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Fusing question

Leon

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Dec 15, 2019
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Hi, I'm hoping to get some advice on how to provide adequate fuse protection for this system I am developing. It is based on the Electrodacus SBMS0, which is why the solar array is weird - no series strings. I understand the need to protect the wiring from overcurrent caused by a short or other defect. The current sources are the solar array, which should max out at about 130A, and the battery, which could provide thousands of amps in a worst case scenario.

I gather that a Class T fuse is necessary to reliably disconnect the battery. Maximum current drawn by the 3500W inverter would be at full load when the battery is at lowest voltage, right? So 3500/20 = 175A. Would a 200A fuse provide enough headroom? The 50mm2 cable is rated to 246A so I could go to 225A for that fuse.

The wiring from the solar array is where I am confused. The individual wires (6mm2, rated to 70A) from the parallel pairs of panel would see about 22A in freakish conditions from solar input. If I shorted a pair out on a sharp bit of roofing, they would see the full force of the battery until the DSSR20 burned out. That's a pretty unlikely scenario and not outrageously dangerous. I have sketched some generic breakers which I think would be much more convenient to use as switches than fuses would be. Does that make sense?

The 35mm2 cables from battery to the charge bus need protection only from solar output, assuming my Class T is already in the circuit. This means I could use a less costly fuse type to protect these cables, right?

The Electrodacus designer does not want a fuse between the battery and the shunts. What about I put the Class T on the negative battery terminal and the other fuse (ANL maybe?) on one of the 35mm2 cables next to either charge bus?



system 1-5.png
I'd really appreciate an input that will help me avoid burning my shed down.

Leon
 
THe shunt I used goes on the negative wire and fuse on the positive wire. In fact, all the shunts I looked at say negative wire. If you can get a shunt that goes on the negative side, you can fuse on the positive side.

Here is a site resource for fusing:


Really don’t understand why a 32 amp breaker. The string going in looks like its either 9 or 18 amps.
 
THe shunt I used goes on the negative wire and fuse on the positive wire. In fact, all the shunts I looked at say negative wire. If you can get a shunt that goes on the negative side, you can fuse on the positive side.

Here is a site resource for fusing:


Really don’t understand why a 32 amp breaker. The string going in looks like its either 9 or 18 amps.
I don't think the shunts themselves care whether they are on the positive or negative wire, but the BMS that senses them does. The SBMS0 I am using needs them on the positive side, so no option there.

Dead right about the 32A breaker, they are too big for the 18A load. I'll probably go 25A to cover any cold, sunny days.

Thanks for the link.
 
I don't think the shunts themselves care whether they are on the positive or negative wire, but the BMS that senses them does. The SBMS0 I am using needs them on the positive side, so no option there.
Does that BMS have a problem with running cells 3P8S?

I am building my first lithium battery, and I am going with 8S with 280 ah cells with a 100 amp BMS. I will build two of these.
 
The SBMS0 will just treat it as a single 8S pack. The 3P groups will effectively become single 450Ah cells.
 
Does that BMS have a problem with running cells 3P8S?

I am building my first lithium battery, and I am going with 8S with 280 ah cells with a 100 amp BMS. I will build two of these.
Are you thinking of going with the Electrodacus system?
 
I have purchased the Overkill BMS. This will give me up to 100 amps from each battery.

Each day I'm on here I see all the other options available. I'm happy with the Overkill.
 
What about I put the Class T on the negative battery terminal and the other fuse (ANL maybe?) on one of the 35mm2 cables next to either charge bus?
I think with the ANL fuse, look at the specs. I use Blue Sea ANL, and they are only rated to 32 VDC. I use breakers from Midnite Solar here:


Those are 150 VDC, but only go to 6 AWG wire. There is a 80 and 100 amp breaker they make that accepts 2 AWG.

You don't need a breaker there, but IMO something needs to be there to shut the current off. If this is a permanent installation, a disconnect may be required by code for things like if a firefighter needs to shut down the power.

I'm also missing the SCC that is going to reduce the current and voltage to what's needed. Should go between the solar panels and the battery. If you have an inverter / Charger, the panels could hook into there.

I don't think the Class T on the negative is a good idea, I think the first thing after the shunt on the positive would be the place to put it. If the electrdaulus support is responding to you ask them.
 
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