LiFePO4 Packs and Invidual Cell Fusing...

[Will Prowse]

UL listed/safety certified lifepo4 parallel packs (consumer and commercial grade) do not have individual cell fusing.

If cells are matched, good quality, charged/discharged at similar c rate in similar ambient temperatures... I would imagine that they would last a VERY long time.. and that a shorted cell would be extremely rare..

But what happens when a lifepo4 cell shorts?

I have read that the pack will have reduced capacity, but can still work. I know that lifepo4 do not create enough heat to cause the typical thermal run away effect like NMC or NCA cells.. but how bad is the effects of a shorted lifepo4 cell in these instances? Is the pack unusable? Or does it still work, just at a reduced capacity?

I would imagine that the other cells in the pack would dump into the shorted cell, which is the norm in most parallel packs.

I have never seen a lifepo4 cell short. Anyone care to share their experience?

Really hard to find information on this. Have been reading battery safety studies and it's the same information that we have known for ages on this chemistry.

 

[electric]

Assuming no external physical force like puncture or crushing, LFP cells do not just short. A situation where parallel cells are happy and then one of them goes hard short is fictional, never happens in real life. Here is what could potentially happen:
1. Cells are over-discharged deeply, all the way to zero volts and held like this for a while. All cells in parallel are affected together. Lithium plating happens, dendrites grow thru the separator and create a hard short. Cells are all dead and become essentially a buss bar, passing current with minimal resistance. There is no more stored energy, so no chance for fire or explosion, unless a very high current is passed for a long time, heating up internal material to the point of ignition, which can't happen if there is any kind of BMS protection. Even without a BMS in this situation any decent charger won't go to bulk because battery voltage is too low. Also, with LFP cells the point of ignition is way too high, difficult to reach.
2. Cell has internal manufacturing defects causing weak shorts thru separator as dendrites start to grow. Because these shorts are weak, they are quickly fused by the remaining energy in the cell and/or parallel cells, hence cell loses capacity slowly, but never has a chance to develop a hard short.

BTW, as part of UL 1642 testing cells are subjected to these and many other destructive scenarios and must pass without fire or explosion, although venting smoke or vapor is acceptable. So, if you can, buy cells tested to UL 1642.