MRBF fuse size

[48Rob]

I have (3) 220 Amp Hr LFP batteries. Each has a BMS of 150 Amps.
Each battery terminal has an MRFB 200 Amp fuse. 4/0 Cables from the batteries will each tie to a 1000 Amp busbar, which is then protected with a 500 Amp Class T fuse before the disconnect switch.

Am I correct in believing that 150 Amps BMS x 3 batteries, 450 Amps potential, needs a 500 Amp fuse?
And that each battery cable is correctly fused at 200 Amps?

 

[FilterGuy]

I have (3) 220 Amp Hr LFP batteries. Each has a BMS of 150 Amps.
Each battery terminal has an MRFB 200 Amp fuse. 4/0 Cables from the batteries will each tie to a 1000 Amp busbar, which is then protected with a 500 Amp Class T fuse before the disconnect switch.

Am I correct in believing that 150 Amps BMS x 3 batteries, 450 Amps potential, needs a 500 Amp fuse?
And that each battery cable is correctly fused at 200 Amps?

You should determine fuse size by load, not by battery capacity.

What is the system voltage and what loads do you have?

 

[48Rob]

Thank you.

I have a 2000 watt inverter, and 85 Amps of 12 Volt DC circuits. 3019.2 Watts/251.6 Amps max load in worst case scenario. But if one battery shorts out, then the other two try to take the (potentially much larger than 150 Amp) load, no?

 

[TacomaJoe]

Keep in mind that your 251A max load will be split between the 3 batteries (~84A each). If one battery goes off line, the other two will pick up the load at about 126A.

As noted, the fuse is to protect the wire.

 

[48Rob]

Thank you.

The nearest size to the potential 126 Amp draw is a 150 Amp fuse, which will protect the 4/0 wire, while not blowing the fuse if one battery goes bad?

The 4/0 cable coming off the busbar, protected by the class T fuse, is rated at 378-440 Amps, depending on temperature.
Max load on the wire fed by the batteries is 251 Amps. So, a 300 Amp fuse would be correct?