If the battery has stopped working, is not charging, or appears “dead”, even though everything seemed fine before — most likely the BMS protection has been triggered.

BMS is a Battery Management System. It ensures that all cells operate without overload, overheating, or short circuit. If something goes wrong, it simply “turns off” the battery to prevent damage to both the battery and the connected equipment.

Why the BMS locks

1. Over-discharge or overvoltage.

If even one cell is discharged below the allowed level or the voltage exceeds the norm, the system disconnects the battery. This is a common situation when the battery has been unused for a long time or is charged with an inverter that is too powerful.

2. Overheating or overcooling.

At temperatures below 0 °C, charging is prohibited because lithium crystals form inside the cells. In hot conditions above 50 °C, a lockout may also occur — to prevent ignition.

3. Excessive load or short circuit.

If the system “sees” that the current exceeds the allowed value, it disconnects instantly. This often happens due to improper inverter connection, especially if there are startup surges.

4. Cell balancing problems.

Over time, cells age unevenly and their voltages drift apart. The BMS detects this and reacts — especially during charging to 100%.

5. Loss of communication or firmware error.

In complex systems with CAN or RS485, any communication failure may cause a lockout, even if the cells themselves are fine.

How to unlock the system

1. Check the voltage at the terminals.

If the voltage is “0”, it does not mean the battery is empty. The BMS has simply disconnected the output. First, connect a control multimeter to each section or use the diagnostic port if available.

2. Try to “wake up” the BMS with a short charge.

In most cases, applying a small charging current (0,05–0,1 C) for a few minutes is enough for the system to exit sleep mode. Some LiFePO4 packs “wake up” even after a few seconds of applied voltage.

3. Cell balancing.

If the lockout is caused by voltage imbalance, the battery must be left on a charger in balancing mode (sometimes for several hours). The BMS will automatically level the cells and allow discharge.

4. Reset via service software.

Most “smart” BMS units (JBD, Daly, ANT, Seplos, Overkill Solar, etc.) have PC or smartphone apps. Through them, you can read errors and manually reset the lockout. However, it is important to eliminate the cause first, otherwise the system will lock again immediately.

5. Protection against deep discharge.

If the battery has been unused for a long time (especially for several months), the voltage may drop so low that the BMS “falls asleep”. In this case, you need a laboratory power supply with adjustable voltage to gradually “raise” the cells to a safe level.

6. Firmware update (for experienced users only).

If the BMS is “frozen” and unresponsive — re-flashing may help. However, do this only if you are confident in your skills, because a mistake can permanently damage the controller.

How to avoid repeated lockouts

• Do not allow deep discharge — use a power management system or backup charging.
• Do not use the battery in freezing temperatures without heating.
• Every few months, check cell balance through the app or communication port.
• If the battery is not in use, keep the charge level at 50–60% and store it in a dry place at +15…+25 °C.
• And most importantly — never short the terminals manually in an attempt to “bypass” the BMS. This can cause smoke and burning smell.

Conclusion

BMS lockout is not a failure — it is protection from failure. It responds not to “whims”, but to real threats to the battery. If you understand the causes and act calmly, 90% of cases can be resolved without replacing the battery.

The main thing — do not panic, do not randomly poke wires, and always have a good multimeter or monitoring app at hand.