Batterie lithium bloquée : Débloquer BMS PACE Delong + Deye

Lithium batteries from the Delong Top Energy range equipped with a PACE BMS (Protection And Control Electronics) are high-performance, reliable storage solutions. However, like any sophisticated protection system, the BMS can sometimes trip and block the battery, leaving the user with a confusing situation: the battery communicates with the inverter, displays a SOC (State of Charge) of 50% or more, but delivers no output voltage (0V).

In this technical article, we will detail the causes of this blockage, explain how to correctly interpret the monitoring data from a Deye installation, and above all, guide you step by step through the procedures for unblocking the PACE BMS.

🔍 Understanding the PACE BMS: Architecture and Role

The PACE BMS (used by Delong, Seplos, and other manufacturers) is an advanced battery management system that performs three critical functions:

• Cell protection: Real-time monitoring of the voltage, temperature and current of each LiFePO4 cell.
• Active balancing: Redistribution of energy between cells to maintain pack homogeneity
• Communication protocol: RS232/RS485/CAN interface to communicate with the inverter (Pylontech, Deye, Victron, etc.)

Key point: The BMS uses MOSFETs (power transistors) as "electronic switches" between the cells and the output terminals. When a protection is activated, these MOSFETs open instantly, physically breaking the circuit. This is why you measure 0V at the terminals while the battery contains 52V inside.

📊 Case Study: Analysis of a Real Blockage

Let's take a concrete example of a Deye 6kVA single-phase installation + Delong 15kWh Battery where the battery refuses to wake up. Here are the data read in the Deye monitoring interface:

🔬 Technical Diagnosis

Analysis of this data reveals a BMS software lock characterised by:

1. Voltage discrepancy: 0.72V measured vs 52.68V internal → Discharge MOSFETs are open
2. Zero discharge limit: The BMS is actively communicating with the Deye and sending it the formal command "No current allowed to be drawn".
3. Load allowed: The BMS would accept a load (195A), which rules out overheating or overvoltage.

Conclusion: The BMS went into protection following a short-circuit detection (probably during an attempt to connect to the inverter's loaded DC bus) and did not automatically reset this protection.

⚠️ The 5 causes of a PACE BMS trip

1. Short Circuit Protection

Trip : The BMS detects a sudden current draw greater than 300-400A for a few microseconds.

Typical scenario: Closing of the battery circuit breaker while the Deye inverter's DC bus (50V, empty capacitors) creates an electric arc. The BMS interprets this arc as a short circuit and opens the MOSFETs in less than 50µs.

💡 Tip: Deye inverters have 1000-2000µF filter capacitors that behave like an instantaneous short circuit when empty. This is the #1 cause of BMS blockages when adding a battery to an already powered system.

2. Under Voltage Cell

Trigger: One or more cells drop below 2.0V (critical threshold for LiFePO4).

Why it's misleading: Even if the overall SOC shows 50%, an unbalanced cell may be at 1.8V while the others are at 3.3V. The BMS switches off everything to protect this weak cell.

3. Over Temperature

Trip: Internal temperature > 60°C (warning) or > 80°C (definitive shutdown).

False positives: If an NTC sensor is disconnected, the BMS can read -40°C or +120°C and block the battery for safety reasons.

4. MOSFET failure (MOSFAIL)

Symptom: The battery refuses to charge OR discharge (only one direction blocked).

Cause: One of the power transistors (charging or discharging) is burnt out following a transient overvoltage (lightning, reverse polarity, etc.).

5. Communication error (Communication Loss)

Effect: If the BMS loses contact with the inverter for more than 60 seconds, some models go into safety standby (Discharge Limit = 0A).

🛠️ Unlocking Procedure Step by Step

Step 1: Visual diagnosis on the LCD screen

Before doing anything, consult the LCD screen of the Delong battery (use the MENU, ENTER, DOWN, ESC buttons to navigate):

• Red LED "ALM" (Alarm):
  • Steady light → Protection active (MOSFETs open)
  • Flashing → Warning (battery working but limit parameter reached)
  • Off → No alarm
• Cell Voltage" menu: Check that all cells are between 2.8V and 3.6V (16 cells for a 51.2V battery)
• Menu "Protect": Identify which protection is active (SC = Short Circuit, UV = Under Voltage, OC = Over Current).

Step 2: Hard Reset via the Physical Button.

According to the official Delong Stackable Series manual, the ON/OFF button (silver below the screen) has a built-in "Reset" function:

1. Isolate the battery:
   • Open the battery's DC circuit breaker (if present)
   • Disconnect the RJ45/CAN communication cable (to prevent the Deye from sending commands during the reset)
2. Forced shutdown:
   • Hold down the ON/OFF button for more than 5 seconds
   • All SOC and ALM LEDs must go out completely
   • The LCD screen goes out
3. Standby: Wait 30 seconds (allows the BMS microcontroller to clear RAM memory)
4. Restart:
   • Press ON/OFF for 1 to 2 seconds (short press)
   • The battery performs a self-check for 5-10 seconds
   • Observe the LEDs: if the red ALM LED remains off after start-up, you've got it!
5. Checking:
   • Measure the terminal voltage with a multimeter: it should read 52-53V (instead of 0V).
   • Reconnect the communication cable

⚠️ Important: If the red "ALM" LED lights up again immediately after the reset, this means that a hardware protection is really active (cell undervoltage, faulty temperature sensor, or damaged MOSFET). Proceed to step 3 (PC diagnostics).

Step 3: "Soft" reconnection with the Deye UPS

Once the battery has woken up (52V at the terminals), you must prevent it from reverting to a safe state when reconnecting to the system. Here is the pre-charging procedure:

Case A: You have several batteries in parallel

1. The Deye inverter's DC bus is already live (~52V thanks to the other batteries).
2. If you suddenly close the circuit breaker of the "dead" battery, it will see a potential difference and a current draw → new blockage
3. Solution:
   • Switch off the Deye inverter (AC and DC) → DC bus at 0V
   • Close the circuit breaker on all the batteries (including the one that was blocked)
   • Switch the inverter back on → it will "gently" charge its capacitors with the current from all the batteries in parallel

Case B: You only have one battery

1. Switch off the UPS completely (AC + DC)
2. Close the battery circuit breaker
3. Switch the UPS back on → no risk of arcing because the bus was empty

🔌 Pre-charge resistor: Deye inverters have an internal pre-charge circuit (resistance of a few ohms that limits the inrush current for 2-3 seconds). However, if you connect/disconnect the battery quickly, this system does not have time to activate. Hence the importance of switching off the UPS before doing anything.

Step 4: Software Unlock via PC (Advanced Method)

If the Reset button does not work, the BMS is in "Hard Lock". You must use the PC configuration software.

Hardware required :

• RS232 to USB cable (serial adapter) or RJ11 to USB cable depending on the port available on your BMS
• PBMS Tools" or "JBD Tools" software (supplied by the manufacturer or downloadable on request from Wattuneed support).

Procedure:

1. Connect the cable to the RS232 port on the battery (RJ11 telephone type socket on the side of the BMS)
2. Run the PBMS Tools software on your Windows PC
3. Select the correct COM port (e.g. COM3, COM4) in the "Settings" menu
4. Click on "Connect" → the battery parameters will be displayed
5. Look for the "Protection Status" or "MOSFET Control" tab.
6. You will probably see "Discharge MOSFET: LOCKED" or "Short Circuit Flag: TRUE".
7. Click on the "Clear Protection" or "Unlock MOSFET" button
8. Confirm and disconnect the cable → the battery should restart with 52V at the terminals

📱 Intelligent Monitoring: Preventing Blockages

A well-monitored installation prevents 90% of BMS blockages. Here are the parameters to monitor in the Deye interface (via LCD screen, Wi-Fi Stick, or cloud platform):

Use the Wattuneed sizing tools to calculate the maximum discharge current of your installation and check that your BMS is not undersized.

🔄 BMS PACE compatibility with Deye UPS

Delong batteries with BMS PACE communicate via the Pylontech CAN 2.0 protocol (also known as "PYLON Protocol"). This protocol is natively supported by Deye UPS since firmware version 4000+.

Recommended configuration on Deye

1. Menu "Advanced Settings" → "Battery Type" → Select "Lithium".
2. Menu "Battery Voltage Type" → Select "LV-48V" (Low Voltage 48V)
3. Menu "Lithium Battery Protocol" → Select "PYLON" or "PYLONTECH
4. Menu "BMS Baudrate" → Leave on "Auto" (or 9600 bps if communication problem)

📖Full guide: See our dedicated article Commissioning Guide Deye Batterie Pylontech UPS which details the configuration of the communication protocol (applicable to Pylontech compatible batteries such as Delong).

Communication check

Once configured, the Deye UPS displays the following information if the communication is working:

• BMS Voltage: 52-53V (real internal voltage)
• BMS SOC: 0-100% (consistent with the battery LCD screen)
• Charge/Discharge Current Limit: Dynamic values (change according to temperature and SOC)
• Battery Factory: Displays "PYLON" or "DELONG".

If the UPS displays "Battery Factory: Unknown" or "BMS Voltage: 0V" while the battery is on → wiring problem or incorrectly configured protocol.

❓ FAQ : Frequently Asked Questions

1. My Deye inverter displays "Battery Voltage 0V" but the battery is on, what should I do?

Answer : This is the classic symptom of a BMS in protection. Follow the "Hard Reset" procedure (ON/OFF button for 5 seconds). If this doesn't work, check the battery LCD screen to see if an alarm is displayed (red LED). An undervoltage cell (< 2.5V) prevents the BMS from activating the discharge MOSFETs.

2. The Deye's "Activate Battery" mode does nothing, why?

Answer: The Deye's "Activate Battery" function sends a small charge pulse (a few amps for 10-20 seconds) to "wake up" a lithium battery in deep sleep (cells at 2.0-2.5V). But if the BMS is in 'Hard Lock' protection (Discharge Limit = 0A), this function fails because the MOSFETs remain open. The BMS must first be unlocked using the physical Reset button.

3. Can I force the Deye inverter to ignore the BMS limits?

Answer: No, and this is a fundamental safety feature. If you configure the inverter in "Lead-Acid" mode instead of "Lithium", it will ignore the BMS commands and apply its own charge/discharge thresholds. Result: risk of cell destruction by overcharging/deep discharging. Never use a lithium battery in "Lead-Acid" mode.

4. How many times can I reset the BMS before damaging the battery?

Answer: The "Hard Reset" (physical button) is a safe software operation, you can do it as many times as necessary. However, if you have to reset the BMS several times a week, there is an underlying problem (weak cell, faulty temperature sensor, unstable wiring). Contact Wattuneed technical support for a detailed diagnosis.

5. What is the difference between "Battery Voltage" and "BMS Voltage" in Deye monitoring?

Answer:

• "Battery Voltage": Voltage measured by the inverter at the physical terminals of the Plus/Minus cables (can be 0V if MOSFETs open).
• "BMS Voltage": Internal voltage communicated by the BMS via the CAN protocol (reflects the true cell voltage).

If these two values differ by more than 1V, the MOSFETs are open (active protection).

6. My BMS displays "MOSFAIL", is this repairable?

Answer: "MOSFAIL" means that the BMS has detected a power transistor failure (burnt-out charge or discharge MOSFET). This cannot be repaired by software. The BMS board must be replaced (under warranty if manufacturer's fault) or the complete battery, depending on age and cause. Contact Wattuneed support immediately with your serial number.

🎯 Good Practices to Avoid Blockages

1. Initial installation: Always connect batteries BEFORE powering up the UPS (avoids current draw from empty capacitors).
2. Adding a battery: Switch the UPS off completely, add the new battery, then switch it on again (automatic precharge).
3. Preventive maintenance: Carry out full balancing (100% charge for 4-6 hours) every 3 months.
4. Temperature: Install the batteries in a frost-free (> 5°C) and ventilated (< 35°C) area.
5. Monitoring: Activate email/SMS alerts on the Deye interface to be warned if the "Discharge Limit" falls below 50A.
6. Firmware update : Regularly check BMS (via PC software) and Deye inverter updates.

🛒 Wattuneed Solutions for Worry-Free Installation

To ensure perfect compatibility and avoid BMS communication problems, Wattuneed offers pre-configured and tested packs:

🔋 SEE PACK DEYE 6KVA + DELONG 5KWH

This pack includes:

• Deye 6 kVA single-phase hybrid inverter (Pylontech protocol settings included)
• Delong Top Energy 5.12kWh lithium battery (51.2V 100Ah) with BMS PACE
• Pre-wired CAN communication cables
• Detailed commissioning guide
• 7/7 Wattuneed technical support

To speed up the processing of your request, please have ready :

1. Battery serial number (label on the side)
2. UPS model and firmware version ("System Info" menu)
3. Screenshots of Deye monitoring ("Battery" and "BMS" pages)
4. Photo of the battery LCD screen ("Cell Voltage" and "Protect" menus)