Battery fuse question

[JustDanD]

Hi. I am new (here and to this stuff) and want to start out with the easy questions.
I have 4 LiFePO4 batteries (total 420ah @ 24v) that I am going to hook each to a Victron Distributor. There are so many amp numbers involved that I am unsure which one is the main focus. My question is "What size fuse to use in the Distributor for each battery?"

All 4 batteries are identical
Each battery has it's own 300amp breaker built in
Each battery has it's own BMS
Each battery has an SB175 Anderson connector
24v
120ah
3kWh
130A max continuous discharge current
3328W max continuous power
350A (6 seconds) max discharge peak current
20-28.8V range
28.8v suggested charge voltage
100A max charge current


Second question... is the "28.8v suggested charge voltage" the same as the "float voltage" that everything wants you to enter?

Thanks
Dan (JustDanD)

 

[Johncfii]

Welcome Dan,
As you surely know, fuses and circuit breakers are for protection against potentially catastrophic failure. For example, to protect against molten metal splatter in you eyes in case you drop or brush a wrench across bus bars. But, unlike for house wiring, there are no national electrical codes that mandate amp ratings.

It is appropriate to select breaker and fuse ratings on the basis of your normally expected maximum current charge or discharge to/from the battery, and the size of your battery cable. The built-in 300 amp breaker is really quite a large amp rating for such a small battery. In my opinion it provides no meaningful protection.

For example, If your load from each battery will never exceed 100 amps, then a 125-amp breaker, backup up by a 150-amp fuse would be reasonable, IF your wiring to each battery can safely carry 150 amps.

If you don’t want to spend the money to install a reasonably sized breaker, then just install a 100, or 150-amp fuse as the first device in line with each battery, IF your battery cables can safely carry that sustained current. But realize that you will need spare fuses, because the breaker will never trip.

The 28.8 volts “suggested charging voltage” is NOT the FLOAT stage voltage. It is what most manuals, and forum users, call the ABSORB voltage, or ABSORPTION stage voltage.

 

[time2roll]

What wire is connected?

 

[JustDanD]

Thanks Johcfii. I had planned on using 150a but was confused with all the different numbers. I agree that the built-in 300a fuse seems too large but that is from the manufacturer.

Time2roll. Nothing has been connected at this time. I have 1/0 wire to use since I like overkill.

Thanks all.

 

[Sanwizard]

Thanks Johcfii. I had planned on using 150a but was confused with all the different numbers. I agree that the built-in 300a fuse seems too large but that is from the manufacturer.

Time2roll. Nothing has been connected at this time. I have 1/0 wire to use since I like overkill.

Thanks all.

1/0 is rated for 150A, so you should fuse for that as max current.

 

[time2roll]

Unless you are really going to use 400+ amps at 24v or 9600+ watts I would keep the fuse on each battery a little tighter at 80 to 100. 1/0 wire can go close to 300 amps so you are fine with that. Verify the limits of the Victron distributer.

 

[time2roll]

Second question... is the "28.8v suggested charge voltage" the same as the "float voltage" that everything wants you to enter?

28.8v charging is different than float. Float is probably better down around 26.0 to 26.4 volts.
Is this a solar controller getting adjusted?

 

[JustDanD]

time2roll. Since max charge is 100a and max continuous discharge is 130a, seems like an 80 or 100amp fuse would not allow the battery to use its full potential. ??

 

[time2roll]

That is 80 amps per battery right? 80 x 4 = 320 amps. Are you going to charge at 300+ amps? That is a lot of power.
My preference is to keep the fuses a bit tight based upon the actual situation.

 

[JustDanD]

time2roll. I see your point. Nothing is pulling that much power. The battery bank is sized for longevity not high amp usage. So many things to consider.

 

[Just John]

time2roll. Since max charge is 100a and max continuous discharge is 130a, seems like an 80 or 100amp fuse would not allow the battery to use its full potential. ??

I use the Blue Sea calculator.

Circuit Wizard - Blue Sea Systems

24 volts, 145 amp continuous, terminated in a fuse = 2 gauge (no fuse is 4 gauge)
I used 200 amp breakers and class T fuses for each pack, and 2 gauge wire. 200 amp breaker and fuse should be just right to protect the wiring, and avoid nuisance trips. (at $20 a fuse, that can be important). So far I haven't blown any fuses or tripped any breakers.
Each battery pack in my setup should easily be able to handle the load of my 3000 watt inverter.

 

[JustDanD]

Just John. Thanks. I downloaded the app and will play around with the numbers. Designing a system is so different than working on one.