Input filter capacitance of MultiPlus-II 48/5000/70-50 120V need for pre-charge resistor calculation

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Hi all. I am building a 48V LiFePO4 battery system using a MultiPlus-II 48/5000/70-50 120V (North American version, UL1741 listed) as my inverter/charger.

I am designing a manual pre-charge circuit to safely charge the inverter’s input capacitors before closing my main DC disconnect. This protects my JK BMS FETs from inrush current damage on startup.

To calculate the correct pre-charge resistor value I need to know the input filter capacitance of this specific unit. The formula is T=RC, and without knowing C I cannot determine the correct resistance value with confidence.

The datasheet does not list this figure. Could someone from Victron or a knowledgeable community member provide the input filter capacitance in microfarads for the MultiPlus-II 48/5000/70-50 120V specifically?

For context I want to confirm:

  • The correct resistance value in ohms

  • The correct minimum wattage rating for the resistor

  • Approximately how long pre-charge should take at that resistance before it is safe to close the main disconnect

Thank you.

2

25 ohms

50 watts

1 second.

That will limit precharge current from a 48v battery to 2A.

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3

Im usually using a 33ohm 10watt resistor, wired in series with a push button, but of course you can also automate that

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4

Hey, check out this video: [YouTube] Soft Start Your Inverter using light bulb.

Just use a 48V light bulb, no calculations needed. You can buy these bulbs with E27 connection (a common standard, at least in Europe). As shown in the video, when the bulb stops emitting light, your circuits are precharged. Easy peasy.

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