question

I don't think a Victron DC-DC would be any different to your current DC-DC.
The problem is due to the high capcitive input that most DC-DC converters will present at their input. The high capacitance causes a high inrush current.
The MPPT load output senses this high current, assumes a fault and switches the output off.

One method which might help is to use a ICL (Inrush Current Limiting) NTC (Negative Temperature Coefficient) resistor in series between the MPPT load output and the DC-DC converter. One of these may help you, but bear in mind that it is a resistor and it will waste energy in heat. How hot it gets depends upon how much continuous current is flowing through it.

I have used an Epcos B57127P0508M301 NTC in the past and it could start high power lightbulbs but it would get hot at high current. 5A load is probably acceptable, but of course the wasted heat rises exponentially as the current rises, so maybe do some calculations first or experiment for yourself.

Thank you for your very helpful reply,

The constant load is under 2amps @ 24v so I guess it wont hurt to try a NTC resistor. A few more losses would be acceptable on the scale of things.

Q, did you use just one NTC resistor, in series on the negative side of the mppt load output?

Im also considering other options to read 'system load' if victron cant help me, I have a cheap but very cheerful smart bms it gives a 'battery load'. Which is accurate when theres no solar input, If I take the 'battery load' reading from the bms and add to that the 'PV input' from the Bluesolar MPPT and i should be close to 'system load'. i guess?

suggestions always welcome

Since you dont seem to need to disconnect the DCDC, Switch Load to always on or very low off settings and precharge the DCDC before connecting to Load output. Maybe even with a switch and resistor directly from the battery or two switches from the load output, one direct one plus one with resistor in line for precharging for future ressetting.

A bit more of a hassle when it actually disconnects to reconnect but it shows (rough) data.

I solved this problem with a 100/20. Seems to be less whingey about in rush current.

Thats great thank you, In the case of your 100/20 did you use a normal type resistor.?. ie not NTC.

I dont have any as low as 0.5 ohms, but then i guess i could have a go with a larger resistor in waiting to order. If i understand correctly, Your idea of change over switches means the resistor or thermistor wont be in the final circuit. So losses through the resistor/thermistor are less important.

Thank again for the feed back