Introduction
A customer switches on the inverter.
The LiFePO4 battery powers the load normally.
Lights work.
The inverter works.
Battery voltage looks normal.
But when solar or grid charging begins:
Charging current remains at 0A.
This seems contradictory.
If the battery can deliver power, why can it not accept power?
The reason is that many LiFePO4 Battery Management Systems can control charging and discharging separately.
The BMS may allow: discharge = ON
while keeping: charge = OFF
This is not necessarily a BMS failure.
It can be an intentional response to:
- High cell voltage
- Low temperature
- High temperature
- Charging overcurrent
- Charger voltage
- Communication commands
- Hardware faults
Understanding direction-specific BMS protection can prevent unnecessary battery replacement.
Charging and Discharging Are Two Different Operations
A lithium battery does not simply have one universal:
ON / OFF
state.
Depending on BMS architecture, it may separately control:
- Charging path
- Discharging path
This can be implemented using:
- MOSFETs
- Contactors
- Other power-switching architecture
Therefore, several states are possible.
Normal Operation
Charge: ON
Discharge: ON
Charge Disabled
Charge: OFF
Discharge: ON
Discharge Disabled
Charge: ON
Discharge: OFF
Full Protection
Charge: OFF
Discharge: OFF
This explains why a battery can still power equipment while refusing charging current.
Why Would the BMS Disable Charging Only?
Imagine one cell is already near its maximum permitted voltage.
Allowing more charging could push that cell beyond the protection limit.
But discharging the battery would reduce cell voltage.
Therefore, the logical BMS response may be:
Stop charging.
Continue allowing discharge.
This gives the battery a way to recover from the condition.
High Cell Voltage Is a Common Cause
Suppose a 51.2V battery contains 16 LiFePO4 cells.
Fifteen cells are normal.
One cell reaches the upper protection threshold earlier.
The total pack voltage may still look reasonable.
However, the BMS sees the individual high cell and disables charging.
The battery can still discharge because discharging moves the high cell away from the upper limit.
The Total Battery Voltage Can Be Misleading
A customer may say:
“The battery is only 55V, so it cannot be overvoltage.”
But the BMS does not monitor only total voltage.
It also monitors individual cells.
For example:
Most cells may be at one voltage.
One cell may be significantly higher.
The total pack voltage alone can hide the imbalance.
When charge is disabled, always check:
- Maximum cell voltage
- Minimum cell voltage
- Cell-voltage difference
BMS Apps Often Show Charge and Discharge Status
Depending on the battery system, BMS software may display items such as:
- Charge MOS: ON/OFF
- Discharge MOS: ON/OFF
- CHG
- DSG
- Charge Enable
- Discharge Enable
- Contactor status
If:
Charge MOS = OFF
while:
Discharge MOS = ON
the next step is to find out why charging is disabled.
Do not immediately replace the BMS.
Low Temperature Can Create Exactly This Symptom
LiFePO4 batteries often have a higher minimum temperature for charging than for discharging.
For example, a battery may still be able to power loads at a low temperature while charging is prohibited.
A winter customer may therefore report:
“The battery runs my inverter but solar cannot charge it.”
The BMS may simply be applying low-temperature charge protection.
Once cell temperature rises into the approved charging range, the BMS may automatically enable charging again.
High Temperature Can Do the Same Thing
During hot-weather charging, the BMS may detect excessive:
- Cell temperature
- MOSFET temperature
- Internal temperature
It may stop charging while still allowing controlled discharge.
This can happen in:
- Outdoor cabinets
- Hot equipment rooms
- High-current solar charging
- Tropical climates
Check BMS temperature before diagnosing charger failure.
Charging Overcurrent Protection
Suppose a battery BMS permits a maximum charging current of: 100A
The inverter or multiple charging sources attempt: 140A
The BMS may disable the charging path.
The customer then sees:
- Battery can discharge
- Charger is connected
- Charge current = zero
Possible charging sources may include:
- Solar MPPT
- Grid charger
- Generator charger
- External charger
Always calculate the combined current.
One Charger May Be Fine—Several Together May Not Be
Example:
MPPT charger = 60A
Grid charger = 60A
External charger = 30A
Potential combined current:
60 + 60 + 30 = 150A
Each charger alone may be below the battery limit.
Together they may exceed it.
The BMS does not care which device supplied the current.
It sees the total battery current.
Charger Voltage Can Trigger Charge Protection
If inverter charging voltage is set too high, the battery may repeatedly reach its BMS high-voltage threshold.
Possible behavior:
- Charging starts.
- One cell rises too high.
- BMS disables charge.
- Cell voltage relaxes.
- BMS enables charge again.
- Cycle repeats.
The customer may see:
0A → charging → 0A → charging
This is not desirable normal charging.
Check charging-voltage settings.
Incorrect Lead-Acid Settings Are a Common Cause
A LiFePO4 battery connected to an inverter previously used for lead-acid may encounter:
- High equalization voltage
- Long absorption stage
- Incorrect float behavior
- Temperature compensation
These settings can push the battery into BMS protection.
When upgrading from AGM/GEL to LiFePO4, review the inverter configuration completely.
Low-Voltage Protection Produces the Opposite Behavior
Now consider the other direction.
If one cell reaches low-voltage protection during discharge, the BMS may:
disable discharge
while:
allowing charge
This makes sense because charging is the action needed to recover the battery.
So:
High-Voltage Condition
Charging may be blocked.
Low-Voltage Condition
Discharging may be blocked.
This direction-specific logic is useful when diagnosing BMS behavior.
What If Both Charge and Discharge Are Disabled?
Possible causes include:
- Severe protection condition
- BMS shutdown
- Sleep mode
- Hardware fault
- Main switch off
- Internal contactor open
The troubleshooting process is different.
First determine the exact BMS state rather than assuming the battery is “dead.”
Communication Can Also Disable Charging
In closed-loop systems, the inverter and battery communicate through:
- CAN
- RS485
The BMS may report:
maximum charging current = 0A
The inverter then intentionally stops charging.
Possible reasons include:
- Temperature protection
- High SOC
- High cell voltage
- Internal battery alarm
In this case:
- Charge MOS may still be physically available
- But the inverter follows a zero-current command
This distinction is important.
BMS Charge Limit = 0A Does Not Automatically Mean a Fault
Suppose:
SOC = 100%
Cells = balanced
No alarm
BMS charge-current limit = 0A
This may simply indicate that charging is complete.
Now suppose:
SOC = 40%
BMS charge-current limit = 0A
Then further investigation is appropriate.
Always interpret one BMS value in context.
What If the Inverter Says “Battery Full” but SOC Is Low?
Possible causes include:
- Communication mismatch
- Incorrect battery protocol
- Inverter interpreting voltage incorrectly
- SOC synchronization issue
- BMS charge limit at zero
- Incorrect charging-voltage settings
Check both: inverter data and BMS data
Do not rely on only one display.
Charger Detection Can Also Be the Problem
Some chargers will not start unless they detect a minimum battery voltage.
If the BMS charging path is open, the charger may see unusual or no battery voltage and refuse to start.
The battery itself may still power loads through its discharge path.
This creates a confusing situation:
- Battery works
- Charger says “no battery”
A manufacturer-approved activation or charging procedure may be required.
Check Charger Voltage Before Blaming the Battery
Use a multimeter to confirm whether the charger is actually producing the expected output.
Possible charger-side faults include:
- AC input missing
- PV voltage insufficient
- Charger fuse open
- Incorrect polarity
- Charger disabled in software
- Charge-current setting = 0
- Wrong battery profile
A BMS cannot accept charging power that the charger never provides.
Polarity Should Be Checked Carefully
Reverse polarity can:
- Prevent charging
- Damage equipment
- Trip protection
Do not repeatedly reconnect a charger until polarity is confirmed.
In high-current ESS systems, polarity mistakes can be severe.
Use proper commissioning procedures.
Example: Charge MOS Off Due to High Cell Voltage
Suppose:
Pack SOC: 92%
Maximum cell: high
Minimum cell: noticeably lower
Charge MOS: OFF
Discharge MOS: ON
Likely explanation:
One cell reached the high-voltage protection threshold.
A controlled discharge may allow the high cell voltage to decrease.
But if the condition repeatedly returns, investigate:
- Cell balance
- Charging voltage
- BMS balancing
Do not simply raise the charger voltage.
Example: Charge Disabled at 35% SOC
Suppose:
SOC: 35%
Charge MOS: OFF
Discharge MOS: ON
Battery temperature: −3°C
This strongly suggests low-temperature charging protection.
The solution is not:
- Higher charging voltage
- Larger charger
The battery must return to an approved charging temperature.
Example: Charge Disabled During Midday Solar
Battery SOC: 55%
Temperature: normal
No cell overvoltage
BMS log:
Charge Overcurrent
Possible explanation:
Multiple charging sources are active or inverter current is set too high.
Reduce charging current to an approved level and investigate the system design.
What About a Failed Charge MOSFET?
Hardware failure is possible.
Symptoms may include:
- Charge command enabled
- No relevant protection alarm
- Correct charger voltage
- Normal temperature
- Charging current remains zero
- Discharge works normally
However, hardware failure should be diagnosed only after normal protection conditions have been excluded.
Do not replace the BMS based only on the symptom:
“Battery discharges but does not charge.”
Contactors Can Also Fail
Larger ESS batteries may use contactors rather than only MOSFETs.
Possible issues include:
- Contactor not closing
- Coil fault
- Precharge sequence failure
- Auxiliary-contact feedback error
These systems should be serviced according to manufacturer procedures.
Do not bypass internal contactors.
Why Bypassing the BMS Is Dangerous
A user may try:
“I will connect the charger directly to the cells.”
This defeats battery protection.
The BMS may be blocking charging because of:
- Cell overvoltage
- Temperature
- Overcurrent
Bypassing it can create a dangerous condition.
Use only manufacturer-approved recovery procedures.
The Correct Diagnostic Order
When a battery can discharge but will not charge, check the system in this order.
Step 1: Confirm Charger Output
Check voltage and charger status.
Step 2: Read BMS Alarm
Look for:
- Cell overvoltage
- Charge overcurrent
- Low temperature
- High temperature
Step 3: Check Charge/Discharge Enable Status
Determine whether charging is intentionally disabled.
Step 4: Review Cell Voltages
Compare highest and lowest cell.
Step 5: Check Temperature
Review all available sensors.
Step 6: Check Charge-Current Limit
In a communication system, see whether BMS CCL is zero or reduced.
Step 7: Review Inverter Settings
Check voltage and current.
Step 8: Review Alarm History
The present condition may have recovered, but historical logs can show why charging stopped.
Ask for Data Before Requesting a Battery Return
For distributors, this type of complaint should not immediately become an RMA.
Ask the customer for:
- Battery model
- Battery SOC
- Pack voltage
- Individual cell voltages
- Battery temperature
- Charge MOS status
- Discharge MOS status
- BMS alarms
- Inverter model
- Charging voltage
- Charge-current setting
- Screenshot of inverter power flow
This information can often identify the issue remotely.
Why Alarm History Is Important
Suppose charging has already stopped.
The cell that caused protection may have relaxed.
Current BMS screen may show:
No alarm
But the event log may show:
Cell Overvoltage Protection – 13:42
Without the history, the cause may be missed.
For intelligent ESS batteries, alarm logs are one of the most useful after-sales tools.
Is a BMS Reset the Solution?
Resetting a BMS can clear certain temporary states.
But reset should not be used to hide an unresolved protection event.
If the battery repeatedly returns to:
- high-voltage protection
- charge overcurrent
- overtemperature
the root cause must be corrected.
Resetting the battery every day is not normal operation.
For Parallel Battery Banks
If one battery in a parallel bank stops accepting charge while others continue charging, inspect that module individually.
Possible causes include:
- Different SOC
- High cell voltage
- Temperature
- Communication address
- Branch breaker
- Cable connection
- BMS protection
Total bank charging current can make it difficult to notice that one battery is offline.
Check module-level data.
Why This Matters for Large ESS
In a multi-module system, one battery dropping out of charging can cause the remaining batteries to accept more current.
For example:
Four batteries share:
200A
approximately 50A each.
One battery disables charging.
The remaining three may now need to share much more current depending on inverter control.
A master BMS should reduce total charging power where appropriate.
This is one reason communication and current monitoring are important in modular ESS.
Common Misdiagnoses
“The Cells Must Be Bad.”
Not necessarily.
Check BMS protection first.
“The Charger Is Too Small.”
A small charger should still normally produce some current.
Zero current suggests another issue.
“The Battery Can Discharge, So the BMS Must Be Fine.”
Not necessarily.
Charging and discharging may be controlled separately.
“I Should Increase Charging Voltage.”
This can make a high-cell-voltage problem worse.
“Resetting Fixes It.”
Only if the original cause has also been resolved.
A Useful Troubleshooting Table
| Symptom | Possible Cause |
|---|---|
| Discharges, no charge, battery cold | Low-temperature charge protection |
| Discharges, no charge, one cell high | Cell overvoltage |
| Charge starts then immediately stops | Charger voltage/current too high |
| Charge limit = 0A via CAN | BMS command or protection |
| Charger says no battery | Charge path open / detection issue |
| No alarm, charge path still off | Possible configuration or hardware issue |
| One parallel module won’t charge | Module-level protection or branch issue |
The table provides starting points—not final diagnosis.
Preventing the Problem
Many charge-disabled events can be prevented by:
- Correct inverter settings
- Appropriate charge current
- Correct CAN/RS485 communication
- Good thermal management
- Proper cell balancing
- Coordinated multiple charging sources
The BMS should protect against abnormal conditions.
It should not be expected to correct a poorly configured charging system every day.
Conclusion
A LiFePO4 battery can sometimes discharge normally while refusing to charge because the BMS treats charging and discharging as separate operations.
Charge-only protection can be triggered by:
- High cell voltage
- Low temperature
- High temperature
- Charge overcurrent
- Communication control
- Hardware issues
The correct troubleshooting approach is to ask:
Why has the charging path been disabled?
rather than immediately concluding that the entire battery has failed.
HIZN LiFePO4 energy storage batteries use intelligent BMS systems to monitor cell voltage, current, temperature and charging/discharging status, with CAN/RS485 communication available on applicable models.
If a battery can power your inverter but will not accept charging current, send us the BMS alarm screen, individual cell voltages, temperature, inverter model and charging settings. This information can help distinguish a protection event from an actual hardware fault.