Pylontech US-Series DIS-UV — Discharge Under-Voltage Protection

What it means: The BMS has cut off discharge because the pack reached the bottom of its voltage window. DIS-UV 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, so identify the fault by the light pattern described here. Pylontech's US-series troubleshooting states that when the battery discharges to 44.5 V or less the battery protection will turn on, and gives the remedy as charging the battery until the red light turns off. The published discharge window is 43.5 to 53.5 Vdc on the US5000 spec table and 44.5 to 53.5 Vdc on the US3000C. On its own, one occurrence of this after a long outage is the protection doing exactly its job. It becomes a real problem when it happens most nights, or when the module is then left flat and will no longer power on.

Why does Pylontech show DIS-UV?

• The bank simply ran flat — a long outage with no grid and no sun to recharge it.
• A bank sized for a shorter outage than the one it met, or a heavier evening load than it was quoted for.
• The inverter's own low-battery cut-off set below the BMS trip point, so the inverter keeps drawing until the battery has to protect itself instead of the inverter backing off first.
• No BMS communication, so the inverter has no real state of charge and no discharge limit from the pack and simply pulls until the voltage collapses.
• On Growatt SPF and M series pairings specifically, Pylontech's compatibility list requires depth of discharge to be set to 80 percent or lower, warning that otherwise it may lead to system shutdown and the need to manually recharge the battery.
• A module left discharged after the event. The manual requires the battery to be recharged within 12 hours after it is fully discharged.

The Pakistan factor

In Pakistan this is the most ordinary Pylontech alarm there is, because it is what a battery does at the end of a long load-shedding evening. That is not a defect. What turns it into a defect is what happens next. A bank sized around a four-hour outage meets an eight-hour one, the protection trips, and then the module sits flat for days waiting for a technician or waiting for the grid, against a manual that asks for a recharge within twelve hours. Houses shut up for weeks over the summer or during Eid are the other version of the same story, and the six-month storage rule is almost never explained at the point of sale here. The settings angle matters too. Where there is no BMS handshake — very common on the Axpert-platform hybrids sold under local brand names — the inverter is running on a low-voltage cut-off somebody typed in once, and if that number sits below the BMS trip the battery is being taken to its floor every single night rather than the inverter stopping first. A pack that reaches under-voltage protection nightly is being worn out on purpose.

How to fix it (safe to try yourself)

1. Safe for a homeowner — recharge it. Pylontech's own solution for this condition is one line, charge the battery until the red light turns off. Grid or solar, whichever you have first.
2. Safe for a homeowner — do not leave it sitting flat waiting for a technician. The manual requires a recharge within 12 hours of a full discharge, and requires a top-up above 90 percent state of charge at least every six months for any module in storage. Time at empty is what turns a routine protection trip into a dead module.
3. Safe for a homeowner — if this happens most nights, the problem is sizing or settings, not the battery. Note how many hours of backup you actually get and roughly what you are running, and take those numbers to your installer.
4. Installer work — set the inverter's own low-voltage cut-off above the BMS trip so the inverter stops first and the BMS never has to. On a Growatt SPF or M series pairing, Pylontech asks for depth of discharge at 80 percent or lower.
5. Installer work — establish BMS communication if there is none. Discharging on a guessed voltage threshold is what walks a pack down to this point again and again.
6. Stop here if the module has been flat and now will not power on at all — you press the SW button and no lights come on. Recovery from deep over-discharge is not a homeowner job. Pylontech's procedure requires a 48 to 53.5 V source and, where the terminal voltage is at or below 45 Vdc, a charge rate of 0.05C or less specifically to avoid damaging the pack's state of health. Getting that wrong costs you the battery. Call your installer or the distributor and tell them the module is over-discharged and will not start.
7. Never try to wake a flat module by wiring it straight to the solar array. Pylontech's safety instructions prohibit connecting the battery to PV solar wiring directly. Do not open the module either.