Project Lithium Nexcell Teardown

That was indeed interesting. Is there a similar disassembly of a Prius NiMH battery? It would need to be cut open, presumably, but it would be useful for comparison.

What I took away from it were:

1. Cells are soldered to each other in series.
2. End bus bars are copper, the longer one appears to be shrink wrapped, painted, or in some other way coated/insulated.
3. Connecting studs are copper instead of steel. (Presumably zinc or some other coating on it, since it as not copper colored.)
4. Sense/balance wires are soldered to each cell.
5. Case has some sort of sealant, but can be opened nondestructively.
6. Case does not appear to have any sort of ventilation holes or other heat dispersion features, like a heat sink or metal side.

I'm going to assume that the bus bars and battery cell connectors are all sized so that they do not heat significantly from the maximum current that might go through them.

Overall it is pretty much what I expected inside except for two things.

1. Each balance wire (presumably copper) appears to be soldered flat to its cell connector. That is, no through hole. I say this because of the angle at which the wire approaches that connector, and because of the huge blob of solder surrounding each such connection. If there is some sort of mechanical target for this wire (a post or raised tunnel) it is completely buried under the solder blob.

2. The cell connectors are soldered to each other flat side to flat side.

This is I suspect where things may eventually go wrong with age and vibration exposure. It may well be that the failure times are much longer than the expected life of the car though, so there might not be any real problem.

Solder joints on macroscopic components tend to be more reliable if there is a mechanical connection in addition to a solder connection. I don't see any, but maybe they are there? Naively, I would have joined the cells by bending each contact into an L shape, with the verticals abutting, then folded that over a couple of times, pressed the surfaces together hard (to maximize contact) and then solder (or spot weld?) that assembly. Not sure if copper sheets like those can be welded together. Even if it was possible, and not prohibitively expensive, it might be too much heat near the battery itself, so not an option.

If I had to bet money though, the most likely thing to fail in one of these is the wire itself, or the big blob solder joint at the end of, the longest balance wire. The yellow one visible at 6:14 in the video. It does not look like there is anything present to prevent that from moving (slightly) in response to car vibration, which will very slightly torque that end going into the solder back and forth. I don't know if this would ever really be an issue. It just looks like if there is a weakest link, that is probably it. Suppressing that sort of vibration is often done by by applying some potting compound.

I have never seen a cut open Prius prismatic blade, but the odds are good that internally it contains no solder junctions.

 

Does he have a schematic (to share) of the resistor network card with values?

 

I'm sure he does, otherwise how could they build it? That said, this is not an open source project, it is a proprietary device, so it would not surprise me much if these sorts of detail are never made public. This is not a criticism, it is true for many such devices. Ever seen the internal circuit diagram (not the "how to hook it up" diagram) for an aftermarket alarm? Exactly.

 

Thanks, I edited my question to be more clear. Assuming he thinks it proprietary, it should be easy enough for anyone that has a blade, like the disassembler on the other thread to measure it.

 

Clearly a lot of thought and tooling money went into making that "blade". The injection mold for the case alone would pay my wages for a few months.

However, as I watched the video'ed disassembly of the product I noticed that there was no attention paid to thermal issues or containing flame spread if one cell over-temps itself.

On the flame spread topic it seems that they might have thinned the wall of the molded housing a bit (for example, deleting the diamonds on the external faces) and put some stamp sheet metal between the "blades" to contain flame spread.

I suspect that Toyota's NiMH battery is a bit tougher than a pouched lithium battery, explaining why Toyota continued to use NiMH battteries long after lithium cells became available. In a steep downhill regen situation maybe NiMH cells can ride through a short term over-charging event but I wonder about these lithium substitutes.

Was any thought given to a more robust battery management circuit board that has the ability to divert excess charge current around a cell that's already at its upper voltage limit?

In this thread someone already commented about high-wattage/high temperature soldering being done at the ends of pouched lithium cells. Tooling up to implement cell interconnection with thermally friendlier spot welding would not be cheap, but spot welding would avoid damaging the cells with the soldering iron's thermal assault.

My hat's off to the Nexcell guys for undertaking such a sophisticated project. I'm eager to hear about the reliability of their solution.