Prelude conversion project - some questions

[jonescg]

Hehe, yeah all good on that front. Plus the internals of the battery will be fully potted with polyurethane resin, so it should be waterproof and electrically insulated.

[OzSpider]

Enjoying the progress. @jonescg You mentioned in another post the downside to this chemistry is the higher internal resistance and the additional heat that generates.

With no chill plate and encapsulation in resin, are you concerned about heat on high ambient temp days?

Curious in case I follow a similar path away from LiFePO.

[jonescg]

It's not the chemistry causing a high DCIR, it's the cell format. The ratio of metallic conductors to energy-storing black stuff is tipped in favour of energy storage. Cylindrical cells always have a limit to how much current they can deliver through the internal tabs, although some cylindrical cells are capable of almost 10 C.

The encapsulated battery will mean the whole module will have a more homogenous temperature, which is good, but also the resin itself has a reasonably high specific heat capacity (for a polymer). However if you were to soak heat out of these modules, through the top and bottom face is the best path since it's about 50% solid copper and aluminium through that plane.

Ambient heat is a problem, yes. Without a cooling plate they will get hot on a hot day, and get hotter in use. I can't afford to install a cooling plate at this stage but maybe later on.

[Leonford]

Hey Chris I have a Kweld sitting here if you want to use it?
This is far superior than the Junko!

[jonescg]

If its available I'll happily give it some work!

[jonescg]

Second module ready for spot-welding.

2nd modole built.jpg (167.09 KiB) Viewed 916 times

The thermally conductive epoxy resin is in Perth - I happened to miss the delivery, so with a bit of luck I'll get it on Monday.

[jonescg]

I finished putting the balance wires on the second module and gluing up the sides. It's ready for spot-welding now.

2 modules ready for welding.jpg (189.23 KiB) Viewed 903 times

I started to assemble a third module, and I used the first polycarbonate capture plate the guys at the plastic machining shop did. It was an older cutter, so the holes were ever so slightly smaller than the rest, meaning it was a tight fit. Unfortunately, the tight one was the top sheet, not the bottom, which was the only one I'd drawn a reasonable map for. Knowing this capture plate would be tougher to populate (as I needed to tap each cell into position with a rubber drift) I populated it first, carefully following the mirror image of the original plan

I did OK and put the second capture plate on (the bottom one) and tapped it into position with a wood block and hammer. Flipped it over and started to check the arrangement.

Balls .

Two cells right in the middle were in the wrong orientation. Well boy oh boy removing that capture plate was an epic battle! I got there in the end without breaking it, swapped the two cells and put it back on with ease. Spent the afternoon screwing the busplates down. This time the BMS wiring needs to exit the opposite side as the previous two modules.

Pretty good momentum now. These four modules will be welded, glued and potted by the end of next weekend I think. Then it's time to re-assemble the battery pack and install it into the car. Cooling plates are all a bit too hard right now, but it's at least an upgrade option ahead of the following summer.

[jonescg]

Okay so this is a huge job. It's taking me about a minute to weld each tab to the busbar, and maybe 10 seconds to weld it to the cell top.
Either way, there's about 2 weeks work assuming no sleep or work...

So I'm putting the call out to anyone who has a welder to come and help me do this. I want the car out of my garage soon, and it's really very close to completion. But man, I will be busy.

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[brendon_m]

Id be happy to lend a hand but I don't have a spot welder.
I guess I could glare at the cells...

[jonescg]

I've found a welder - you're in!

[antiscab]

for additional welders, have you considered something like this:
https://www.aliexpress.com/item/1005001 ... web201603_

One fairly solid 12v battery (I have some A123 AMP20s in my shed), a decent heatsink for the fets and it should be plenty of capacity

[jonescg]

Yeah, but I'm trying to get the car out of the garage by the end of February. No good ordering stuff now given Chinese New Year. I worked out I can do a busplate (46 tabs) in 40 mins. So that's 8 days work of 8 hours a day. With four welders and four people, I should be able to knock that down to about 2 days, especially if some of the welders are better than the Junkko.

[francisco.shi]

And I thought that assembling my packs with all those cooling plates and pipes was too fiddly.

[brendon_m]

and then there was me, complaining about my 26s 3p pack with bolt on terminals and no cooling...

[jonescg]

Well this latest community transmission detection has resulted in a week of not going to work, which means a week of no pay. At least that means I can work on the battery modules in a hot shed all day.

first module bottom side welded sml.jpg (185.28 KiB) Viewed 773 times

I've finished spot welding one entire side of a single module. The slowest part is welding the tabs to the aluminium as it only makes the tiniest of welds; so I need to zap it at least 6 or 8 times. And all this zapping means the electrodes wear down fairly quickly. I'm almost through my first set of electrodes after just 1/8th of the work.

Anyway, next step after testing continuity on all welds was to slather the bottom face with thermally conductive epoxy resin and stick the sheet of FR4 on. It took a lot more resin than I first thought - almost 900 grams. I also worked out I need a better supply of disposable cups and stirrers. If ever there was a time to need a clean paddlepop stick in urgency...

I had a blurry photo of the resin application, which is probably about right given the rush I was in, but this is the module on the home stretch:

bottom layer on.jpg (123.46 KiB) Viewed 773 times

[SPanna]

I think you may have originally underestimated the time it would take, I'm sticking to my 192 60Ah pouch cells

[jonescg]

There's a lot of learning as I go along too. I finished the other side of this module in a day, but yeah its a huge job. If the nickel stuck to the plated aluminium like I want it to I could have done all 4 by now.
In future I'll use 21700 cells, as they're 30% less work.

[jonescg]

top side welded.jpg (168.34 KiB) Viewed 605 times

Module glued up.jpg (143.62 KiB) Viewed 605 times

Okay, so the first module is more or less done. Some key observations about this battery building technique.

1. It's painfully slow, but only because I have a small welder with tiny electrodes. I feel it would be faster if I only needed to zap each tab once or twice, and not 7 or 8 times.
2. It's incredibly compact and energy dense, even with the resin and potting. While it won't be a powerful pack, it will be rugged.
3. It's probably only good for about 2 C - 6 amps per cell. For this battery, that's still about 50 kW, which is what it takes to go up Greenmount hill.
4. It's able to be cooled via a cooling plate which takes heat quite effectively through the thermal epoxy.
5. Nickel plated aluminium is not fun to work with.

On point 5, I have a feeling nickel plated copper might have been a better option. The waterjet cutters weren't confident I could get a reasonable finish using copper, so I opted for plated aluminium. But even if I did use copper, it would still need to be plated with nickel so I can spot-weld nickel tabs to the plates. Both copper and aluminium soak up so much heat that the welds are stopped in their tracks. The layer of nickel makes for a more resistive path, and generates a localised hot spot where the two metals can fuse. But it's still pretty ordinary.

Nickel plated aly sucks.jpg (146.22 KiB) Viewed 605 times

Wherever there's a dark shmutz on the plate, the tab won't stick, and when it does stick, it's easy to lift it up with a fingernail of effort. I really hope the tabs stay put!

[francisco.shi]

I think the problem with Al is that it is very difficult to get a metallic bond to other metals. Copper on the other hand likes to alloy.

[nuggetgalore]

On point 5, I have a feeling nickel plated copper might have been a better option. The waterjet cutters weren't confident I could get a reasonable finish using copper, so I opted for plated aluminium. But even if I did use copper, it would still need to be plated with nickel so I can spot-weld nickel tabs to the plates. Both copper and aluminium soak up so much heat that the welds are stopped in their tracks. The layer of nickel makes for a more resistive path, and generates a localised hot spot where the two metals can fuse. But it's still pretty ordinary.

Nickel plated aly sucks.jpg

Wherever there's a dark shmutz on the plate, the tab won't stick, and when it does stick, it's easy to lift it up with a fingernail of effort. I really hope the tabs stay put!

I wonder why you cannot get copper lazer cut?
I had no trouble finding operators that do both lazer and water jet, lazer much cheaper, absolutely perfect, accurate to within less than 0.1mm (not that I needed that accuracy),
no distortion, no burrs,no discolouration. I doubt nickel plating copper is more difficult than aly,as far as I know some plating may even need a copper primary (or platinum if the matrix is non metallic, but that is a different story).

[francisco.shi]

Nickel plating on copper is dead easy. Anyone can do it.
Plating Al is not something I want to try.

[jonescg]

Copper is highly reflective, so it needs to be a fibre laser so the optics don't get burned out. They can't cut nickel for this reason either.
Waterjet cutting has a kerf of about 0.5 mm which is probably OK for this stuff, but the sheer number of holes and cuts made it expensive and slow. I'd block up the machine for a week.
As for welding to nickel plated copper, the layer of nickel is so thin, spot welds might not stick as the copper soaks away all that heat. Even welding two bits of 0.15 mm thick nickel with a piece of aluminium behind it doesn't make an ideal bond, but better than what I currently have.

[nuggetgalore]

Copper is highly reflective, so it needs to be a fibre laser so the optics don't get burned out. They can't cut nickel for this reason either.

Strange that no one in Perth has a laser that can do reflective material, of the 6 laser cutting businesses within a half hour from me two could do it. There would be a dozen or more in Melbourne. I would have had to wait three weeks+ for my busbars to be cut by waterjet, with laser I could pick them up in less than a week.

[jonescg]

Oh there may well be one, but I was fast running out of time (still am). Plus I have no experience with nickel plated copper and how it welds.

[SPanna]

did you use electroless nickel plating or traditional?