Pylontech US-Series PERM-FAIL — Permanent Protection, Module Locked Out

⚠ DANGER — this fault involves lethal voltage. Do not attempt a DIY fix. Call a qualified technician.

What it means: The module will neither charge nor discharge, the red alarm light is solid, and the room temperature is normal. PERM-FAIL is this site's label for that condition and not a code the battery displays — the US-series front panel carries only RUN, ALM and SOC lights and a buzzer, and shows no fault code at all. Pylontech calls this state permanent protection, and states the conditions that latch it — the voltage of a single cell has been higher than 4.2 V or lower than 1.5 V, or the temperature has been higher than 80 degrees C. The instruction the manufacturer gives for this state is one sentence long, and it does not include anything you can do on site. Switch off the module and contact your local distributor for repair. This is not a nuisance trip and it is not clearable by a power cycle, by the inverter, or by any setting. The BMS has latched itself off because a cell has already been outside the envelope in which it can be trusted.

Why does Pylontech show PERM-FAIL?

• Sustained over-charging that took a cell above 4.2 V — usually an inverter charge voltage set too high, or a lead-acid charge profile left in place after a battery upgrade.
• Deep over-discharge that took a cell below 1.5 V — typically a pack that ran flat and was then left flat, against a manual that requires a recharge within 12 hours of a full discharge.
• Cell temperature above 80 degrees C, which is a thermal event, not ordinary summer heat.
• Any of the above happening on a system with no BMS communication, where nothing was limiting what the inverter did to the pack.

The Pakistan factor

This state arrives in Pakistan by two routes, and both of them are preventable and both are somebody's install rather than the battery's fault. The first is the tubular-to-lithium upgrade where the inverter kept its lead-acid charge profile and held the pack above its window until a cell finally passed 4.2 V — often months later, long after everyone has forgotten the upgrade. The second is the pack that ran completely flat during a long outage, or in a house shut up for weeks over the summer, and was then simply left there. Pylontech requires a recharge within 12 hours of a full discharge and a top-up above 90 percent state of charge at least every six months in storage, and neither rule is commonly explained at the point of sale here. Ambient heat shortens both routes. Once the module has latched permanent protection there is nothing a local technician can safely do on site, and the battery revival services advertised around the electronics markets are genuinely dangerous on a latched LiFePO4 module — bypassing the latch means running a cell the BMS has already judged unsafe. Take it to the distributor with the paperwork instead.

How to fix it

1. Safe for a homeowner, and do this first — switch the module off at its power switch. On a US5000-B, switch the front-panel breaker off as well.
2. Safe for a homeowner only where the isolation is a switch or a breaker — open the disconnection device between the battery and the inverter, if one is fitted, so the rest of the bank is not left running with a permanently protected module still wired into it. Do not isolate it by pulling the power connectors. The US-series power connectors are self-locking and live, they require the lock button to be held while pulling, and that is installer work. If there is no switch or breaker to open, leave the wiring alone and say so when you call.
3. Do not attempt to reset or revive it. Do not power-cycle it repeatedly, do not try a recovery charge, and do not connect a bench charger or another battery to it. Every one of the three conditions that latches permanent protection means a cell has already been taken outside its safe operating envelope, and the latch exists precisely because that cell can no longer be trusted.
4. Do not open the module. Pylontech prohibits opening, repairing or disassembling the battery by anyone other than Pylontech or a party it has authorised, and states it accepts no responsibility for the consequences. The module's own label warns of arc flash and shock hazard from the DC voltage inside.
5. Call your distributor. This is the manufacturer's instruction for this state, not a fallback. Have the serial number, the invoice and the installation date ready, and tell them the module is in permanent protection.
6. If the module is hot, swollen, deformed, leaking, smells unusual, or has been wet, treat it as a damaged battery. Keep people away from it, move it no more than you must, and say those symptoms explicitly when you call. Pylontech's own emergency guidance states that damaged batteries may leak electrolyte or produce flammable gas and are not fit for use.
7. If a battery cell is on fire, the manufacturer's instruction is to cut off the external power source first and then use a large amount of water. For a fire in the cabling or other non-cell components, cut the power and use a dry powder or carbon dioxide extinguisher. Get everyone out of the room and call emergency services. Do not stay to fight a lithium battery fire.