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Discussion Starter #1
So my dad and I are working on converting a '77 VW Rabbit to electric. We plan on using the AC-20 (with the original transmission), a Curtis 1238E-7621, 4x batteries from a Tesla Model S/X (to reach 96v), and use a 115v charger from the wall. The car will not be driven in a hot climate.

The BMS is the only thing we really have no idea what to do with. I found this PDF from Orion which seems very helpful:
https://www.orionbms.com/manuals/pdf/tesla_modules.pdf
But I'm not sure I understand what it is I'm supposed to get. So let me lay it all out:
* Each battery contains 6 cells in series (which for the sake of easy terminology, I'll refer to as a "set").
* According to the PDF, the BMS is only concerned with the number of sets.
* The battery I want to get has 444 cells, suggesting there are 74 sets.
* According to Orion, the BMS2 can be [directly] configured up to 180 sets.
* We intend to use 4x batteries, which would be 296 sets.

So here are my questions:
1. Unless there's something I'm not understanding, are we expected to get more than one BMS to accommodate all batteries? Or even 1 BMS per battery? Seems like that'd get very expensive very fast.
2. Do we even need a BMS, considering we're not doing anything too extreme?
3. Is there a better option other than Orion?

Much thanks in advance.
 

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1-You only have 6 cells in serie in each 444 cells module (6 ''set''). So you need a BMS for 24 cells only. All parallel group of 74 cells (444/6) could be consider as a single cell.

2-Yes. Some errors with other lithium type of cell can cause problem, but with tesla cell the more serious problem can be your complet car/house in fire.

3-I like Zeva bms.

Good luck and learn as mush you can before tempting dangerous stuff.
Don't hesitate to share pictures of your project.
 

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Discussion Starter #3
1-You only have 6 cells in serie in each 444 cells module (6 ''set''). So you need a BMS for 24 cells only. All parallel group of 74 cells (444/6) could be consider as a single cell.

2-Yes. Some errors with other lithium type of cell can cause problem, but with tesla cell the more serious problem can be your complet car/house in fire.

3-I like Zeva bms.

Good luck and learn as mush you can before tempting dangerous stuff.
Don't hesitate to share pictures of your project.
1. Ah, I had a feeling that was the case. It seemed a little wrong to me that I needed to compensate for 296 "sets". But, this is why I asked.
2. I will heed your advice seriously. A 24-cell BMS isn't too terribly expensive for what would otherwise be a very costly mistake.
3. Cool, I'll look into that. I assume Zeva counts cells the same way (meaning, I would need it configured to check for 24 cells)?

Thanks for the quick reply.
 

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We use a zeva bms in a static 14 cell home energy recovery system using nissan leaf cells.

Zeva is easy to set up, easy to install, and so far faultless and reliable.

Check out zeva bms australia.

Anthony.




1. Ah, I had a feeling that was the case. It seemed a little wrong to me that I needed to compensate for 296 "sets". But, this is why I asked.
2. I will heed your advice seriously. A 24-cell BMS isn't too terribly expensive for what would otherwise be a very costly mistake.
3. Cool, I'll look into that. I assume Zeva counts cells the same way (meaning, I would need it configured to check for 24 cells)?

Thanks for the quick reply.
 

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A "string" of cells or packs is the standard term for units connected in series.

I've seen "group" used a lot for cells in parallel, especially at the lowest level. Bank for the overall end result at the top level.

"Blocks" and "modules" can be used for sub-bank level units if needed, ideally in a precisely defined way for a given bank's layout / design.

Pack is usually reserved for units original to an EV when recycling.
 

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The ideal design as far as BMS cost is concerned is each cell as large as possible to get to the needed Ah without paralleling.

I assume for an EV of any range, 200+ Ah cells would be small, I've seen House banks on boats use 1700Ah cells, with diesel used for propulsion.

Maximum two strings total if redundancy is desired, but ideally just one.

Thus every cell is individually monitored by the BMS.

When going to a design involving hundreds of tiny cells, obviously that becomes impractical.

But with expensive BMSs, I would expect them to cost at least as much as the cells in order to get decent granularity.

Maybe consider building the lower-level modules with cheap easily replaced BMS, and then only use the expensive ones at the top treating each module as if it's a single cell?
 

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Discussion Starter #7 (Edited)
Thanks for the great input everyone.

Those Zeva BMSs seem a lot cheaper compared to the Orions, even with the additional accessories. The manual also seems very easy to follow.

So if I understand correctly, I need 1 of each of their "Electric Vehicle Management System V3" products, except I need 2 of the BMS modules?

Any advice on wiring it up to Tesla batteries, or should it be pretty much work the same way as the Orion PDF I linked to earlier?


Maybe consider building the lower-level modules with cheap easily replaced BMS, and then only use the expensive ones at the top treating each module as if it's a single cell?
I'm not sure I understand what you're suggesting here.
 

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I don't know the details on the Tesla packs, so just generalizing.

Monitoring each cell is just not affordable with those pricey BMSs.

If possible using the cheap under $100 BMSs may be more so.

Each smaller subset could then (maybe) be treated as if a single cell, allowing that "top level" layer to be managed by a single (or at least a smaller number) of the expensive BMSs.

Dunno how to word it more simply than that, and as I said just spitballin, those who know more please chime in if the concept is impractical.
 

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Discussion Starter #9 (Edited)
I don't know the details on the Tesla packs, so just generalizing.

Monitoring each cell is just not affordable with those pricey BMSs.

If possible using the cheap under $100 BMSs may be more so.

Each smaller subset could then (maybe) be treated as if a single cell, allowing that "top level" layer to be managed by a single (or at least a smaller number) of the expensive BMSs.

Dunno how to word it more simply than that, and as I said just spitballin, those who know more please chime in if the concept is impractical.
Haha well, considering I was questioning if I needed a BMS at all, I'm actually not especially concerned with monitoring every single cell. I just assumed I had to based on the way that Orion PDF phrased things, but I was corrected on what it was implying.

It seems other people have got the ZEVA system to work with Tesla batteries (though, I haven't found a lot of in-depth details yet). So as far as I'm concerned, going with ZEVA ought to give me sufficient safety and features. At this point I guess it's just a matter of how to hook up the BMS to the batteries. It doesn't seem hard, I just don't have the info laid out in front of me. I'm sure I could figure it out by myself but I for one don't like to experiment with expensive and dangerous equipment if there's already something documented.
 

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Discussion Starter #13 (Edited)
60V max per module. 700V max per system.
By the way, if you have some $ left, buy the monitor and the current sensor to have access to full information during driving/charging and be able to reprogram your bms.
Ah ok, that makes a lot more sense.
And yes, I was thinking of getting the monitor and current sensor. I know they're not a necessity but to my understanding, the monitor is also the only way to access the 2 outputs, which we want to hook up to the dashboard lights for stuff like the temperature warning.



What about using the tesla BMU boards already on each module with Tomdb's master BMS controller board?

Here is the link in the classifieds,
https://www.diyelectriccar.com/forums/showthread.php?t=198263

and he has links to github to find the technical details thread "Tesla BMS"
I do have some experience working with Arduinos, but not so much working on electric cars and high-wattage batteries. That being said, I'm not really willing to experiment on something where I'm not 100% confident correctly setting up a commercial system. It seems Tomdb's solution is still very much a work-in-progress too; some of the source code was updated just a few days ago. Considering this car doesn't even have airbags and the BMS is pretty much the only real safety feature this has, I'm not so sure it's a good idea for me to be taking chances on that.

But, thanks for bringing it to my attention anyway.
 

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I have an Orion BMS on my Fiat X1/9. It uses a single box of electronics which can be optioned up to manage strings of lithium cells of almost any length. My system can support 48 cells. It talks to my charger via CAN bus using standard OBD2 protocols so I have a Bluetooth dongle hanging on the OBD2/CAN interface which allows me to monitor the system with a tablet or my phone.


The Zeva BMS appears to be similar in principle but uses smaller, cheaper modules each capable of 12 cells maximum. The ZEVA system uses CAN bus so I'm guessing you could do a similar monitoring setup to my Bluetooth arrangement.

In your situation your need to manage a string of 24 cells (ignoring that each "cell" is actually 74 cells in parallel) so you need an Orion fitted for 24 cells or 2 Zeva monitoring modules (plus the other bits). If you read the Zeva manual carefully it says the 60V maximum is for each 12 cell Zeva module, the pack limit is 600V when using multiple modules.



A cheaper option may a "distributed" BMS like this:
https://evparts.com.au/ev-power-bms.html
The distributed system has one cheap module for each cell (so you will need 24 of them) which are linked together into chain with some other electronic bits to tell the charger to turn off (or tell you the battery is nearly flat so pull over!). It's very simple and does the basics of protecting your battery, but doesn't give you any clever stuff like reporting individual cells voltages.


I think the other people in this thread recommending Zeva may well be correct as the price looks pretty good.


Good Luck!
 
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