BMS Expectations for Dead Short

[hammer_64]

I have a 48V battery bank using 8 Battle Born BB5024 (24V) mounted in a cargo trailer converted to a camper. Without getting into the details, there was a dead short on one of the batteries causing a total meltdown. There was a lot of smoke and no small amount of drama. Fortunately, other than the acrid smoke and mess from the discharge of a couple of fire extinguishers, there was no other damage.

My question: How should the internal BMS perform in this situation?

The BB website states (for a 12V battery): “Our internal battery management system is rated for three different levels.

- 100 amps continuous (1200 watts at 12 volts) – this means you can pull 100 amps out of the battery when you need it until the capacity is all used up.
- 200 amps for 30 seconds (2400 watts at 12 volts) – if your device has a surge, an individual battery can deliver 2,400 watts for 30 seconds.
- ½ second surge up to the max capacity of the battery. If you have a high moment over 200 amps, the battery will handle this for ½ second.”

Later on the same page, the website states: “The battery will disconnect during a moment of high currents in the system and try to establish reconnection again after 5 seconds.” So, could the scenario have been: max surge for ½ second, 5 second reset, max surge for ½ second, and so on until the battery melted down?

I guess what I am asking is given a very undesirable situation, what protection (if any) should be expected?

Thanks

 

[wpns]

Without knowing how the firmware for their BMS was written, and the details of the short (it probably wasn't 0.00 ohms), and how you have your 24V batteries combined into a 48V bank, everything I can imagine is sheer speculation. Are there fuses? Are there breakers? Did you drop a tool? Connect the battery backwards?

Honestly, most of the firmware for this stuff is outsourced to Elbonia so I'm not surprised they didn't code something in there to deal with a short. Nor that no-one tested the corner cases.

Google gets a lot of hits for Battle Born batteries failure
but then probably so does every mfr.

 

[hwy17]

It depends how good the dead short is. Most BMS can interrupt an accidental sparking touch of a short.

If it's a good dead short like a strong switch closing, I don't think you can expect any BMS to interrupt that. It's a job for a fuse.

 

[Supervstech]

Any bms will deal with an actual dead short by failing. No electronics can handle the output of a large bank of lfp cells. At 12V the output can liquigy a large steel wrench in under a second...
You HAVE to use fuses to protect from dead shorts.

 

[robbob2112]

What I learned recently is that you either fuse both ends of the cables to the batteries or you fuse the end next to your bus bar and depend on the BMS in the event there is a short in the middle of the wire.

More details on your setup would be needed to answer any specific questions - like a wiring diagram - (draw.io is ideal for this) - and also pictures, lots of pictures, of the wires, bus bars, any fuses, inverter...

And of course a description of what happened to cause the short - assuming you were around when the failure happended?

 

[Quattrohead]

I would have expected the BMS to deal with it appropriately, but a lot is going to depend on how exactly they were wired and what exactly melted down on you.

 

[Skypower]

I would think that the bms would over current protect and disconnect wait and reconnect, protect, a vicious cycle that will continue indefinitely until ultimate failure. Failure could end up with MOSFETs failing in closed condition which would commence a showing of magic smoke. This is why you always want to have an appropriately sized T class fuse as close to either battery terminal as practical. I’m no fan of BB batteries however all bms’s are the last line of defense and have a limited number of rescues.

 

[Supervstech]

Exactly...

A bms has a few jobs, but the primary one is cell imbalance protection. The next two are over and under discharge protection.

A bms has a specific current they are designed to allow, and there is always SOME leeway, but NO electronic safety board will be able to handle a dead short.

Likely no conductor will either...

Use fuses.

Molten metal tends to start fires.

 

[time2roll]

Is there a reason the fuse did not open? Can't speak for BB BMS but this BMS should not be the only protection.
120a to 150a fuse on each string would be best depending on the wire and other conditions.

 

[hammer_64]

Thanks for all your replies! Seems the answer is that in the case of a continuous dead short, the BMS will not protect the battery.

 

[robbob2112]

Thanks for all your replies! Seems the answer is that in the case of a continuous dead short, the BMS will not protect the battery.

It will not. Best thing you can do is fuse at both ends of the cables between the batteries and the bus bars. In particular with more than 2 parallel batteries/strings. After that from the bus bar to inverters, MPPT, and anything else should be fused accordingly. If you choose not to fuse both ends fuse the end on the bus bar. The bms may short but it will kill the power.

A server rack battery with a breaker or fuse built in is great and only needs fused at the bus bar end. But for standard battery I would put a MRBF on the post and a class T or breakers on the bus bar. Expensive? Yes, but a one time expense. If they are sized right they will never blow unless there is a serious problem.