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bob-bobson avatar image
bob-bobson asked

MPii (3kVA 48V) DC input terminal question

Hi, All

I am a new owner of a 3kVA Multiplus II (48VDC, 220VAC 50 Hz), and I have a few basic questions about the DC input terminals on these units.

1. My unit's positive terminal is a big shiny hunk of metal, whereas the negative terminal looks to be just a wide piece of tinned PCB track.

This looks a bit flimsy, considering that the OEM recommends a 125 A DC breaker on this line, and that at a moderate load we expect tens of amps to pass through this connection. Am I missing something here?

2. Following on, I have done a temporary setup for configuration and testing, passing only 4 to 8 amps or so with a temporary battery connection consisting of a 1.5 mm**2 cable (10 A fused) looped around the post and clamped under the nuts, and today I was sorting out something more permanent with crimp lugs. When I removed the old connection on the negative terminal, I noticed that where the copper has been in contact with the tinned PCB track, the track looks a matt grey colour instead of the shiny tin of the rest of the strip. The contact resistance is very small (fraction of a millivolt with a few amperes flowing) but I am concerned I might have damaged something during my temporary setup period. Any thoughts?


Thank you in advance!

battery connection
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2 Answers
Alexandra avatar image
Alexandra answered ·

@Bob_Bobson

You fuse for cable size not load/inverter size. So use the recommended battery cable as well as fuse. Fusing is never there for the equipment, it has its own protections for that.

The discoloration may have been from arcing as a possibility. Wrapping is not good contact. But should be ok.

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bob-bobson avatar image bob-bobson commented ·

Of course, you are quite right about the purpose of fusing being to protect the cable. I am fusing (conservatively) to protect a battery cable that can carry only about 15A in a temporary configuration, to be able to set up and afterwards run some important small loads with very unreliable AC mains power, until such time as I can finish making my permanent cables up and commission the system properly. It is not pretty, but it is safe and includes earth leakage protection and appropriate overcurrent protections.

Not ideal, these temporary DC cables, but cannot be helped, and we don't want some idiot plugging a big appliance into the AC while we're inverting and making incandescent battery cables ...

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bob-bobson avatar image
bob-bobson answered ·

To help answer my own question, the electrodes appear to be very soft, so for example once the nuts are tightened down, any text engraved onto lugs gets transferred to the electrode surfaces. I think my "discoloration" was just fine indentations from the wire loop, now most probably smooshed down again by the newly fitted lug.


Any input from @Victron about the real reason for the electrode asymmetry would be greatly appreciated, but I think that the bar on the +ve electrode is an appendage of the heat-sink of the battery charger (or inverter?) which possibly has transistors with internally connected cases screwed right onto it.

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