Battery Modules Questions.

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coulomb
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Re: Battery Modules Questions.

Post by coulomb » Sat, 13 Apr 2019, 21:51

weber wrote:
Sat, 13 Apr 2019, 21:28
I also need to be able to divide them electrically into two equal (or nearly-equal) half packs, for charging.
Are you wanting to retain the "max ELV between any two points with the power off" criterion as well? That means a contactor every 29 cells max (using 4.1 V as the maximum cell voltage). I think we used 28 cells maximum between contactors in the original design, since it was conceivable that LFP could go to 4.2 V. That could complicate the battery design considerably.
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weber
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Re: Battery Modules Questions.

Post by weber » Sun, 14 Apr 2019, 06:49

Yes Mike, I'd want to preserve something like that. I'm willing to stretch it to 34 NMC cells (143 V) between contactors for this exercise. The limit of 28 LFP cells was only because we had many rows of 14 cells due to the chassis rail spacing, so 28 was convenient. We could have gone to 32 LFP cells for ELV-segments because more than 3.65 V would be cell abuse and would only occur if multiple protections had failed.

Francisco, I find it hard to understand how you could get numbers almost twice what Mike found (and which I agreed were correct). I notice that 85 = 5 * 17 and 102 = 2 * 3 * 17. So I guess you are using purely per-cell dimensions, stacking them 17 high in the 215 height, and ignoring the overheads for the manifolds in all dimensions. How is this valid?
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Re: Battery Modules Questions.

Post by francisco.shi » Sun, 14 Apr 2019, 09:27

We can change a few things to make the cells fit.
One is to change the manifolds so that they indent into the cell this would make the end tabs flush with the manifolds.
Also we could make the cooling plates hold two cells instead of one. This would save over 12mm of width.
I have a quick model of how they could fit in the 1092x253x215 box. (6x17 cells)
The width from the end of the square pipes is 263mm (instead of 253)
test1.JPG
test1.JPG (134.45 KiB) Viewed 618 times
Here is a top view with the box constraint.
test2.JPG
test2.JPG (51.19 KiB) Viewed 618 times
Here is a side view with the box constraint.
test3.JPG
test3.JPG (113.36 KiB) Viewed 618 times
It would still need some development to make sure everything fits and that we can connect the tabs together.
The distance between cells is 12mm in height. I am not sure if we can get a screw and nut to fit in this gap without shorting to the cell clamp above.
Also the cooling plates will not be as efficient but I do not think this will be a big problem.
And I do not know where you would put the BMS.

Anyway. This may give you more ideas.

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Re: Battery Modules Questions.

Post by weber » Sun, 14 Apr 2019, 09:52

I wrote the following before receiving your latest above.

I believe we can only stack the cells 16 high in the 215 height because we need a 16 mm overhead for strapping and manifold. n x 12 + 16

I see from your other thread,
https://www.aeva.asn.au/forums/viewtopi ... 178#p70178
that the cells are 120 mm wide. I understand the tubes are 6 mm. Allow another 2 mm for the cooling plate wrap and the strapping. That's 8 mm each side, bringing the total to 136 mm. And perhaps the 154 or 153 mm dimension of the manifold can come down to 152 mm. That would make the formula
k x 136 + 16, giving the series 152, 288, 424, 560, 696, 832, 969, 1104. The other dimension is
m x 354 + 120, giving the series 474, 802, 1182.

So the best we can do in the 1092 x 253 space (under the boot) is 802 x 152 which is 2 x 1 x 16 = 32 cells. However this happens to be the only box in the car where it is possible to expand it (at the risk of having too much weight on the rear wheels) but only in the 253 dimension. If we expand that to 288, we can have 2 x 2 x 16 = 64 cells.

In the 678 x 500 space (under the bonnet) with a missing corner that brings those dimensions down to 586 and 376 in places, the best we can do is 560 x 474 which is 4 x 1 x 16 = 64 cells.

When you look at the MX-5's battery layout diagram, you may wonder why the two fuel tank boxes couldn't be combined into one box. The main reason is that you couldn't get them in or out if they were in one large box. To get the rear one out, you must first lift out the forward one and slide the rear one into its place.

So in the two 678 x 253 spaces (fuel tank), the best we can do is 474 x 152 which is 1 x 1 x 16 = 16 cells in each.

That's a total of 160 cells. 32 more required.

None of the remaining spaces can take any cells in the laid-flat orientation. If instead they are standing on edge, with the 153 dimension in the 215 height, then in the 632 x 130 space (radiator) we can fit 474 x 124 which is 1 x 9 x 1 = 9 cells.
In the 908 x 130 space (behind rollbar) which I really don't want to use, due to sun exposure, we can fit 802 x 124 which is 2 x 9 x 1 = 18 cells.

That's a total of 187 cells, and none of the remaining spaces can take any cells in any orientation.

I could probably live with the voltage of 187 NMC cells, but I'd still have cells exposed to sun, and I'd still have 6 separate battery boxes. I was hoping to get that down to 4 or less.
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Re: Battery Modules Questions.

Post by brendon_m » Sun, 14 Apr 2019, 10:11

Looking through the build I'm thinking what you need is a liquid battery pack that can just be poured into the car to fill all the nooks and crannies

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Re: Battery Modules Questions.

Post by weber » Sun, 14 Apr 2019, 11:07

brendon_m wrote:
Sun, 14 Apr 2019, 10:11
Looking through the build I'm thinking what you need is a liquid battery pack that can just be poured into the car to fill all the nooks and crannies
Hee hee. You're sooo right. :)

Jeff Owen and Mike Van emmerik did a brilliant job of helping me squeeze cells into every nook, and quite a few crannies. But I wouldn't want to have to repeat the exercise. And nor would they!
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Re: Battery Modules Questions.

Post by weber » Sun, 14 Apr 2019, 11:14

Stacking them 17 high doesn't leave room for aluminium bar or rectangular-tube to distribute the strapping pressure. 16 is the max.

Removing the tubes from between rows of cells not only reduces cooling but also structural strength.

Reducing the manifolds from complete end-plates to individual square tubes reduces structural strength and eliminates possible mounting points.

I don't see how a 20 gee collision rating could possibly be achieved with all these reductions.
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Re: Battery Modules Questions.

Post by francisco.shi » Sun, 14 Apr 2019, 12:44

I think you are right.

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Re: Battery Modules Questions.

Post by weber » Sun, 14 Apr 2019, 13:28

Thanks for trying. It's just bad luck with the available dimensions in the MX-5.
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Re: Battery Modules Questions.

Post by francisco.shi » Fri, 31 May 2019, 19:46

I am about to order the E63 cells.
A rough estimate on the price is US$42 per cell and about A$3.20 per cell for the freight with the current quantity.
This is around U$180/kwh
The exchange rate at the moment is 0.69 so including freight it would be around A$271/kwh.


If anyone is interested in getting some please let me know.

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