fusing batteries

[mtnman1]

Sorry if this has already been answered, but I didn't find it searching. With 3 parallel 24v batteries running a 100a overkill bms on each battery, is it better to fuse each battery with a 100a class t fuse before the busbar or should I fuse the entire bank after the busbar?

 

[pullin_gs]

Both.
Fuse each of 3 batteries at maximum rated output of each pack (or max amp load of wiring), and also fuse between busbar and inverter at max amps rating of inverter (or max amps of wiring feeding inverter).

 

[mtnman1]

Thanks

 

[sparklehunt]

if you fuse each parallel battery, I would think that the fuse after the bus-bar to the inverter is unnecessary. Each battery is already protected.

There is no situation where any individual fuse would fail to trip, when your hypothetical combined battery fuse would trip. In other words, the combined fuse is not adding any extra protection (besides redundancy).

Please correct me if I am wrong.

@mtnman1 can you edit your title to be more descriptive for future learners. I was searching this same question. Maybe rename it "fusing on parallel batteries"

 

[timselectric]

You need a fuse at each battery and after the bus bar. (Unless the inverter and wire is rated for the total output of all batteries)

 

[sparklehunt]

You need a fuse at each battery and after the bus bar.

Why after the bus bar?

(Unless the inverter and wire is rated for the total output of all batteries)

"Total output of all batteries" makes no sense. That threshold is very high and is not the determining factor in highest current in a system.

Did you mean if the wire can safely carry the max load current, as calculated by Wattage * 1/efficiency * 1/batt voltage * 1.25 nuisance. In this case, any well designed system should have this.

 

[timselectric]

Why after the bus bar?


"Total output of all batteries" makes no sense. That threshold is very high and is not the determining factor in highest current in a system.

Did you mean if the wire can safely carry the max load current, as calculated by Wattage * 1/efficiency * 1/batt voltage * 1.25 nuisance. In this case, any well designed system should have this.

All wires need to be protected, at their rating.
If the wire going to the inverter is rated for 300a (total of the 3 batteries) , then no fuse is needed.

 

[sparklehunt]

All wires need to be protected, at their rating.

Yes agreed.

If the wire going to the inverter is rated for 300a (total of the 3 batteries) , then no fuse is needed.

But then what do you size the wire to? This implies that we size wires based on what the battery bank can output. I don't think so.

Example: what if you have 100 batteries in parallel, you're not going to size the wire to what the battery can output. You can't

You size it to the greatest load.

 

[Danke]

WRT rack batteries, when I look at all the setup here (and Will‘s) , I never see each battery with a fuse. Individual rack batteries have breakers and a fuse from the busbar to the inverter.

Am I overlooking something?

 

[mtnman1]

This thread might be useful as it has more information on the specific topic of fusing and T class fuses. In my situation I am fusing the individual batteries as well as the inverter cables with t-class fuses based on Hedges feedback. I suspect to some extent it ultimately becomes a question of how must safety you want to design into your system.

T-Class Fuses

 

[timselectric]

Yes agreed.

But then what do you size the wire to? This implies that we size wires based on what the battery bank can output. I don't think so.

You size the wire for the expected current. And then you protect the wire at its rating.

Example: what if you have 100 batteries in parallel, you're not going to size the wire to what the battery can output. You can't

You could, but I wouldn't recommend it.
So..... you protect it at its rating.

You size it to the greatest load.

And protect it at its rating.

 

[sparklehunt]

You size the wire for the expected current. And then you protect the wire at its rating.

You could, but I wouldn't recommend it.
So..... you protect it at its rating.

And protect it at its rating.

I think we are agreed about protecting at it’s rating.

So the original question still stands does a OCPD at every individual parallel battery constitute “protection of the wire” after the combination of the parallel batteries?

 

[timselectric]

I think we are agreed about protecting at it’s rating.

So the original question still stands does a OCPD at every individual parallel battery constitute “protection of the wire” after the combination of the parallel batteries?

If the wire is rated for the full available current of all batteries combined.