Introduction
Four 12.8V LiFePO4 batteries are connected in series to create a nominal 51.2V battery bank.
The installer needs a 12V supply for:
- Router
- Lighting
- Security equipment
- Fan
Instead of installing a DC-DC converter, the installer connects the 12V load directly across Battery 1.
The system appears to work.
Several weeks later:
- Battery 1 reaches low SOC first
- Battery 1 repeatedly enters undervoltage protection
- The complete 48V system shuts down early
- Other batteries still show significant SOC
The installer asks:
“All four batteries are in series, so why is only Battery 1 becoming empty?”
The answer is the midpoint or individual-battery tap.
Taking power from only part of a series string creates unequal energy removal.
The same problem can appear in more complicated series-parallel banks and can be difficult to identify if the auxiliary load is small but operates continuously.
1. Normal Series Operation Uses the Same Current Through Every Battery
Four:
12.8V 100Ah
batteries connected in series produce:
51.2V 100Ah
When the 48V inverter draws:
20A
that same 20A passes through:
- Battery 1
- Battery 2
- Battery 3
- Battery 4
The batteries therefore experience the same main-string Ah throughput.
2. What Changes When a 12V Load Is Connected to Battery 1?
Now Battery 1 supplies:
Main 48V system current
plus:
Additional 12V auxiliary current
Batteries 2–4 supply only:
main system current
Battery 1 therefore loses more energy.
Over time, its SOC falls below the rest of the series string.
3. A Small Continuous Load Can Create a Large Daily Imbalance
Suppose the 12V auxiliary load is only:
5A
It operates:
24 hours per day
Daily extra capacity removed from Battery 1:
5A × 24h = 120Ah
For a 100Ah battery, this is more than one nominal battery capacity of additional throughput per day.
Even a much smaller continuous load can create serious imbalance.
4. Example With a 1A Auxiliary Load
A 12V device draws:
1A continuously
Daily:
1A × 24h = 24Ah
Battery 1 loses an extra:
24% of a 100Ah nominal capacity
every day relative to the other batteries.
That is enough to create a rapidly growing battery-level SOC difference.
5. The Other Batteries Cannot Automatically Refill Battery 1
This is the critical point.
In a series connection, Batteries 2–4 do not have a direct parallel path to Battery 1.
They cannot simply transfer their excess SOC into Battery 1 during rest.
The imbalance remains.
Normal 48V charging also sends the same series current through all four batteries.
It does not selectively give Battery 1 an extra 24Ah to replace the midpoint load.
6. The Imbalance Can Grow Every Day
Imagine the bank begins perfectly matched.
Day 1:
Battery 1 ends lower.
Day 2:
It begins lower and again powers the auxiliary load.
Day 3:
Difference grows further.
Eventually Battery 1 reaches:
- Low cell voltage
- Low battery SOC
- BMS undervoltage protection
before the rest of the bank.
7. One Battery Can Shut Down the Complete 48V System
Because the batteries are in series, current must pass through every module.
When Battery 1’s BMS opens:
the entire 48V current path is interrupted.
The inverter shuts down.
Meanwhile:
- Battery 2 may show 40%
- Battery 3: 45%
- Battery 4: 42%
That remaining energy becomes inaccessible because Battery 1 has reached protection first.
8. The Customer May Blame Battery 1
A common warranty complaint is:
“Battery 1 is defective. It always empties first.”
But if Battery 1 is powering an additional 12V load, it is being used differently from the other modules.
Replacing Battery 1 will not solve the problem.
The new Battery 1 will eventually develop the same SOC imbalance.
9. Midpoint Loads Can Also Be 24V
Suppose four 12.8V batteries form a 48V bank.
The installer takes:
24V
from Batteries 1 and 2 to power equipment.
Now:
- Batteries 1 and 2 supply extra energy
- Batteries 3 and 4 do not
The 48V bank gradually splits into:
lower-SOC half
and:
higher-SOC half.
The principle is the same.
10. Midpoint Connections Become Even More Complicated in Series-Parallel Banks
Consider:
4S2P
Eight batteries total.
Two 48V strings operate in parallel.
A technician connects a 12V device to Battery A1 in String A.
Now not only is:
Battery A1
being discharged more than A2–A4,
but String A may also begin behaving differently from String B.
This can produce both:
- Series imbalance
- Parallel-string current imbalance
at the same time.
11. One Small Midpoint Load Can Affect the Complete Bank
As String A becomes weaker:
- String B may carry more current
- String A reaches protection earlier
- System current redistributes
- Remaining string may become overloaded
What started as a small 12V accessory connection can therefore affect the performance of the entire ESS.
12. Midpoint Grounding and Midpoint Loading Are Different Issues
A midpoint may sometimes exist in specially engineered electrical systems for:
- Voltage reference
- Grounding architecture
- Monitoring
That is not the same as using part of the battery string to supply a continuous load.
The system design must explicitly define whether midpoint connections are allowed.
Do not assume that because a physical midpoint exists, it can be used as an auxiliary power source.
13. The Better Solution: Use a DC-DC Converter
If the main battery bank is:
48V
and the system needs:
12V
use an appropriately rated:
48V-to-12V DC-DC converter
connected across the complete 48V bank.
Now the converter draws current from the entire series string.
Every series battery experiences the same main current.
The 12V load no longer discharges only one module.
14. Example
12V load:
120W
Incorrect Method
Connect directly to Battery 1.
Battery 1 alone supplies approximately:
10A at 12V.
Better Architecture
48V bank
→ DC-DC converter
→ 12V load
The 48V side may draw only a few amperes depending on efficiency and operating voltage.
That current passes through the complete series string equally.
15. DC-DC Converter Sizing
Select the converter based on:
- Input voltage range
- Output voltage
- Continuous output current
- Peak current
- Efficiency
- Environmental temperature
- Isolation requirement where applicable
Do not size only for average load.
Some 12V devices have startup surges.
16. The Converter Input Must Match the Actual Battery Voltage Range
A nominal 48V LiFePO4 bank does not stay at exactly 48.0V.
Its operating voltage changes with:
- SOC
- Charging
- Load
- BMS limits
The DC-DC converter must support the complete approved input voltage range.
17. What If the System Needs Both 12V and 24V?
Possible architecture:
48V battery bank
→ 48V-to-24V converter
and:
→ 48V-to-12V converter
Alternatively, a properly engineered distribution system can be used.
Do not casually create:
- 12V tap from Battery 1
- 24V tap from Batteries 1–2
because different loads will create complex imbalance.
18. Existing Midpoint Loads Can Be Hard to Find
In a problem installation, the main inverter may appear correctly connected.
But one hidden accessory may be connected to an individual battery.
Check for:
- USB charger
- Router
- LED lighting
- Security alarm
- Fan
- Relay coil
- Monitoring device
- GPS tracker
Even a small continuous load matters over time.
19. BMS Power Leads Can Also Be Misunderstood
Some external monitoring or control equipment may use a low-voltage auxiliary supply.
Installers may conveniently take this from one battery.
That still creates unequal load.
Use a system-approved auxiliary power solution.
20. Diagnosing a Suspected Midpoint Imbalance
Measure each series battery:
- At full charge
- Mid-discharge
- Near shutdown
- After rest
A pattern such as:
Battery 1
always lower
while:
Batteries 2–4
remain close
suggests Battery 1 is experiencing different energy throughput.
Now inspect for an external load connected directly across Battery 1.
21. Example Measurement
After charging:
- B1: 13.40V
- B2: 13.41V
- B3: 13.41V
- B4: 13.40V
After 12 hours:
- B1: 12.90V
- B2: 13.30V
- B3: 13.31V
- B4: 13.30V
If a 12V load is connected across B1, the explanation is straightforward.
22. Check Current With the Main Inverter Off
Turn off the main 48V load according to safe procedures.
Then measure whether one battery still has current because of an auxiliary circuit.
This can reveal a hidden midpoint load.
23. The Problem Can Look Like Self-Discharge
If the auxiliary device remains unnoticed, Battery 1 appears to:
“self-discharge overnight.”
But when isolated from the auxiliary load, its voltage may remain stable.
This is why external loads should be eliminated before diagnosing abnormal battery self-discharge.
24. The Problem Can Also Look Like Capacity Loss
Battery 1:
- Charges quickly
- Discharges quickly
- Reaches 0% first
These symptoms resemble a degraded battery.
But if it is continuously powering an additional load, the apparent reduced runtime may be entirely installation-related.
25. Rebalancing After Removing the Midpoint Load
Once the incorrect load has been removed, the batteries may still have different SOC.
Simply reconnecting the main inverter does not instantly correct the accumulated imbalance.
Follow the battery manufacturer’s approved procedure for bringing series modules back to similar operating conditions.
Do not arbitrarily increase the total charger voltage.
26. Why Increasing 48V Charge Voltage Is the Wrong Fix
If Battery 1 is low:
- B1 needs more energy
But Batteries 2–4 may already be near full.
Increasing total charger voltage can push the higher batteries toward:
- Cell overvoltage
- BMS charge protection
before Battery 1 catches up.
Correct battery-level imbalance separately according to manufacturer guidance.
27. Do Not Add Another 12V Charger Permanently to Battery 1 as a Shortcut
This can create:
- Multiple charger interaction
- Grounding issues
- Uneven control
- Maintenance complexity
A properly engineered DC-DC architecture is usually cleaner than continuously compensating for an intentionally unbalanced load.
28. Midpoint Taps Can Increase Cycle Count on One Battery
Battery 1 experiences:
- Main 48V cycle throughput
- Extra 12V accessory throughput
Its BMS may therefore accumulate:
- More Ah throughput
- More equivalent cycles
than Batteries 2–4.
A higher cycle count on one series battery can sometimes reveal an unequal auxiliary load.
29. Temperature Differences May Also Appear
If Battery 1 supplies additional current continuously, it may operate:
- Slightly warmer
- With greater SOC movement
than the other modules.
Temperature and cycle-count data can therefore support the diagnosis.
30. Series-Parallel String Imbalance From a Midpoint Load
In a 4S2P system:
String A contains the tapped battery.
String B does not.
Over time:
String A reaches lower SOC earlier.
During parallel operation, String B may begin supplying more of the inverter current.
Now the original midpoint load indirectly creates:
unequal current between the two complete strings.
This can become a system-level reliability issue.
31. Critical Systems Should Avoid Unapproved Midpoint Loads
In:
- Telecom backup
- UPS
- Commercial ESS
- Remote solar installations
one hidden midpoint load can reduce available backup duration.
For critical installations, every auxiliary voltage should be shown on the system schematic.
There should be no undocumented battery taps.
32. Design Checklist
Before connecting a low-voltage auxiliary device to a series battery bank, ask:
| Question | Requirement |
|---|---|
| Is the load connected across the whole bank? | Preferred |
| Is a DC-DC converter available? | Use appropriately |
| Is midpoint loading approved? | Must be confirmed |
| Does auxiliary load run continuously? | Include energy calculation |
| Is converter input range adequate? | Verify |
| Is converter output current adequate? | Verify |
| Is protection included? | Design correctly |
| Are auxiliary circuits documented? | Yes |
Frequently Asked Questions
Can I take 12V from one battery in a 48V LiFePO4 series bank?
It should generally be avoided because it creates unequal battery discharge. Use an appropriately designed DC-DC converter.
What happens if I use only one series battery for lights?
That battery loses SOC faster and may reach BMS low-voltage protection before the others.
Can the other series batteries automatically balance it?
Normally no. Series connection does not provide a direct energy-equalization path between complete batteries.
Can I take 24V from the middle of a 48V bank?
This can create imbalance between the two halves unless the system is specifically engineered for midpoint loading.
Can midpoint loading cause the whole inverter to shut down?
Yes. One low battery can open its BMS and interrupt the complete series string.
Does this matter in series-parallel systems too?
Yes, and it can also create unequal current between complete parallel strings.
Conclusion
A midpoint or individual-battery load is one of the easiest ways to create hidden imbalance in a series LiFePO4 battery bank.
The system may work perfectly at first.
But over repeated operation, the tapped battery experiences additional energy throughput and gradually moves to a lower SOC than the rest of the string.
The result can include:
- Early low-voltage protection
- Reduced usable capacity
- Unequal cycle counts
- Series-string shutdown
- Parallel-string imbalance
For systems that need 12V or 24V auxiliary power from a 48V-class bank, an appropriately designed DC-DC converter across the complete battery bank is generally the cleaner approach.
HIZN Lithium supplies LiFePO4 batteries for 12V, 24V, 48V and 51.2V applications, including modular ESS solutions for solar, UPS, telecom and off-grid projects.