Charging MOSFET Breakdown Issues and Solutions | BMS Optimization Guide

In energy storage systems and lithium battery applications, charging MOSFET breakdown is one of the most common issues reported by users. Recently, after analyzing several cases, we identified the root causes and developed effective solutions, including firmware upgrades and hardware optimizations.

This article provides a comprehensive overview of the problem—from the phenomenon and causes to practical solutions—helping engineers and users prevent MOSFET breakdown and improve system reliability.

1. Problem Phenomenon: Frequent Charging MOSFET Breakdown

Some users reported that the BMS charging MOSFET frequently fails or breaks down during operation. This not only affects the stability of battery charging and discharging but may also cause the entire energy storage system to shut down, resulting in extra maintenance costs.

2. Root Cause Analysis: Overvoltage & Inverter Reverse Current

Through in-depth analysis, the core mechanisms leading to MOSFET breakdown are as follows:

Overvoltage Protection Triggered
When the battery voltage reaches the overvoltage threshold, the BMS closes the charging MOSFET to cut off the charging circuit.

Inverter Reverse Current (Backfeed)
Some off-grid inverters (such as Victron, Voltronic, etc.) may generate reverse voltage (backfeed) when the MOSFET suddenly shuts down, feeding back into the battery side.

Transient Overstress
If the reverse voltage is too high, the charging MOSFET and TVS diode are exposed to transient stress simultaneously, making them highly prone to breakdown and permanent damage.

In short: BMS passive shutdown + inverter reverse current = MOSFET breakdown.

3. Solutions: Firmware + Hardware Optimization

To completely address the issue, we provide a two-pronged solution:

(1) Firmware Optimization (Version V1.5)

Active Current & Voltage Control:
When the single cell voltage reaches 3.55V × N (N = series count), the BMS actively sends commands to the inverter:

Request Current: 0A (instructs inverter to stop charging)

Request Voltage: 3.55V × N (instructs inverter to reduce target voltage)

With this logic, the inverter gradually decreases or stops charging, avoiding direct overvoltage protection and eliminating hard MOSFET shutdown, which significantly reduces reverse voltage risk.

(2) Hardware Optimization

Shunt Resistor Improvement: Optimized design for better current sharing among parallel MOSFETs, reducing stress on individual devices.

Faster Shutdown Speed: Enhanced driver circuit accelerates MOSFET turn-off, shortening reverse voltage duration and improving device resilience.

4. Implementation Recommendations

Upgrade Firmware: Users are strongly advised to update to V1.5 or above, which greatly reduces MOSFET breakdown risk.

Inverter Compatibility: The upgrade is optimized for inverter brands on Seplos’ compatibility list. For other brands, please confirm with Seplos.

New Equipment: All new shipments come with Seplos V1.5 firmware preloaded, along with optimized shunt and driver circuits.

User Best Practices:

Avoid frequent disconnection under high-power charging or near overvoltage conditions.

Set the inverter to “Battery Priority Control” mode to ensure proper execution of BMS current/voltage requests.

5. Conclusion

The root cause of charging MOSFET breakdown lies in the combination of BMS passive shutdown and inverter reverse current backfeed. With active firmware control and hardware-level enhancements, the issue has been effectively resolved.

For system integrators and end-users, upgrading firmware, using compatible inverters, and following best practices will greatly improve the safety, reliability, and stability of energy storage systems.