Cool student project!

Batteries, Chargers, and Battery Management Systems.
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jonescg
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Cool student project!

Post by jonescg » Jan 08, 2018 11:40 pm


Rube
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Re: Cool student project!

Post by Rube » Jan 10, 2018 5:05 am

Very interested in the cell fusing and materials used for statement "cells are actively cooled with a CNC machined cooling plate mounted to the terminal plates through electrically isolating material'' Thanks for sharing.

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Re: Cool student project!

Post by jonescg » Jan 10, 2018 9:19 pm

^^ Me too - I really like this approach. Very compact and the cooling, although perhaps not as good as a radial approach, will still work quite effectively.

Might write to them and see how they got on.

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Re: Cool student project!

Post by jonescg » Jan 11, 2018 12:23 am


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fechter
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Re: Cool student project!

Post by fechter » Jan 11, 2018 10:51 am

Wow, that's a crapload of wires coming out of the battery. Seems like way more than you would need for 22s. Hope all of them get hooked up to the right spot. :shock:

Nice pack design otherwise.
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Re: Cool student project!

Post by jonescg » Jan 11, 2018 8:48 pm

fechter wrote:
Jan 11, 2018 10:51 am
Wow, that's a crapload of wires coming out of the battery. Seems like way more than you would need for 22s. Hope all of them get hooked up to the right spot. :shock:

Nice pack design otherwise.
Yes I noticed that - balance wires would explain ~23 of them, but the rest of them - :?

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Re: Cool student project!

Post by Rube » Jan 12, 2018 5:59 am

Perhaps the temperatures were logged across the pack?

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Re: Cool student project!

Post by suecy » Jan 12, 2018 6:47 am

Formula student rules require measuring at least 30% of cell temperatures

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Re: Cool student project!

Post by jonescg » Jan 12, 2018 7:16 am

That's epic. So much data! No wonder they go for low voltage battery packs :lol:

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Re: Cool student project!

Post by suecy » Jan 12, 2018 10:43 am

30% of all individual cells therefore less cells in series do not reduce the amount of needed sensors.
Actually most top teams utilize the maximum allowed 600V (144S2P of 6.6Ah Melasta LiPo or similar).
Last season i was in charge of my teams BMS and managed to measure and balance all cells without wires by placing the temperature sensors directly onto our BMS PCB and screwing it on top of our busbars.

There`s lots of good info in the FSG rules and check out the video from one of the Audi Sport LMP1 battery designers: https://youtu.be/krqpMNPHqMg?t=32m20s

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Re: Cool student project!

Post by fechter » Jan 12, 2018 2:37 pm

Seems like a silly requirement. I'll bet a Tesla doesn't have nearly that many temp sensors. I guess they are doing this in the interest of safety. Using larger format cells so you can run 1p would be an advantage here.
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Re: Cool student project!

Post by coleasterling » Jan 12, 2018 2:59 pm

Most teams don't use the max 600V. There's actually a pretty good distribution with most teams between 300V and 500V. The best teams are typically around 450V.

The pack design linked above is for Formula Hybrid in the US, which has a 300V limit along with FSAE-E at Lincoln, NE. FH has some significant rules distinctions from either FS or FSAE-E, as well. There have been a lot of complaints over the years about how the FH and FSAE-E rules committees are keeping voltage limits artificially low in order to keep FS teams from dominating the US competitions. Having won the FH competition in '11, I think they are just a little more paranoid about safety. Either way, US teams would definitely choose to run higher voltages if the rules allowed it.

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Re: Cool student project!

Post by jonescg » Jan 12, 2018 8:29 pm

Thanks for the background guys! Yeah Tesla has 32 temperature sensors (two per module) and they keep an eye on the coolant temperature in and out.

System voltages are largely determined by the motor/inverter choice, and most of these are in the 400-700 V DC range.
Voltron Evo runs a 700 V max battery because that's the rules - 168 cells in series. The numerology works out perfectly: 4 blocks of 42s, and 42s / 2 is 21s, which puts the terminal at the opposite side as the start of the run, so you can turn around and come back. To build a lower voltage version of the bike would require 88 cells in series (326 V nominal, 369 V max). Which is *just* outside the voltage limit of the PM150DX... Same system voltage as a Mitsubishi EV powertrain...

Still - I really like their cooling approach. I'd love to know how they got on.

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Re: Cool student project!

Post by coleasterling » Jan 13, 2018 12:16 am

They failed miserably, having never passed tech with a car that was on a 2 year development cycle. Here's a link to car info: https://urchinz.github.io/FH_Web/content/VMR-2.html

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Re: Cool student project!

Post by foolsday » Jan 14, 2018 10:33 pm

Hi all,

Came across this post by random chance and though I'd chime in. I oversaw the design of the team's battery and co-designed the BMS that year. I'm really glad people found it interesting!

It's been a while, but I can try to answer a few of the questions:

The electrical isolation material used was commercially available Sil-Pad (thermally conductive electrical insulator). I don't remember the brand or model # in particular, but I believe it had a conductivity of around 3 W/(m-K).

We designed and tested our own fusible link geometry so we could integrate them into the collector plates. The design we settled on was basically a necked down section of material with a "D" shaped tab on the end for welding to the cell. These tabs were pushed down through an insulation plate to make contact with the cell terminal.
Tab cross-section.png
We validated the fuses by paralleling a few low voltage high current power supplies together and running current profiles through the links in-situ on a mock-up cold plate to simulate the pack conditions as closely as possible.
fusing_link.jpg
fusing_link.jpg (19.76 KiB) Viewed 92 times
One of our concerns was a failure mode where a link could pass enough current to hover near melting point without fusing. This could potentially melt through the sil-pad insulator and break isolation between the pack and cooling plate. The quick and cheap solution we settled on was adding an aluminum heat spreader on top of each collector plate. We tested this and found that it spread the heat evenly enough to not overheat the insulator.
heat_spreader.png
heat_spreader.png (69.06 KiB) Viewed 92 times
Crapload of wires is right. Almost all of them are going to temperature sensors glued between sets of three cells. The reason for such excessive numbers is that we wanted every sensor location to be double redundant. We knew it'd be an hugeundertaking to attempt maintenance after welding collector plates, and didn't want to risk disqualification if one failed in the future. Only one sensor in each of the 44 sites is actively monitored by the BMS. The wiring took days, but the data was satisfying.

Our pack voltage was directly determined by a motor we already had and a donated charger we could have never afforded on our own (thanks Magna Power http://www.magna-power.com/blog/2016/ba ... ybrid-team). Having a lower voltage but higher p-count was also nice in terms of single cell failure redundency, even with the hit to efficiency.

suecy's solution is definitely cleaner! Our centralized BMB approach and space limitations made it very difficult to locate sensors at the ends of the cells. If we had more space, it would have been nice to run sensors to the bottoms of each cell (opposite the cooling plate) to monitor the hottest point and increase cell packing density.

coleasterling: Well that's a little harsh, but you are correct. Like many FH teams, we didn't pass tech (though the design of our battery was not the culprit there as far as I know). At least in my time, our team was often guilty of focusing more on fun engineering projects than the challenge of getting a car finished in time with the resources and budget at our disposal ;) .That was my last year with the team, but the current group of students are working on some really awesome stuff - I'll try to get them to update the blog more than once every 1.5 years.

Sorry for the long post, hope that filled in some of the gaps!

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Re: Cool student project!

Post by jonescg » Jan 15, 2018 12:38 am

foolsday wrote:
Jan 14, 2018 10:33 pm
Sorry for the long post, hope that filled in some of the gaps!
Thanks so much for the insight! The fusing is an interesting point - a cell failure resulting in the neighbouring parallel cells dumping current will heat up the fusible link, but as you point out, there's a chance it might set fire to stuff. The heat spreader also improves the ampacity of the buslink, so double win.

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Re: Cool student project!

Post by Rube » Jan 15, 2018 7:30 am

Hot dam Foolsday that was a very insightful post :D I read the cells are welded to nickel terminal plates. Is that pure nickel at 0.13 mm? If I read correctly (aplogies on my phone) the heat spreader was directly affixed to the nickel terminal plates? Cheers

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Re: Cool student project!

Post by coleasterling » Jan 15, 2018 4:18 pm

foolsday wrote:
Jan 14, 2018 10:33 pm
coleasterling: Well that's a little harsh, but you are correct. Like many FH teams, we didn't pass tech (though the design of our battery was not the culprit there as far as I know). At least in my time, our team was often guilty of focusing more on fun engineering projects than the challenge of getting a car finished in time with the resources and budget at our disposal ;) .That was my last year with the team, but the current group of students are working on some really awesome stuff - I'll try to get them to update the blog more than once every 1.5 years.
I guess it depends on your perspective. As an individual, you probably learned quite a bit and significantly improved your engineering skills. If that's your metric for success, then you did extremely well. I'm with the others in thinking your pack design was very innovative.

I get very frustrated at Hybrid because so few teams compete every year. Some teams have been doing Hybrid from the start of the competition and still don't pass tech consistently. Please don't take this as an assault on you or your team. I think it has far more to do with project organization and culture than individual teams. The FH officials have essentially said it is okay to fail to pass tech and that deadlines don't matter. That doesn't sit well with me.

Not passing tech means teams didn't read the rules well enough or didn't seek clarification on items that could be confusing. Or, they prioritized the wrong projects instead of insisting the car as a whole be finished. From an outside perspective, FH seems to push pie in the sky science projects instead of promoting timely and efficient engineering design. Unfortunately, that has repeatedly led to management failures that sideline otherwise very hard working teams at competition. All of that fantastic detail design doesn't mean much if the team can't validate it.

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Re: Cool student project!

Post by foolsday » Jan 15, 2018 10:12 pm

jonescg: Yeah, that's a great point about the ampacity. We should have stuck a milli-ohm meter on there and seen what the typical resistance ended up at.

Rube: Yep, those were (close to) pure nickel terminal plates. Sorry, the illustration isn't very clear. 0.13mm was the thickness of the thermally conductive electrical insulator. The terminal plates were 1.28mm (.050"). The heat spreaders were affixed directly to the plates. Nickel really wasn't a very good material for the job. The resistivity is ~2.5x higher than copper, melting point is ~350K higher than copper, and it was very expensive. We only settled on it because of the challenges of welding copper to cells with our DIY discharge welder...

coleasterling: No offense taken, I share many of your same frustrations with the Formula Hybrid competition. Designing and building a reliable, competitive vehicle on a deadline is in many ways a more valuable and difficult challenge. I agree that most teams fail to get there not for lack of effort or talent, but because their project and resource management skills just aren't there. From the perspective of one who participated for 4 years, I feel that the inconsistency, vagueness, and flawed logic surrounding many of competitions rules exacerbates this problem when teams arrive for tech inspection.

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