Introduction
A 48V energy storage system is sometimes built from four 12.8V LiFePO4 batteries connected in series.
After installation, the system owner may also need 12V power for equipment such as:
- CCTV cameras
- Internet routers
- LED lighting
- Relays and contactors
- Communication equipment
- Security systems
- Small DC pumps
- Monitoring devices
This leads to a common installation question:
Can a 12V load be connected directly to one battery in the 48V series string?
Although this connection may power the load temporarily, it should generally not be used. Drawing power from only one battery creates unequal state of charge within the series string. Over time, the selected battery becomes more deeply discharged than the other batteries, causing reduced usable capacity, repeated BMS protection and premature failure.
The correct solution is normally to install a properly sized 48V-to-12V DC-DC converter across the complete battery bank.
Understanding a 48V Series Battery Bank
Consider four identical 12.8V 100Ah LiFePO4 batteries connected in series.
The completed bank provides approximately:
- Nominal voltage: 51.2V
- Capacity: 100Ah
- Nominal energy: 5.12kWh
In a series connection:
- Battery voltages are added together.
- Amp-hour capacity remains unchanged.
- The same main current flows through every battery.
- Each battery should remain at a similar state of charge.
A properly balanced series string depends on all four batteries being charged and discharged together.
If a separate 12V load is connected to only Battery 1, that battery must supply:
- Its share of the main 48V inverter load.
- The complete current required by the additional 12V load.
The other three batteries supply only the main inverter current.
This immediately creates unequal loading.
What Is a Battery Midpoint Connection?
In a series battery bank, the connections between individual batteries are sometimes called midpoints or intermediate voltage points.
For example, in a four-battery series string:
- Battery-bank negative to the first connection: approximately 12.8V
- Battery-bank negative to the second connection: approximately 25.6V
- Battery-bank negative to the third connection: approximately 38.4V
- Battery-bank negative to the positive end: approximately 51.2V
These intermediate voltages may look convenient for powering lower-voltage equipment.
However, the midpoint should normally be used only for approved monitoring or balancing functions—not as a power supply for auxiliary loads. Manufacturer wiring guidance warns that taking a lower-voltage load from a series-bank midpoint creates an imbalance much larger than a typical battery balancer can correct.
Why Midpoint Loads Cause Battery Imbalance
Assume a 12V communication system consumes 8A continuously.
If it is connected to Battery 1, that battery supplies an additional:
12V × 8A = 96W
During ten hours of operation, the auxiliary load consumes approximately:
96W × 10 hours = 960Wh
This energy is taken from only one battery.
The complete 48V battery bank may still show a relatively high total voltage, but Battery 1 can be at a much lower SOC than Batteries 2, 3 and 4.
During the next charging cycle:
- Battery 1 requires more energy to reach full charge.
- The other batteries reach their upper voltage limits earlier.
- One of the higher-SOC batteries may trigger overvoltage protection.
- The charger stops before Battery 1 has fully recovered.
During discharge:
- Battery 1 reaches its low-voltage limit first.
- Its BMS disconnects the complete series circuit.
- Significant energy may remain in the other three batteries.
The customer may then believe that the entire battery bank has insufficient capacity, even though the real problem is unequal auxiliary loading.
Why the BMS Cannot Automatically Correct the Problem
Each 12.8V battery may contain its own BMS, but the BMS is primarily designed to protect the cells inside that battery.
It may provide:
- Cell overvoltage protection
- Cell undervoltage protection
- Overcurrent protection
- Short-circuit protection
- Temperature protection
- Cell balancing
The BMS cannot transfer a large amount of energy from Batteries 2, 3 and 4 into Battery 1.
If Battery 1 is continuously powering a 12V auxiliary load, the imbalance may be many amp-hours per day. Normal passive balancing currents are far too small to compensate for this continuous external load.
Can a Battery Balancer Solve the Problem?
A battery balancer can help correct relatively small voltage or SOC differences between series-connected batteries.
However, a balancer should not be used as permission to power a significant load from one battery.
Battery balancing equipment is intended to correct limited differences caused by:
- Manufacturing tolerances
- Slightly different self-discharge rates
- Temperature variation
- Small capacity differences
- Normal ageing
It is not designed to replace the energy continuously removed by a router, lighting system, pump or other auxiliary load.
The correct approach is to draw auxiliary power from the complete bank.
The Correct Solution: Install a 48V-to-12V DC-DC Converter
A DC-DC converter accepts the voltage of the complete battery bank and provides a regulated 12V output.
The input is connected across:
- Main battery-bank positive
- Main battery-bank negative
Because the converter draws from the full 48V or 51.2V bank, every series-connected battery carries the same input current.
The converter then supplies the required 12V power to the auxiliary equipment.
This arrangement provides several advantages:
- Balanced discharge of all series batteries
- Stable 12V output
- Protection from changing battery voltage
- Better auxiliary-load reliability
- Easier fuse coordination
- Clearer maintenance and troubleshooting
- Reduced risk of early BMS disconnection
Isolated Versus Non-Isolated DC-DC Converters
DC-DC converters are available in isolated and non-isolated versions.
Non-Isolated Converter
In a non-isolated converter, the input and output negative connections may be electrically common.
Advantages may include:
- Lower cost
- Smaller size
- Higher efficiency
- Simpler design
However, a common negative may affect grounding, communication and fault-current paths.
Isolated Converter
An isolated converter provides electrical separation between the 48V input and 12V output.
Advantages may include:
- Better separation between systems
- Reduced risk of unwanted ground loops
- Greater flexibility in grounding the 12V circuit
- Improved compatibility with sensitive communication equipment
The correct type depends on:
- Battery grounding design
- Inverter architecture
- Telecom or monitoring equipment
- Required electrical isolation
- Local electrical rules
- Equipment manufacturer requirements
Do not assume that isolated is always necessary or that non-isolated is always acceptable. The complete grounding architecture must be reviewed.
How to Size the DC-DC Converter
Start by listing every 12V load.
Example:
| Auxiliary Load | Normal Power | Starting or Peak Power |
|---|---|---|
| Router and modem | 25W | 30W |
| CCTV equipment | 45W | 55W |
| LED lighting | 40W | 40W |
| Relays and controls | 15W | 25W |
| DC pump | 80W | 180W |
| Total | 205W | 330W |
The converter should not be selected only according to normal power.
Consider:
- Continuous output power
- Peak or starting current
- Operating temperature
- Ventilation
- Input-voltage range
- Output-voltage adjustment
- Efficiency
- Derating
- Environmental protection
- Future load expansion
In this example, a 12V 20A converter provides approximately 240W and may be too small for the pump’s starting demand. A 12V 30A or larger converter may be more suitable, subject to its actual surge rating.
Confirm the Converter Input-Voltage Range
A “48V battery” does not remain at exactly 48V.
A 51.2V LiFePO4 system may operate over a voltage range determined by:
- Cell voltage
- BMS limits
- Inverter settings
- Charging profile
- Load current
- Temperature
The converter must accept the complete real operating range, not only the nominal voltage.
For example, verify that it can tolerate:
- Maximum charging voltage
- Normal resting voltage
- Low-voltage operating range
- Short voltage transients
A converter designed for a fixed industrial 48V supply may not always be suitable for the full voltage range of a LiFePO4 battery bank.
Recommended Wiring Arrangement
A professional arrangement is:
Battery-bank positive
→ DC-rated input fuse or breaker
→ DC-DC converter positive input
Battery-bank negative
→ Negative busbar
→ DC-DC converter negative input
Converter 12V positive output
→ 12V output fuse box
→ Individual 12V loads
Converter 12V negative output
→ 12V negative distribution bus
→ Individual 12V loads
Do not connect the converter input by stacking unsuitable lugs directly on one battery terminal. Use an approved DC distribution point or busbar.
Input and Output Protection
The converter installation may require protection on both sides.
Input Protection
The input fuse or breaker protects:
- The 48V supply cable
- The converter input circuit
- Wiring between the battery bus and converter
It must be rated for the system’s DC voltage.
Output Protection
The output fuse protects:
- The 12V distribution cable
- Individual auxiliary circuits
- Connected equipment
Each significant 12V branch may require its own fuse.
Protection ratings must follow:
- Converter manufacturer instructions
- Cable ampacity
- Maximum current
- Fault-current capability
- Local installation requirements
Installation Procedure
Step 1: Confirm Series Compatibility
Verify that the 12.8V batteries are approved for the planned series quantity.
Step 2: Inspect the Existing Battery Bank
Record:
- Individual battery voltage
- Total bank voltage
- Battery SOC
- BMS alarms
- Existing midpoint loads
- Cable condition
Step 3: Remove the Midpoint Load
Switch off and isolate the auxiliary equipment before removing its connection.
Step 4: Select the DC-DC Converter
Confirm:
- Input-voltage range
- Output voltage
- Continuous current
- Surge capability
- Isolation requirement
- Cooling requirements
- Environmental rating
Step 5: Install Protection Devices
Install the input protection close to the battery distribution point and output protection close to the converter or 12V distribution panel.
Step 6: Complete the Wiring
Check polarity with a multimeter before closing any breaker.
Step 7: Energise the Converter
Where required:
- Energise the battery bank.
- Close the DC-DC input protection.
- Measure the converter output.
- Confirm stable 12V voltage.
- Add auxiliary loads one at a time.
Step 8: Rebalance the Series Batteries
If a midpoint load has been used for a long period, the batteries may already be at different SOC levels.
Before returning the bank to normal service:
- Measure every battery.
- Review BMS cell data.
- Charge batteries individually if required.
- Follow the supplier’s balancing procedure.
- Do not reconnect the series string until the batteries are suitably matched.
Common Installation Mistakes
Taking 12V from the Lowest Battery
This creates continuous SOC imbalance.
Alternating the 12V Load Between Batteries
Manual rotation does not provide reliable balance and increases wiring risks.
Installing a Small Battery Balancer
A balancer cannot normally compensate for a substantial continuous auxiliary load.
Selecting a Converter by Output Current Only
Its input-voltage range, surge rating and thermal derating are equally important.
Ignoring the Converter’s Standby Consumption
A permanently connected converter consumes some energy even when the 12V loads are light.
Using an AC Breaker on the DC Input
The protective device must have an appropriate DC rating.
Connecting the Converter to a Series Midpoint
The converter input should normally connect across the complete battery bank.
Frequently Asked Questions
Can I temporarily connect a small 12V load to one battery?
Even a small permanent load can create imbalance over time. Use a full-bank DC-DC converter for regular operation.
What about a load that operates only a few minutes per day?
The risk is lower, but the connection still creates unequal discharge. A correctly designed converter remains the preferred solution.
Can I use a 48V inverter’s USB or 12V auxiliary output?
Only when that output has sufficient power and is approved for the intended equipment.
Does the converter need CAN or RS485 communication?
Most basic DC-DC converters do not. More advanced managed systems may use communication for enable, shutdown or current control.
Can a 24V load be taken from the midpoint of a 48V bank?
It creates the same fundamental imbalance problem. Use a 48V-to-24V converter.
Can I connect a 12V charger to only one battery while the series bank is operating?
This can create unsafe or unpredictable current paths. Charging arrangements must follow the battery manufacturer’s approved design.
Conclusion
A lower-voltage load should not normally be powered from one battery or one midpoint in a LiFePO4 series string.
The connection may appear to work, but it can cause:
- Unequal state of charge
- Early BMS disconnection
- Incomplete charging
- Reduced usable capacity
- Battery ageing
- Difficult troubleshooting
The recommended solution is a properly sized DC-DC converter connected across the complete battery bank.
When requesting a battery system proposal from HIZN Lithium, provide:
- Main battery-bank voltage
- Battery model and quantity
- Required auxiliary voltage
- Continuous auxiliary power
- Peak auxiliary power
- Grounding requirements
- Inverter model
- Installation environment
This information allows the battery bank, DC-DC converter and protection system to be evaluated as one complete installation.