Pylontech US-Series CHG-DIS-OC — Charge / Discharge Over-Current Protection

What it means: The BMS has opened the current path because charge or discharge current went past its limit. CHG-DIS-OC 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. Charge and discharge over-current is one of the protections Pylontech lists as a basic BMS function on these modules, and the US-series troubleshooting gives the reference figure plainly — if current exceeds 90 A, battery protection will turn on, and the remedy is to check whether the current is too large and change the settings on the power supply side. Read that figure alongside the per-model ratings rather than instead of them, because they are not the same number. The US5000 spec table gives 80 A recommended, 100 A maximum continuous, 101 to 120 A for 15 minutes and 121 to 200 A for 15 seconds. The US3000C is rated far lower, at 37 A recommended and 74 A maximum continuous. A US3000C can therefore be worked well beyond its rating without the protection ever firing. On a US5000-B there is also a 125 A type C breaker on the front panel that can trip instead of, or before, the BMS.

Why does Pylontech show CHG-DIS-OC?

• Too few battery modules for the inverter's output. Pylontech's multi-group instructions state that each pair of power cables holds a maximum of 100 A constant current and that enough pairs must be connected based on the calculated system current.
• A heavy motor start — a deep-well or booster pump, an air-conditioner compressor, a welding set — drawing a surge the bank cannot supply.
• The inverter's maximum charge or discharge current left unlimited, which is what happens by default when there is no BMS communication telling it what the pack will allow.
• High cell temperature. The manual's footnote to the ratings table states that the recommended and maximum continuous operating currents apply for a battery cell temperature within 10 to 40 degrees C, and that outside that range the operating current is derated. A hot pack therefore protects at a lower current than the same pack would when cool.
• Undersized, over-long or badly terminated battery cabling, which both limits current and heats the joints.
• On a US5000-B, the front-panel breaker releasing. The manual's reminder is that when the breaker has released for protection you check the root cause of the current surge and the cable connection between battery and inverter first, and only then try to connect again.

The Pakistan factor

Two Pakistani habits collide in this fault. The first is the load mix — a 1 to 1.5 HP water pump and one or two inverter air-conditioners running off the same bank is completely normal here, and the pump's starting surge is what takes a single-module bank past its limit. The second is buying modules one at a time as budget allows while fitting the full size inverter from day one, so a 5 or 6 kW inverter sits on a bank rated for a fraction of what it can pull. Heat then makes both worse in a way most owners never connect to the symptom. Because the rated currents only hold for a cell temperature of 10 to 40 degrees C and are derated outside it, a bank in a 45 degree store room protects at a lower current in June than the same bank did in March, with no change in the load at all. If your system started tripping when the weather turned, look at the room before you look at the battery.

How to fix it

1. Safe for a homeowner — note what was actually running at the moment it tripped. A trip that always happens when the water pump starts, or when the second air-conditioner comes on, has already told you the answer. Write the pattern down.
2. Safe for a homeowner — reduce simultaneous load. Stagger the motor loads so the pump and the compressors are not starting into the same bank at the same time.
3. Safe for a homeowner on a US5000-B — if the front breaker has released, follow the manufacturer's own order. Find out what caused the surge and have the cabling between battery and inverter checked before you close it again. Repeatedly re-closing a breaker that keeps tripping is how a bad joint becomes a fire.
4. Installer work — check the inverter's charge and discharge current limits against how many modules are actually installed, and confirm that enough pairs of power cables are fitted for the system current rather than one pair carrying the lot.
5. Installer work — establish BMS communication so the pack sends its own charge and discharge current limits instead of the inverter working from a number somebody guessed.
6. Installer work — check cable gauge, lug crimps and terminal torque. Pylontech asks for the power connectors, grounding point, cable and screws to be checked every year after installation for looseness, breakage and corrosion. A hot joint at a battery terminal is a fire risk, not a voltage-drop nuisance.
7. Stop and call your installer if the protection trips at what should be a normal load, or if any cable, lug or terminal is discoloured, deformed or hot to the touch. Do not keep resetting it and do not open the module.