Hi everyone,
I'm hoping for some help diagnosing what appears to be an inverter DC-bus capacitor inrush / LiFePO4 BMS protection issue after replacing my old flooded lead-acid bank with a WattCycle lithium battery.
System
Schneider Conext SW2524 inverter/charger
MidNite Classic 150 charge controller
Whizbang Jr.
WattCycle 24V 314Ah LiFePO4 battery
Schneider/MidNite-style E-Panel
250A DC breaker between battery and inverter
The previous battery bank was a lead-acid bank had no problem powering up the Conext. The problem appeared only after changing to the WattCycle LiFePO4 battery.
Current problem
With the WattCycle connected, the solar charging side is working normally. The Classic can charge the battery, the WattCycle BMS shows normal operation.
The problem occurs when I try to connect the Conext inverter to the battery by closing the 250A DC breaker. As soon as the inverter DC connection is made, the WattCycle battery trips to protect itself. The Conext display itself does not have time to power up.
Perhaps the Conext capacitors are drawing a very high instantaneous current when the breaker closes and the WattCycle BMS is interpreting that as a danger?
The Classic has been adjusted for the new custom lithium settings. Equalization is disabled. The Whizbang Jr. has been reconfigured as well.
I'm considering installing a manual pre-charge circuit across the 250A DC breaker consisting of:
Momentary pushbutton
Appropriate resistor
Small fused pre-charge wiring
The idea would be:
Leave the 250A breaker OFF.
Press and hold the pre-charge button.
Allow the Conext input capacitors to charge through the resistor.
Close the 250A breaker.
Release the pre-charge button.
Finally, My Questions Are...
Does this symptom sound consistent with Conext capacitor inrush exceeding the WattCycle BMS instantaneous current limit?
Has anyone successfully used a pre-charge resistor with a Conext SW inverter?
Is there anything in the Classic 150 settings, Conext configuration, or startup sequence that could avoid needing a dedicated pre-charge circuit?
Is there anything else you would check before concluding that this is simply a BMS/inverter inrush incompatibility?
Thanks for any insight.
This is a common issue with some inverters. You need to precharge the capacitors using a resistor or other methods. so your plan sounds pretty good. You only need to do it for the initial start up . Some lithium batteries have a precharge circuit in them , guess yours doesn't. There should be info on youtube or solar forums on how to do the precharge. You are correct that the inrush current to capacitors exceeds the protection amps the bms is designed for , so it shuts the battery off. Another solution would be to get another battery and then you might have enough amps and this won't happen or need the precharge . One of my batteries included a 50 watt 20 ohm resistor with it , but I never needed to use it .
Larry