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In a large battery pack of lithium-based cells for an electric vehicle or grid storage system, how are failed cells handled? Answers to another question indicate these cells are usually hardwired in parallel blocks (which are then connected in series and balanced) so that resistance isn't added in the path of high current.

What happens when a cell fails and acts as a short circuit?

It seems this would short out a block of parallel cells and decrease the capacity of the overall pack significantly. If the dead cell could be removed from the pack by an electronic or safety system (power transistor, fuse, physical removal &/or replacement) the rest of the parallel cells could continue functioning with a much smaller decrease in usable pack capacity.

Lead-acid batteries seem to be replaced on a timed maintenance schedule, or when certain usage metrics are exceeded, say in data center usage. With lithium batteries being so much more expensive, are there any typical electronic design features which handle failed cells automatically rather than by rotating out the entire pack before end of life?

Are chemistries such as LiFePO4, with 2000+ cycles to 80% capacity, reliable enough that cells failed to short circuit are rare enough to ignore as an electronic design issue?

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    \$\begingroup\$ In pack with large cells, there would be a fuse in series with each cell. If a cell fails shorted, the remaining cells have sufficient current capability to blow that fuse open and operation continues, although with reduced capacity. \$\endgroup\$ – Dwayne Reid Apr 7 '15 at 0:11
  • \$\begingroup\$ @Dwayne, what kind of fuse/fusable link should be used in that application? I'm not sure what I would do. I don't think i would want it to be user serviceable. \$\endgroup\$ – Sean Boddy Apr 7 '15 at 16:27
  • \$\begingroup\$ My experience is with LiPo batteries in consumer devices. The largest I have dealt with is 4 Ah. This is a single cell per device. Each cell has a protection circuit that shuts off if charge current or discharge current exceed a limit. Also, discharge will be cut off if cell voltage drops below a low threshold, and charge will be cut off above 4.25V or so. There is also a simple passive fuse or PTC in series. In applications where cells are paralleled, I imagine that the parallel group would have one protection circuit, but each cell would have a fuse or PTC. We used simple 0402 fuses. \$\endgroup\$ – mkeith Apr 8 '15 at 6:19
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    \$\begingroup\$ Apparently someone got a pack from a Tesla S and did a teardown. teslamotorsclub.com/showthread.php/… There is a very novel busway that appears to use fusable links to make each connection. \$\endgroup\$ – Sean Boddy Apr 8 '15 at 17:10
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I imagine from the questions that you have asked that you are planning some project that may require a high capacity lithium iron phosphate battery that you would like to build yourself.

The only large capacity battery pack in high numbers production that uses a number of cells in parallel vs only a couple of large cells in parallel is the Tesla. All of the other EV seem to go with the large capacity pouch cells and then only use a couple in parallel. The advantage to the Tesla is that their power to weight ratio is almost double that of everyone else.

The disadvantage has to be safety. Tesla has 104 patents on its battery pack and most of them have to do with safety. Specifically with how to deal with single cells that short or go into thermal runaway. FYI the Tesla S battery pack uses 74 cells in parallel and then 96 in series.

I have read through their patents and none of them deal with any sort of system to repair or remove damaged individual cells. They just make sure that on the rare occasion that a cell does short or heat up, start fire etc., that it is contained to that cell. They do this by separating each cell by a certain distance, using active and passive cooling, using a fuse at each cell etc.

Tesla brags that they can replace an entire battery pack out of a Tesla S in only 90 seconds, but they make no claims about repairing that battery pack. In fact because of all of the safety features like fire proof foam, it takes a couple of hours for a person to get access to the individual cells of the Tesla battery pack and by that time you have ruined the structure of the battery, so it is no longer useable to the car even if you did replace the destroyed individual cell.

So to answer your question, it appears that they put a lot of effort into preventing a shorted cell from destroying the rest of the pack, but otherwise they leave it there and let the rest of the cells in that parallel group take over.

Remember that the Tesla is demanding a very heavy load from its battery, not only pushing the cells to their limits to get a further distance out of the pack (some owners report cell voltages that dip below 3.0 volts) they also demand a high current for the crazy acceleration the Tesla gets.

From reading their patents, Tesla believes that over charging is much more dangerous than over discharging (this is from tests they have done in their labs). Over charging leads to fires and explosions while over discharging tends to speed up capacity loss.

Good luck in your project. Using lithium iron phosphate cells you are already a magnitude safer.

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  • \$\begingroup\$ @MattB.- I should add that Tesla does claim to repair damaged battery packs, but I haven't heard of it actually happening. They package the batteries in 74 parallel like I mentioned and then put 6 together in series. Then the 6 series are put in series with 15 other banks of 6 series batteries to get the 96 in series total. It is suggested by some Tesla enthusiasts that a repaired pack would entail the replacement of one of the 16 banks of 6 in series. However, the Labour to replace at even that basic level is very high so I doubt that they do it - it would create an imbalance with older \$\endgroup\$ – Filek Apr 10 '15 at 3:59
  • \$\begingroup\$ Banks, and it appears that Tesla might not balance between the 16 banks, but only between the 6 in series. Of course no one knows for sure what happens except that it is insanely labour intensive to replace a single cell. \$\endgroup\$ – Filek Apr 10 '15 at 4:01
  • \$\begingroup\$ After looking over the Tesla teardown photos, it does appear very labor intensive to replace one cell, or even a bank. Maybe they swap any bad pack for a new one (under warranty) and do failure analysis rather than return it to service. Of the approx. 70k Model S units in the field, almost all of them are less than two years old and half are less than one year old. They may not have seen more than a few failures so far. \$\endgroup\$ – Matt B. Apr 10 '15 at 21:32
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    \$\begingroup\$ @MattB. I think your right, and with 104 patents on the battery pack - and this is after the actual 18650 cell has already been finalized - they may not see many failures except those that wear out from use and age. Perhaps they have 'perfected' the battery pack made from small cells. Their energy density is almost double that of the Nissan Leaf and other competitors who use the large cells. I am curious about what you are planning to make! \$\endgroup\$ – Filek Apr 11 '15 at 5:12

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