Extra Battery, How to Integrate with 24kWh Traction Battery?

[mux]

Our modules have cell-top BMSes (well, per module instead of per cell, but *very* similar to Yuasa) that communicate via galvanically isolated CAN. We also have contactors inside the battery box, so everything is completely galvanically isolated from each other, both on the comms and power side.

By the way, this is not something I should specify, this is absolutely necessary for any such system. The approach in the pics above is... well, I mean, I understand it from a prototyping perspective somewhat, this is done in the e-bike community, but it's not something you should do for more than a few test drives. 97 opportunities for something to go terribly wrong.

But to be fair, I don't know if that is what caused the issues. Battery fires generally happen only when overcharging or *generously* exceeding max cell temperatures. And even then, the (old Mn-based) Leaf cells endure 4.4V/cell and 75C apparently indefinitely. They are really, really good in that respect. Shorting them over those tiny wires is unlikely to cause any issues, even if you put, say, 100V excess on a cell, you'd just use those wires as a fuse.

[coulomb]

Our modules have cell-top BMSes ... so everything is completely galvanically isolated from each other, both on the comms and power side.

I assumed so, but I'm glad to hear it confirmed.

But to be fair, I don't know if that is what caused the issues. ...even if you put, say, 100V excess on a cell, you'd just use those wires as a fuse.

But those wires aren't designed as fuses, certainly not DC fuses. One failure mechanism I could see would be a short caused by those wires, the wires get very hot very fast but don't actually fuse immediately, insulation is melted, that starts more shorts and the glowing wires start a fire. Remember that DC arcs sustain at 1V per millimetre once started, so even if the wire fused, the arc could continue for perhaps 300 mm (a foot)!. That's why DC fuses have to be specially designed, with melting sand or other techniques to extinguish the arc.

I know the cells can bulge due to over-voltage, but I would think that a fire would cause the electrolyte to expand before and during catching fire, and that might look much the same as overcharged cells. Granted, a complete failure of the BMS is another possibility, and maybe the extra wires somehow compromised the original BMS.

One more possibility. Maybe the wiring was such that the added battery didn't share charge current very well, and maybe ended up with enough charge current to cause some of the added cells to over-voltage much earlier than the main cells did. The thin wires don't perfectly convey the voltage of the added cells. So an added cell might have been at say 4.5V when an original cell was still at 4.0V. The 0.5V difference was merely dropped across those thin wires. So the BMS saw no reason to cut back the charge current, and the fire stated in the added battery.

[nlspace]

Response from the Flower Mound Fire Dept. concerning fire post on previous page:

There was nothing suspicious about this fire and it was not investigated by the Fire Marshal's office. The fire report does not indicate the presence of any external batteries. I cannot say for certain if these were present or not.

[coleafrado]

Response from the Flower Mound Fire Dept. concerning fire post on previous page:

There was nothing suspicious about this fire and it was not investigated by the Fire Marshal's office. The fire report does not indicate the presence of any external batteries. I cannot say for certain if these were present or not.

The particular fire shown in the video could have been anything - an exploding Galaxy Note 7, a dropped Zippo, a bottle of nail polish remover spilling into the 12V cabin electronics and igniting. The cabin upholstery and seats are dense enough to eventually cause that scale of fire, no need for an extender pack (and no evidence of such).