A: 25.4V
String B: 24.2V
The problem only becomes visible once current flows.
This again shows why series-parallel commissioning must include controlled load testing.
7. Measure Each String Current First
Before measuring every cell, establish the big picture.
Use a suitable DC clamp meter:
String A current
78A
String B current
22A
Now you know the imbalance exists at string level.
Next determine whether the cause is:
- External string branch
- Internal battery/string condition
8. Measure String Voltage Under Load
Record:
- String A terminal voltage
- String B terminal voltage
Because both join the same bus, external connection voltage should be close at the bus.
If one string’s internal terminal voltage differs substantially before its protection/cable path, investigate voltage drop along that path.
9. Measure Individual Batteries Inside Each String
Example under load:
String A
A1: 12.7V
A2: 12.7V
String B
B1: 12.6V
B2: 11.6V
B2 is clearly collapsing.
String B’s low current may be caused by one weak battery.
10. Cause #3: Different Starting SOC Between Strings
Suppose:
String A batteries are both around: 80%.
String B batteries: 40–50%.
The two string voltages may still appear relatively close because LiFePO4 has a flat voltage curve.
But under load their behaviour can differ.
String B may reach low-voltage protection earlier.
Current then shifts almost entirely to String A.
11. What Happens When One Series String Trips?
Before trip:
- String A: 60A
- String B: 40A
String B BMS opens.
Now:
String A must carry:
100A
If String A was designed for only 60–80A continuous operation, it may also trip.
This produces a cascading shutdown.
12. A Series String Is Only as Strong as Its Weakest Battery
If 2S uses:
2 × 100Ah batteries,
but one battery has only:
70Ah real capacity,
the usable series-string capacity is limited by that weaker module.
When it reaches its BMS limit, the complete string disconnects.
The other battery’s remaining capacity cannot bypass it.
13. Cause #4: String Breaker Resistance
Each complete parallel string should normally have suitable protection before joining the common bus.
If String B breaker develops high resistance:
- String B supplies less current
- Breaker heats
- String A works harder
A loaded millivolt drop test across the string breaker can reveal this problem.
14. Cause #5: Loose Series Connection
A loose jumper between B1 and B2 creates resistance in the only current path through String B.
Possible signs:
- One internal terminal gets hot
- String current lower than the other string
- Voltage drop increases with load
- Intermittent string dropout
Inspect every series joint, not just the common busbar.
15. Cause #6: Different Battery Models Between Strings
Example:
String A: 2 × Battery Model X
String B: 1 × Model X + 1 × Model Y
Even if both are:
12.8V 100Ah,
they may have different:
- BMS current limit
- Internal resistance
- Protection threshold
- Capacity
The complete strings will not necessarily behave identically.
For series-parallel systems, matching becomes particularly important.
16. Both Parallel Strings Should Have the Same Series Count
Never parallel:
String A
2 × 12.8V in series
with:
String B
3 × 12.8V in series.
Their nominal voltages are different.
This is not a valid matched parallel-string architecture.
Every parallel string must use the same approved series configuration.
17. Cause #7: One String Reaches Full Charge First
During charging:
String A accepts: 70A
String B: 30A.
Later String A reaches a battery high-voltage threshold and disconnects.
Now almost all charging current tries to enter String B.
If charger current is not reduced, BMS protection may occur.
Series-parallel systems need to be tested in both:
- Charge
- Discharge directions.
18. Check String Current Near Full SOC
An imbalance that appears only: above 90% SOC
may be related to:
- One battery reaching high-cell voltage
- Capacity mismatch
- SOC imbalance
An imbalance present from:
20% through 80%
is more likely to involve:
- Resistance
- Wiring
- BMS current limit
The SOC region where the imbalance begins provides useful evidence.
19. Cause #8: Midpoint Connections
Do not interconnect the midpoint of String A and String B unless the battery system is specifically engineered and approved for that architecture.
For example:
A1–A2 midpoint connected to B1–B2 midpoint.
This can create unintended current paths and complicate BMS protection.
Treat each series string as a complete unit before paralleling at the approved endpoints.
20. Do Not Take a 12V Load From One String Battery
If a 12V device is connected only to B1:
String B develops internal battery-level SOC imbalance.
B1 reaches low SOC sooner.
Eventually the whole String B disconnects early.
Then String A carries the full load.
A small auxiliary load can therefore become a parallel-string imbalance problem.
21. Diagnostic Step 1 — Compare String Current
Under stable load:
- Measure String A
- Measure String B
Do not start by guessing individual batteries.
Confirm whether the strings are actually sharing current abnormally.
22. Step 2 — Compare Complete String Voltage
Measure each string at equivalent locations.
If one shows greater voltage sag, continue inward.
23. Step 3 — Compare Individual Battery Voltage
Inside the weak string, look for one battery showing:
- Larger sag
- Higher voltage during charging
- Lower voltage during discharge
This can identify the limiting module.
24. Step 4 — Compare Lowest Cell Voltage
If B2 appears weak, inspect its cells.
Under load:
- Most cells stable
- One cell falls rapidly
Now the internal cause becomes much clearer.
25. Step 5 — Measure External Voltage Drop
If both batteries appear healthy, check:
- Jumper
- Breaker
- Positive cable
- Negative cable
- Busbar connection
using loaded voltage-drop testing.
26. Step 6 — Check Temperature
Compare:
- Battery terminals
- Jumpers
- String breaker
- Branch cables
A hot connection often identifies the high-resistance location.
27. Practical Test Table
| Load | String A | String B | A1 | A2 | B1 | B2 |
|---|---|---|---|---|---|---|
| 1kW | 18A | 17A | Normal | Normal | Normal | Normal |
| 3kW | 35A | 22A | Normal | Normal | Normal | Slight sag |
| 5kW | 60A | 15A | Normal | Normal | Normal | Large sag |
This pattern points toward B2.
28. What If the Problem Follows the Complete String?
For qualified technicians, after proper isolation and manufacturer approval, compare string connections.
If the imbalance remains with:
String B electrical branch
even after battery conditions are verified,
external branch resistance becomes more likely.
If the behaviour follows one battery wherever installed:
battery condition becomes more likely.
29. Current Sharing Does Not Need to Be Exactly 50/50
A 2P system does not require: 50.0A / 50.0A at every moment.
Small differences are expected.
Focus on:
- Large persistent imbalance
- One string near its current limit
- One string repeatedly disconnecting
- Growing SOC difference
- Abnormal heating
30. Series-Parallel Systems Need More Commissioning Data
For every string record:
- String current
- Total string voltage
- Individual battery voltages
- Lowest cell voltage
- Highest cell voltage
- String protection temperature
This is more detailed than a basic single-battery installation, but it prevents difficult future troubleshooting.
Frequently Asked Questions
Why does one series string carry less current in a 2S2P LiFePO4 bank?
Possible causes include higher string resistance, a weak battery, different SOC, bad series jumper or branch protection resistance.
Does the same current flow through batteries inside each series string?
Yes. Every battery in the same series string carries the same string current.
Do the two parallel strings have to carry exactly the same current?
No, but large persistent differences should be investigated.
Can one weak battery reduce the current of the whole string?
Yes. One battery can increase voltage sag or trigger BMS protection for the complete series string.
Can a loose jumper cause string imbalance?
Yes. The series jumper is part of the complete current path.
Should string midpoints be connected together?
Only if the exact battery/system design specifically approves such an architecture.
Conclusion
A 2S2P or 4S2P LiFePO4 bank combines the challenges of:
series battery balance
and:
parallel current sharing.
When one complete series string carries much less current, do not look only at the common busbar.
The cause may be anywhere inside the string:
- One weak battery
- One weak cell
- Series jumper
- Branch breaker
- Cable
- SOC mismatch
The most effective diagnostic order is:
string current → string voltage → individual battery voltage → cell voltage → external voltage drop.
This structured approach helps installers avoid replacing an entire string when only one connection or one module is responsible.
HIZN Lithium supplies LiFePO4 batteries for 12V, 24V, 48V and 51.2V applications, including modular configurations for solar, telecom, UPS and off-grid systems.