Inverter fuse concerns and recommendations for MultiPlus II 2x120V

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rvnomad asked Jun 16, '24

Inverter fuse concerns and recommendations for MultiPlus II 2x120V

My setup:

50-amp RV with Onan 5500 generator
Renogy 400 watt solar panels with Victron SmartSolar controller
Battleborn 2x100 AH batteries
Victron MultiPlus II 2x120 inverter charger
Victron BMV-712 battery monitor.

I am troubleshooting a considerable voltage drop when running a microwave (150 Amp draw) and have found several sub-optimal details in the work done by the RV shop installer. This 300-amp fuse is installed between the battery (right terminal) and the inverter (left terminal). An IR thermometer shows that a spot on the fuse blade warms up to nearly 100 deg C!

The fuse is rated for 300 amps, but seems rather thin to be conducting such a load and has a voltage drop of about 0.2V across it. It is possible that the heat is coming from the cable and not from the fuse itself, but the cable lug does not heat up to the same degree.

My questions:

1. Is this kind of fuse appropriate? And is there a name for it?

2. Can I replace it with a push button circuit breaker like this one? (Example link: https://www.amazon.com/Hamolar-Circuit-Breaker-Manual-Holder/dp/B095YZN677 )

3. What should the fuse rating be? My highest typical load is about 160 amps from the two lithium batteries. Battleborn says that 200 amps is the normal current spec, with 300 amps for less than 30 seconds.

All suggestions are appreciated, especially for fuses / circuit breakers that are high quality and reliable.

Multiplus-II fuses

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Answer 1 · 2024-06-16T08:43:45Z

pwfarnell answered · Jun 16 at 08:43 AM

Do NOT use one of those breakers you show, there are many reports on this site of people having high resistance on them melting bodies, tripping at the wrong currents etc.

You do not state what type of Multiplus you have, a 3000VA model should be fused with 400A and should have 2x50mm2 cables. I do not like that type of fuse holder with the allen key (hex key), i am not sure that you can torque it up enough. Also, the indicated hot connection does not have a washer under the bolt head, this is very important as without a washer the tightening torque is much less effective and that will make the connection contact poor. Change the bolts to proper hex head brass bolts with washers and torque them up properly.

When you mention 0.2V drop on the fuse was that measured on the bolts, cable lugs or exposed fuse blade. ab300A fuse should itself not have a high voltage drop or get hot at 150A. I am not sure you really have enough battery, again as you do not give your inverter model I can not confirm.

btw, there is nothing wrong with fuses, my 3000VA 12V Multiplus has run for years through a 400A fuse without issue as have most others.

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rvnomad commented · Jun 16 at 09:12 AM

It’s the MultiPlus II 2x120 3000VA

Any particular type of material for the washers? Copper or stainless steel or does it matter?

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pwfarnell rvnomad commented · Jun 16 at 09:19 AM

Copper or brass, this is what Victron supply on their shunts. If you run the inverter at max load you a lot you might like to consider more battery capacity.

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rvnomad pwfarnell commented · Jun 16 at 03:29 PM

Max 150 amps for the microwave, and typically no more than 5-10 minutes at a time.

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rvnomad commented · Jun 18 at 06:21 AM

@pwfarnell

What’s a good torque for these contacts? Battleborn suggests 10 ft-lbs for their battery terminals. Is that a good number for the fuse as well?

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pwfarnell rvnomad commented · Jun 18 at 07:52 AM

I am metric but have checked US units, for M8 (5/16") 9ft.lb and for M10 (3/8") 15ft.lb are figures from Victron equipment (shunts / battery / inverter terminals)

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Answer 2 · 2024-06-25T04:37:37Z

rvnomad answered · Jun 25 at 04:37 AM

@pwfarnell

So I replaced the Allen key bolts with hex bolts and washers and torqued them to about 10 ft lbs.

Result:

The cable lugs no longer heat up.

The ends of the fuse no longer heat up.

The very center of the fuse (where the yellow arrow is pointing) heats up to about 50 deg Celsius. Is this a cause for concern?

Voltage drop across the inverter is still about 1.1 volts when running the microwave at 150 amps.

Next steps:

Check voltage drop across the inverter cable lengths.

Any other suggestions?

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pwfarnell commented · Jun 25 at 07:00 AM

A fuse is a resistor, when the current is high enough it has to melt so running a 300A fuse at 150A will generate some heat. The spec for 300A ANL is 0.35 milli ohm, so at 150A that is 8W so it will warm up a bit. I do not know the expected temperature but 50degC does not sound unreasonable.

Regarding next steps it is just a case of checking voltage drops with a multimeter and connections for temperature. A poor isolation switch is often the cause of voltage drop.

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