So this morning walked into the garage where the inverter setup is and smelled something burning.

Killed the power and discovered the attached cable burn. It is where cable from busbar connects into inverter

 

Setup

Multiplus II 5000

Smart solar 150/85 with 12 by 280w panels in 4 strings

2 by pylontech us3000b.

Using 35mm cable between battery and busbar, busbar to inverter.

Got mersen 100A DC fuses on both poles of battery

Was drawing around 4000w two 2kw kettle, 2kw geyser,  2000w from battery and 2000w from panels. Installation about 2 months old. Dishwasher was on may have kicked into heating at same time as kettle (still training Mrs but she may have checked power and dishwasher element off). Have got software limiter of 4500w on inverter to limit battery draw.

I have a bad crimp, bad connection to board?

Edited September 21, 20196 yr by Paul Oosthuizen
More info

So just realized there was a washer between lug and board, humm presuming that resulted in the problem when there was a big draw. This make sense¿

Dammit small oversight potential big problem

Now to cleanup the mess on the contacts

Edited September 21, 20196 yr by Paul Oosthuizen

1 hour ago, Paul Oosthuizen said:

there was a washer between lug and board

That could do it. But it looks like a bad crimp to me, based on where I see deformation and/or discolouration. 

Also the lug seems quite different to the positive one. There doesn't seem to me much more than 10 mm x say 2 mm = 20 mm² between the crimp and the bolt. That's getting quite marginal; I think 35 mm² battery cable is already a little light on for a 5000 W inverter. As is 100 A for the battery fuses, but that's not relevant to this problem.

So I don't like that style of lug. I'd replace it with a more conventional one like the positive lug. Good luck with the cleaning up of the PCB; hopefully it won't be too damaged, and will have enough copper area after you clean off any discoloured bits.

Just crimp a lug on your yellow cable to for earthing. 

MPII 5K battery cable should be 70mm

MPII 3K battery cable is 35mm

 

Difference in the lugs is simply that I pulled some insulation tape off the black one,  they are identical lugs and cables. 

On the batteries- Pylontech supplies what I guess to be 25mm cable with around 2m length. This goes to busbar (4m round trip) Inverter can only charge 70a and discharge limited to 37A per battery=74A for two. I did software limit to 4500W through inverter through ESS. Thought 100A fuses all good? The battery cables have special clips that are not readily changeable. Seems racks of 4 batteries possible. They seem to feel these cables adequate as no alternatives possible?? So not sure about 70mm here.

Then the 35mm cable with approx 1m round trip run from busbar to inverter. I was getting 2600w odd from mppt and 2000w from battery. 4600w÷48v= 95A total running through washer was not good. Would have thought cable would have been fine for 35mm as 0.2% loss at 180A over 1m run,  36mm m8 Lugs don't believe a problem

Perhaps 50mm with larger lugs though, but this only drops loss from .3 to .2% with 9000w surge- which is below my supply of 3000w from panels and 3500w from batteries. Both seem acceptable losses.

 

The sizes I mentioned above are as per the Victron MPII manual.

mainly just curious 

 

Quote

Was drawing around 4000w two 2kw kettle, 2kw geyser,  2000w from battery and 2000w from panels. Installation about 2 months old. Dishwasher was on may have kicked into heating at same time as kettle

2 kettles? - but why xD

but seriously - 2 kettles, plus 2kw to geyser + dishwasher - that's like 8kw?

assuming the 2 kettles and dishwasher were on the same plug circuit? thats 6kw on the circuit, can the cabling handle that? you must have like a 25A breaker in? train the wife by replacing it with a 15A 😛

 

I am guessing you just meant a 2Kw kettle though, not 2 of - 6kw was still too much though

Edited September 22, 20196 yr by Dex_

2kw kettle, 2kw geyser and 2kw dishwasher. But multiplus grid connected so can draw excess from the grid. This is a grid tied setup

6 hours ago, Paul Oosthuizen said:

Perhaps 50mm with larger lugs though, but this only drops loss from .3 to .2% with 9000w surge- which is below my supply of 3000w from panels and 3500w from batteries. Both seem acceptable losses.

17 hours ago, Sean said:

MPII 5K battery cable should be 70mm

Paul, read what Sean said. 🙂 

 

I have asked the question before and no-one answered: Who adheres to the specifications the manufacturers give?

Bad crimps are most definitely always a problem waiting to happen no matter the cable thickness used.

But 35 mm² on a recommend 70mm² ?
Ps. Note the "2 x " mentioned on some of the specs. Who does that?
Pss. And the recommended cable lengths, who does that?

I don't care what people say, if the manufacturers engineers recommend specification, there is ALWAYS a good reason for that. Nike it.
Unless the specs given just appear to be "off" i.e. read "to low". Then one use common sense and up the ante.

Edited September 22, 20196 yr by Guest

Manufacturers instructions always overule any local regulations, and certainly take presendence over assumptions regarding "looking about right"

3 hours ago, Paul Oosthuizen said:

2kw kettle, 2kw geyser and 2kw dishwasher. But multiplus grid connected so can draw excess from the grid. This is a grid tied setup

ah cool I didn't know that, I would have thought it still was limited to the inverter rating

1 hour ago, Dex_ said:

ah cool I didn't know that, I would have thought it still was limited to the inverter rating

Correct, limited to the inverters grid tied maximum continues output. 

Like a 3kva is limited to 2.4kw continues output depending on temp.

To supply that continuous power, depending on where the ESS software process it is at, it will either take what it can from panels and batts or revert all back to Eskom whilst using all panel power to recharge the batts a bit before using them again. ESS is clever in that sense. 

3 hours ago, Guest said:

Correct, limited to the inverters grid tied maximum continues output. 

Like a 3kva is limited to 2.4kw continues output depending on temp.

To supply that continuous power, depending on where the ESS software process it is at, it will either take what it can from panels and batts or revert all back to Eskom whilst using all panel power to recharge the batts a bit before using them again. ESS is clever in that sense. 

well then by your description i am still right? because 6kw > 5kw? all you are saying is the unit can mix sources, not exceed capacity over the unit rating from grid directly?

So had a bit of this battery fuse cable discussion when planning my install. You can check out the posts on my account. Plonkster and others clarrified draw on the batteries is limited by Pylontech BMS with dvcc activated in ESS when I queried fuses and cabling. I thought having 100A fuses would be safer than 200A when my draw is Max 74A on batteries.

I did see fuse and cabling spec for Multi II 5kw. But that is for 200A draw and up to 5m return- I have 74A battery to busbar (and Pylontech is happy with their cabling even for 4 batteries on their install, so two manufacturers offering different spec here and don't even know how to source 70mm for pylontechs with their proprietary connection). I have 100A fuses on both battery busbar link and mppt busbar link.

I used Victrons downloadable app to check on cabling between busbar and inverter.  No one in this post has questioned the maths I offer? No one here seems to indicate that if my cable was good, doing the maths, it would have fried under the load I was running. Or did I miss something.

Reality is I still have a burnt cable, so either crimp bad- though pretty sure crimp is perfect, or washer where it should not have been  and limiting lug contact to inverter connection was the problem. 

I want to replace cable anyway as insulation damaged so will most likely go bigger and get 70mm run. Does seem over spec though but can't do any harm. 

Even at 6000w we are talking 125A which is well within limits as only 0.1% drop?? And battery was giving 2000w and mppt 2600 when I checked before disconnecting.  I also did have inverter limit set to 4500w as well. If it went above thought it would have switched to Eskom which I have limited to 32A (7000w) but that should be direct pass through.

Seems I just got careless with a washer. Small mistakes not happy consequences. But new 70mm wire going in this week anyways. 

Vd isn't the sole variable to be considered when calculating wire guage.

2 hours ago, Paul Oosthuizen said:

Reality is I still have a burnt cable, so either crimp bad- though pretty sure crimp is perfect, or washer where it should not have been  and limiting lug contact to inverter connection was the problem. 

One or the other, or tightness.

To be sure this was localized heat.

Go with recommended cable sizes, as a rule though.

 

Good idea to borrow someones Flir (thermal camera) once you got a system up and running.

I think you would have caught this.

8 hours ago, phil.g00 said:

Good idea to borrow someones Flir (thermal camera) once you got a system up and running.

I think you would have caught this.

haha borrow someones Flir camera xD

I use this for temp checks