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Battery side Fuses/Breakers sizing
#1
Let me understand how this is calculated:

Let's say you built a 14s100p battery from perfect 2200mAh cells.
That will be a battery of a total 48v and 220Ah. => 3,7V * 2,2Ah * 1400u = 11396Wh

Then for sizing the breaker, the values to take into account would be the total amperage of the battery and the accepted/desired discharge rate of the weakest cell?

On a side note:
Per Ohms law, V1 * I1 = V2 * I2 => 48 * 220 = 230V * I2 => (48 * 220)/ 230 = 45,9Ah
230V * 45,9Ah = 10560W
So 10560W will be the total power available to the house? I think there's something wrong here, it's too much.
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#2
New or old cells? IF old your max is your testing current. IF thats 0.5a then its 50a. At 3.2v thats a max of 3.2*50*14= 2.2kw.

At 230v work TH losses perhaps 1800w.

For new cells you can use their rating from the datasheet. Note that your wiring a d all between need to work with it.
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#3
You would typically size it on three things, the cable and the power source (battery), and maybe the load, whichever is the weakest.

Now on a cell I assume you test it at 1A discharge. Most cells are capable of at least 1C, which is whatever that mah rating is on a cell. So a 2000mah cell will be 2000ma or 2A. With a 100p pack that means a theoretical of 200A. But I typically set a max of .5A, which means for a 100p would be .5*100=50A.

On the cable, if you use a 14ga or 1.6mm2, which is typical of our US household, then it's a 15A breaker. You wouldn't install a 100amp breaker because then your cable would melt before the breaker would trip. So you have to size for that 100amp breaker, whihc would be a 2 gauge wire.
https://en.wikipedia.org/wiki/American_wire_gauge

Then you have your load, which is your inverter. If you have a 3kw inverter and running at 48V, then it's 3000/48 = 62amp. So that means you'd want a breaker that won't trip at 62amps or close to it. Now this isn't typical since as long as you take care of the two above, then nothing will harm your batteries, but if you don't size the breaker high enough it will trip because your inverter is asking for more. I size mine for double it's rating since the inverter can handle 2x its rating for 5 seconds or more. Which means at 62amps, it can run at 120amps for 5 seconds or more. So I find a circuit breaker that can handle at least that.

With the above three, 100p pack, with 2awg wire and a 3000W inverter, I would pick a 100amp breaker.
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#4
Thanks guys for your replies. I was still calculating the single component specs, you gave me even more.

(07-26-2020, 01:42 PM)not2bme Wrote: With the above three, 100p pack, with 2awg wire and a 3000W inverter, I would pick a 100amp breaker.

Wow! I have ordered a 5000W one, and now I'm concerned to wire a single battery bank to it. It should be a big ass breaker and will move the weak point to the powerwall. I'll not have enough time to build more banks to leverage.
Call it a unbalanced system.

Note: In Spain, 1,5mm2 it's the minimum required for lighting, 2,5mm2 min for plugs/general appliances and 4mm2 minimum for kitchen lines. I don't know if those are national or Euro requirements, but 1,6mm2 seems to me so small for wiring the battery to the inverter. IIRC hbpowerwall's example uses something past 30mm2. Holy welding cables!

@daromer, Cells are 2200mAh min. I bought them already tested and they're on their way. And I'm now figuring out the whole scenery. Building a 3d house model with real sun position. Deciding where to put panels will be no easy task.

PS: Should I start a build post, already?
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#5
Its OK to use a lower amp breaker than you need to protect the battery - one sized to your usage.  For example, my battery is capable of 600a @ 48v (at 1a per cell) but my max use is 'at most' 400a - so I use 400a shunt-trip.     You also want to make sure the wiring is lined up with fuse/breaker - e.g. you don't want a breaker that will allow more current than the wiring will handle - or else the wiring could become the fuse in a melt-down. Smile    
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#6
The breaker must be a slightly higher value than your max in-use consumption.

Example: If something drains 30A at max, a 35A breaker is fine.
The wires are a separate thing. When you design something you must thing of sufficient gauge from start not to have current-based overheating problems.

It does not matter if your battery can provide much more current (like 300A). You should choose the breaker at a higher value than your max consumption. This of course assumes the battery is also well designed and can provide sufficient current for your need.
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#7
A breaker is usually rated at the running amps. You can a run a 20A breaker at 20A for several hours before it pops (on a new one). A 20A breaker can also handle 25A for short periods, like about 30 seconds or so, before it pops. Breakers don't work exactly the same way a fuse does in the sense it pops during a slight overload. It'll resist that for a short time. However, it will pop when severely overloaded. 20A with a 40A short, for example.

I have a circuit where I'm using a 20A breaker, and it has my window A/C unit and microwave on it. The A/C runs at about 12A when the compressor is on. The microwave will do about 15A when the magnatron is enabled. The microwave can usually do about 3 cycles before it overloads the breaker. In which case I'm going "ugh!!! not again" and having to make a trip to the other end of the house to flip the breaker Tongue
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#8
I would not rate it above 50a to be honest. You dont know much about those cells to be honest. Bitr the Apple and either dont run the inverter att high load.... Or buy a smaller inverter. But donet overrated the protection.... Its There to peotect you from shit hit the fan issues Smile
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#9
The problem is figuring out the amps transformation from the house line to the battery line:
If I consume 4000W max sustained that would be 17,39Ah. (@230V) How do you calculate at DC side?

I understand I should size the wiring at more than the peak inverters capacity.

@daromer I was recommended to buy the Victron Multiplus II 48/5000 by the seller, I theory after looking all the consumption data I collected the past three years. It's surprising bc I don't even have an air con or electric heating.
I wanted to get the same model but 3000 with slight use offered for 500€, but he said I would overload it and then wouldn't be easy to chain them.
Even though it was a great deal. A very good one. And I plan to get an EV.

** Is a DC two way breaker really needed there? One would say that current flowing to the battery is more controlled and not an issue.

This is a sample of my consumption log. It's curious that you can even see the fridge duty (the serration on the green and cyan lines)
I was having problems with RF comms with only one channel, so that's why the cyan line has blackouts.
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#10
You calculate the DC side the same as the AC side. If you consume 4000W @ 230V, that's 17.3A. (4000W / 230V). Well, you're still going to pull 4000W on the DC side. So on a 14s (48V) system, we'll use 52V as nominal (3.7 * 14s = 51.8V). 4000W / 52V = 76.9A.

Now, there are inefficiencies in the inversion process. We usually calculate about 15-20%. So, 20% of 77A = 92.4A.

This means you'd size the DC side for about 100A.
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