Maximum short circuit current

[Jonndeka]

Hi friends,

I would like to know how can we calculate the maximum short circuit current?
Can we calculated based on maximum discharge rate in datasheet? for example if max. dis. rate is 8C and cell capacity is 20Ah it means our max. short circuit current can be 20*8=160A?

Is it right?

 

[fechter]

160A would be the maximum allowable current. The short circuit current would be much higher and would be the voltage/cell internal resistance (+ any wiring resistance).

 

[Jonndeka]

160A would be the maximum allowable current. The short circuit current would be much higher and would be the voltage/cell internal resistance (+ any wiring resistance).

Can we say maximum short circuit current is 1.5 to 2 times grater than the maximum allowable current?
It means 160*2 = 320 A.

 

[dominik h]

The short circuit current will be 5-10 times higher then the 8C current depending on the internal resistance and of the wiring.
It will be up to 1600A.

If I calculate my battery pack 21S5P build with 20Ah 8C LINMC cells , the short circuit current at the controller will be up to and even over 6700A
Measured resistance at the controller screws is 13mOhm @20°C, includig wiring and fuse. Pack voltage is up to 87,5V.
I=U/R
87,5V/0,013Ohm=6730A

Calculated resistance of my pack is 1,2mOhm/5*21=5,04mOhm (1,2mOhm is the single cell resistance, the rest is 5P and 21S)
The rest to 13mOhm is the wiring, the fuse and the cell connectors.

If the short circuit is direct at the battery the short circuit can reach 17kA

 

[Jonndeka]

The short circuit current will be 5-10 times higher then the 8C current depending on the internal resistance and of the wiring.
It will be up to 1600A.

If I calculate my battery pack 21S5P build with 20Ah 8C LINMC cells , the short circuit current at the controller will be up to and even over 6700A
Measured resistance at the controller screws is 13mOhm @20°C, includig wiring and fuse. Pack voltage is up to 87,5V.
I=U/R
87,5V/0,013Ohm=6730A

Calculated resistance of my pack is 1,2mOhm/5*21=5,04mOhm (1,2mOhm is the single cell resistance, the rest is 5P and 21S)
The rest to 13mOhm is the wiring, the fuse and the cell connectors.

If the short circuit is direct at the battery the short circuit can reach 17kA

Thanks Dominik,

It was very good example.
Two issues are still ambiguity for me.

1- Shouldn't we measure the resistance in high temperature? Because in high temperature (i.e., 45°C) the resistance is lower than 13 mohm. Therefore the short circuit current can be more than the value you calculated.

2- With measuring the total resistance (pack resistance + (fuse, wire, connectors)), it shows the resistance of inside the pack (fuse, ....) is more than the total cells resistance). Is it true?

 

[dominik h]

1. the resistance gets lower when the pack heats up. I measerad my system with 20°C , the highest I have seen in summer is 29°C
My pack is fully isloated and I built in a heating, because I drive more kilometers in the colder half of the year.
2. the inside battery pack resistance is lower. If I calculate a single 1P5S section . I get 17,5kA.
In reality is it not possible to make a perfect short circuit , so I took the measured resistance at the controller including all connectors , the wiring and the fuse.
The only thing to you need to calculate the short circuit current for is to see if you have big enough wiring to survive the short circuit, and if the choosen fuse is able to turn off during a short circuit event.
But even if a calculated everything and it looks ok, I do not want to make a real short circut to " test" if everything works.
Personally I think my BMS will die in flames.

 

[Chalo]

Current well in excess of the cells' rating is to be avoided. I can't think of why you'd even need to know it unless it's to make sure a circuit breaker or fuse value isn't too high.

I recently added a battery module to my collection that was fused 225A. I removed that fuse along with some other extraneous stuff, because I knew that my plugs and cables would burn first. What's the maximum short circuit current for that module? Don't know, don't care. By the time it becomes relevant, my situation would already have crossed over from catastrophic equipment failure to dangerous emergency.

 

[larsb]

A mind boggler i’ve had is that i’m not sure the battery (being a chemical reaction) will supply current corresponding to the initial I=U/R value when short circuited - it is likely load dependent.
Like here (SLA battery, don’t know but i’d expect load dependency for lithium also) https://actec.dk/media/documents/68F4B35DD5C5.pdf