BMS Output Short Circuit Protection Analysis Report

When BMS connected to inverter meet output short circuit protection

BMS short circuit protection is designed to prevent excessive current, abnormal loads, and unsafe operating conditions when battery systems connect with high-power inverters.

Problem:

A 16S-48V-200A BMS to connect a Volitronic 11-kW inverter,  the BMS triggered output short-circuit protection and could not be powered on, making it unusable.

Frequency: 2000PCS/multiple occurrences

Cause Analysis:

1. The system power demand far exceeds the BMS rated output and does not match the calculated total output demand

1. The total power of three 11kW inverters is 33kW. Assuming the inverter efficiency is 90%, the input power is approximately: 36.67kW
2. In a 48V system, the input power requirement is 10.24kW

         51.2v*200A=10.24kW

2. Surge current avalanche effect – the output current is too large, causing the BMS to trigger short-circuit protection

Inverter characteristics:

1. The starting surge current of a single inverter can reach 300A (5-10 times the rated current); so when three inverters are started in parallel, the superposition of the surge current may exceed 900A.
2. Far exceeds the BMS 200A short-circuit protection threshold – short-circuit protection threshold 500A.
3. The opening steps are not standardized.

The inverter was not fully pre-charged, and the voltage at the battery end was always stuck, causing the battery to trigger output short-circuit protection.

We can conclude that:

1. The rated output of the BMS is only 10.24 kW, and the actual required power is 3.82 times the rated value.
2. At the moment of startup, there is no load, and the charging current of the capacitor inside the inverter can trigger the protection. The BMS detects that the current exceeds the threshold and directly determines that there is a short circuit.
3. There was a problem with the power-on sequence operation. There was always a voltage stuck at the battery end, causing the BMS to trigger short-circuit protection.

Solution

1. Reconstruct the system architecture – add BMS, and each BMS group operates with one inverter. This approach helps avoid unnecessary BMS short circuit protection activation caused by inverter overload and startup surge current.
2. This approach helps avoid unnecessary BMS short circuit protection activation caused by inverter overload and startup surge current.
3. Connect the inverter to the grid and let the grid pre-charge the inverter. After the inverter is pre-charged, start the battery. – If you really need to use one battery to power multiple inverters, you must strictly follow the following startup steps. The startup sequence must be as follows:

Connect the inverter to the grid → Turn on the inverter → Turn on the battery → Open the circuit breaker (battery output)

Based on the content of the appeal, the following conclusions can be drawn:

1. The inverter power demand is seriously mismatched with the BMS current threshold, and the combined surge impact leads to serious defects in the protection system design;
2. If you want to follow the existing plan without making any changes, you need to follow the steps below so that the system can be used normally.

Connect the inverter to the grid → Turn on the inverter → Turn on the battery → Open the circuit breaker (battery output)

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