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Battery Bank and DC Circuit breaker tripping on high load


Bobby Kaucic
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Hi guys,
I have a Victron Multiplus II 5000VA (4kW) with MPPT charger and a 48V battery bank (4x12V 100Ah batteries in series x 2 = 200Ah total).

Battery bank positive wire reaches the inverter over a 63A DC Circuit breaker. There is also an 80A fuse on the path.

System is running ESS with power assistance turned ON.

Last night (no sun for the MPPT) I turned on couple of appliances and the load came to around 4800W which is higher then what the inverter can produce, but due to ESS and Power Assist, the portion that was needed was taken from the Grid, hence no overload.

That being said, 4kW was being drawn from the battery bank at the specific moment. I had a look and the amperage being drawn from the battery bank was in around 120A (?!).
Then the DC circuit breaker tripped 🙄
As from what I see - DC and AC Amps are way different.

How would one calculate maximum amount of Amps that can be drawn from the battery bank and for how long? As per the battery bank capacity - I would say that I can get 200Amps for one hour before discharging the bank fully - right?
I am a bit cautions of getting a larger DC breaker as I wouldn't like to burn something 😬
 

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36 minutes ago, Bobby Kaucic said:

Battery bank positive wire reaches the inverter over a 63A DC Circuit breaker. There is also an 80A fuse on the path.

Way too small. That's 60*50 = 3kW (just rounding the numbers to make the math easy). Also, the Multi can exceed 5kw for short periods of time. The fuse needs to be at least 125A as should the breaker.

41 minutes ago, Bobby Kaucic said:

I am a bit cautions of getting a larger DC breaker as I wouldn't like to burn something 😬

The fuse and/or the breaker is there to protect the cable. That is all. A fuse cannot protect the electronics since it usually doesn't blow fast enough. What usually happens is the electronics blows up, causes some kind of short circuit (eg MOSFET that burns through) and that then causes the fuse to blow. If the fuse isn't there, then things get hot and you end up with a fire.

Your cable must be sized for 150A or so (35mm^2 or larger), and then I'd make the fuse even larger, 160A or 200A. Again, the fuse is there to protect the cable, and a cable of that size can hold that kind of current long enough to blow the fuse.

Also, refer to the Victron installation recommendations, which recommends a much larger fuse than I just did!

In your case, since there is a fuse and a breaker, I would not worry about the breaker too much. The fuse will tend to blow first anyway. Fuses blow within 10 seconds at twice their rated current (this depends on the type of fuse, fast blow fuses obviously blow faster). In your case the breaker is there only as a means to disconnect the DC, the fuse is the real overcurrent protection.

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22 hours ago, plonkster said:

Way too small. That's 60*50 = 3kW (just rounding the numbers to make the math easy). Also, the Multi can exceed 5kw for short periods of time. The fuse needs to be at least 125A as should the breaker.

The fuse and/or the breaker is there to protect the cable. That is all. A fuse cannot protect the electronics since it usually doesn't blow fast enough. What usually happens is the electronics blows up, causes some kind of short circuit (eg MOSFET that burns through) and that then causes the fuse to blow. If the fuse isn't there, then things get hot and you end up with a fire.

Your cable must be sized for 150A or so (35mm^2 or larger), and then I'd make the fuse even larger, 160A or 200A. Again, the fuse is there to protect the cable, and a cable of that size can hold that kind of current long enough to blow the fuse.

Also, refer to the Victron installation recommendations, which recommends a much larger fuse than I just did!

In your case, since there is a fuse and a breaker, I would not worry about the breaker too much. The fuse will tend to blow first anyway. Fuses blow within 10 seconds at twice their rated current (this depends on the type of fuse, fast blow fuses obviously blow faster). In your case the breaker is there only as a means to disconnect the DC, the fuse is the real overcurrent protection.

As always - invaluable info :)
Thank you so very much!


I managed to find place to buy the fuses as these things are not so common (I think).

So the main fuse from the battery bank to the positive bus bar is 125A which I think is not enough. I would replace that with 200A. Cable is 35mm2 and it is 2,5m long so i believe that that is fine. EDIT: Fuse is: https://za.rs-online.com/web/p/products/1568709/ but, looking at it now, it is for AC only :( 
Then from the positive bus bar to inverter there is an 80A fuse which I would replace with the 125A that I would have from the above.
Additionally, there is a 100A fuse towards MPPT which I believe is ok.

Does that sound ok?

And another question - is the DC circuit breaker really necessary?
I am struggling to find a 200A DC circuit breaker and the ones I did find are very expensive (over R2k 😮)

Edited by Bobby Kaucic
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Thank you @Jaco de Jongh, but I have the box and everything already mounted and this looks quite big 🙄
If I don't come right, I'll reach out to you 😊

@plonkster
Please can you check my diagram
Would this be OK with regards to Fuses?

In nearby future, I am planning to add a second Victron and parallel them so it would be 2 x 5000VA
Will all this be able to run ok with 2 x 5k Multis setup? 🤔

873280588_SolarDiagram.thumb.jpg.9a456c92a9ebb675abe6ae7f359331e2.jpg
 

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2 hours ago, Bobby Kaucic said:

main fuse from the battery bank to the positive bus bar is 125A which I think is not enough

It's probably okay. Remember that fuses don't blow immediately and take several seconds even at twice the capacity. The Multi can only peak at 10kw for a second before overloading. But if you are going to work on the system, I would suggest increasing it as suggested (160A or 200A).

2 hours ago, Bobby Kaucic said:

Cable is 35mm2 and it is 2,5m long so i believe that that is fine.

Yes.

2 hours ago, Bobby Kaucic said:

Fuse is: https://za.rs-online.com/web/p/products/1568709/ but, looking at it now, it is for AC only :(

See if you can find a DC rating for it. Often something with an AC rating of 450V (ish) will also be suitably rated for 60VDC. But don't just assume it, check the spec sheet. I tried to find a sheet (not very hard though) but was not successful.

2 hours ago, Bobby Kaucic said:

Then from the positive bus bar to inverter there is an 80A fuse which I would replace with the 125A that I would have from the above.

Yeah that sounds fine. For this I like to use a Megafuse or an ANL fuse, but the Bussman fuses are good too (just check that DC rating).

2 hours ago, Bobby Kaucic said:

Additionally, there is a 100A fuse towards MPPT which I believe is ok.

Yes.

2 hours ago, Bobby Kaucic said:

And another question - is the DC circuit breaker really necessary?

It doesn't have to be a breaker, but you need some way to disconnect the DC. The Keto holders Jaco listed are good for this. They are fused disconnects. They hold the fuse, but you can also pull them out to interrupt the DC if needed. They WILL arc if you pull them out under load (so don't be alarmed), but the holder is designed to contain the arc (and not set itself on fire).

The other option is the Mersen fuse holders. They are essentially the same as the Keto ones, and they cost the same too.

Your diagram looks good to me.

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36 minutes ago, plonkster said:

It doesn't have to be a breaker, but you need some way to disconnect the DC.

Important point to remember.

46 minutes ago, Bobby Kaucic said:

but I have the box and everything already mounted and this looks quite big

Please do remember, you or the fireman or your wife will need to disconnect your batteries from the installation in case of an emergency. Something you can easily reach and easily open. 

Once something goes wrong, there is no time to open the box, look for the 13 spanner and then disconnect the fuse, it should be in the form of a Keto or a breaker or Isolator. 

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