Fuse size between battery banks and inverter

[OKA133]

The system I am currently working on will have a single inverter and 4 battery banks.

The inverter is a Deye Hybrid 12kW 3 phase. According to its manual there is a 300A fuse needed for its battery connection. This inverter can charge or discharge with 240A at maximum.

I am going to use a Suntree DC MCCB SM8 320HPV 2P (similar to this one, but 320A) between the battery bank and the inverter. It has a 20kA AIC, which should be more than enough for LiFePo4.

The manual suggests a 60mm² cable for this connection. I will use a 70mm² cable.

The batteries will be 4 units of Deye RW-M6.1 and will be paralleled using a Victron Lynx Distributor. The 4 pairs of battery cables will be connected on the bottom of the Lynx Distributor. The left side of the Lynx Distributor will then connect to the MCCB. Basically this is in reverse for most common usages of the Lynx Distributor. The batteries include the required cables, 25mm², so I will stick with that.

The batteries have a recommended charge and discharge current of 60A. This is also what the inverter is able to do, 240A/4 is 60A, when 4 batteries are paralleled. The maximum is 100A and peak is 150A.

My questions:

1. What fuse should I use in the Lynx Distributor?
2. Is a Mega fuse sufficient because I already have MCCB that can handle the AIC?
3. What size fuse should I use? Normally I would say 125A, but this is based on the maximum of the battery. Should I take the peak into account as well?
4. Can I use a smaller size fuse because I will never exceed what the inverter is able to? But what if one or more battery banks disconnects for whatever reason?
5. Should I use regular busbars and class T fuses, put that inside a nice battery combiner box and skip the Lynx Distributor all together?
6. Did I miss anything?

Thanks ?

 

[yabert]

Should I use regular busbars and class T fuses, put that inside a nice battery combiner box and skip the Lynx Distributor all together

I can't help much for the Lynx Distributor, but if you need low cost / high AIC fuse, take a look at those AMH, AMX fuses.

Eaton AMX and AMH Fuses alternative to Class T

I think I found a really interesting 10-15$ fuse alternative to Class T fuses. Eaton Bussmann AMH specs: -current ratings 350A, 400A or 500A -ultra-high interrupting ratings, up to 20,000 A. -voltage ratings, up to 125 Vdc Eaton Bussmann AMX specs: -current ratings 80A to 350A -ultra-high...

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About fuse size, look at the spec, but most fuse can take 3-5x her nominal amps for several seconds without blowing.
So there is no problem to choose a fuse based on nominal amps and it will take the peak.

 

[DIYrich]

1. What fuse should I use in the Lynx Distributor?
2. Is a Mega fuse sufficient because I already have MCCB that can handle the AIC?
3. What size fuse should I use? Normally I would say 125A, but this is based on the maximum of the battery. Should I take the peak into account as well?
4. Can I use a smaller size fuse because I will never exceed what the inverter is able to? But what if one or more battery banks disconnects for whatever reason?
5. Should I use regular busbars and class T fuses, put that inside a nice battery combiner box and skip the Lynx Distributor all together?
6. Did I miss anything?

Thanks ?

Assuming the wire is rated for it, I would use a 125amp fuse for each battery connection. Although you anticipate 60 amp max under normal conditions, you never know if a battery goes offline, and amps increase for the others. 125 amp will allow the battery to work at 100 amp max, and provide some headroom for 150 amp peak. If 2 batteries go offline, you could be drawing 120 amps from the remaining 2 batteries.

 

[OKA133]

I will use a 125A fuse on each battery.
Maybe I will look into the MCCB mentioned earlier if there is a variant rated for 125A and with a high enough AIC (in the datasheet this is mentioned as Icu).

 

[OKA133]

According to the battery specs the peak current is 150 for a maximum of 2 minutes. Most likely this is an enforcement by the BMS.

I also contacted the manufacturer about the fuse sizes and they told they were working on a new model of the same battery with an included circuit breaker of 200A. After asking why not 125A, they told me that the peak current is the reason.

Should I stick with 125A or go with 200A as the manufacturer says?

 

[yabert]

Should I stick with 125A or go with 200A as the manufacturer says?

A fuse is not a breaker.
125A fuse or 200A fuse, both will do the job as many 125A fuse can pass 200A for over 5 minutes.
The 125A can generate a bit more heat and the 200A will blow at high current (potentially cosign more important damage). So each option have advantages and inconvenients.
At the end it's your choice.

 

[OKA133]

I already had a breaker in mind, so it's a similar way as the manufacturer chose, external but as close as possible to the battery. With this breaker I have options for 125A, 160A, 200A, 225A and 250A.

When I look at some existing LiFePo4 server rack or wall mount batteries, they usually have a 100A max draw, a peak draw of seconds only and a 125A breaker.

In my case I could go for a 160A breaker that should cover all possible current draw. Or stick with 125A. If I read the curve chart of the breaker correctly, then it will trip within 20 milliseconds on an overload of 8A.

 

[OKA133]

A fuse is not a breaker.

Any thoughts on using a breaker and its size?

 

[yabert]

In my case I could go for a 160A breaker that should cover all possible current draw.

Why not?
To me the only downside of a breaker is the potential lower Interrupting Rating capacity or the poor chinese quality.
All this whe compare to an high quality fuse, of course. Poor quality fuse can cause as problem than poor quality breaker.