Mercedes EQS modules with satellite BMS boards

[Daveturpin]

I have these Mercedes EQS modules. I am integrating these into my 1971 Beetle project, and right now I am trying to find the least expensive path forward for charging and BMS.

What I know: These are from a Mercedes EQS. They are CATL modules, 18s1p, 66v, 9+ kWh each. I have five, which I selected because they will fit in custom floor-mounded boxes and give my Beetle decent range and power. The prime mover is a Tesla rear SDU.

What I think I know: The black boxes on each module is a slave BMS. The 26 wires coming into this slave are probably 18 cell voltages, ground, temp, current sensors, etc. The open connector has 4 wires, which I ASSUME is for the CANBus.

What I don't know: So much!

CanBus is 2 wires, CANH and CANL. But there should also be a 12v in and a ground, right? Without 12v the CAN won't wake up and send signals? I have a CANBus logger, but I need help trying to figure out which wire is which. Is there a procedure to figure it out? So, I tried using a multimeter across the pins, and I did see 12v from a couple pins, which diminished to zero. Assuming that I discharged a capacitor? Can I assume that the pin that was 12v temporarily was the 12v in wire? I have 4 other modules that probably have charge in their capacitors still.

I have tried researching these connectors and these BMS, and there is not much info out there. One German website inscrutably calls this a "control module". I also haven't had much luck decoding Mercedes wire colors. Although each wire is orange with a stripe, I would guess that orange/black is B-, orange/red is B+, and orange/green, orange/blue is the CAN signal? I don't want to fry anything by tinkering without more knowledge.

My goal: I want to verify that I am dealing with a BMS slave, and log the CAN signals, so that I can send those logs to Vero Electric to see if they recognize the format and possibly if the Vero BMS 2.0 can run them. That would be an ideal solution.

If I cannot figure that out, I can go a full BMS. I would love to use the OpenInverter BMS cards, but they are 12-channel and ideally I would have 1 18-channel BMS board per module?

This is the advice that I am looking for. Happy to share what I learn, because these EQS modules tick a lot of checkboxes for EV conversions: They are compact, relatively light, sturdy, high-voltage, and at the very least have easy-to-access wiring harnesses!

[tom91]

Open up the slave module and take detailed pictures of the slave boards.

No point speculating.

[Daveturpin]

Thanks in advance for your help!

Okay, what I see: 18 voltage channels, 2 CAN channels?

The traces are so tiny I need a magnifying lens. But it appears the 4 wires in the harness use the outer 4 pins on that connector (out of 8) which each seem to go to a diode and an oscillator? No obvious 12v or ground. Can you make sense of this?

[Daveturpin]

I don’t see any traces on the back so I think those inner 4 are just part of the Hella connector and not used in this circuit.

[tom91]

LTC6813 so its has ISO-SPI

https://www.analog.com/media/en/technic ... 8134fa.pdf

Old chip for something so "new"

[Daveturpin]

Excellent, thank you.

On the one hand getting a CANBus reader was a waste. On the other hand reading the specs of the chip makes more sense. It can handle 18 cells which is probably why Mercedes went with 18s1p in the module.

Does “Passive cell balancing up to 200 mA (max.) with programmable pulse width modulation” mean it can do some balancing without the master IsoSPI controller?

[tom91]

Does “Passive cell balancing up to 200 mA (max.) with programmable pulse width modulation” mean it can do some balancing without the master IsoSPI controller?

No, passive balancing means it balances by switching resistors across the cells. It is a a dumb front end slave so will report data to the IsoSPI master and the master will do the calcs and logic and then send commands out.

[Daveturpin]

Gotcha.

And... Do you know of a BMS that can communicate via isoSPI, or am I going to need to build something custom, centered on this LTC6820 chip I keep reading about?

This unit is pricey but it appears to have the same LTC 6813 chip my slaves use...

https://stealthev.com/product/aem-batte ... em-master/

[tom91]

You will need to write custom code to work with the setup you have. The Dilithium bms https://www.thunderstruck-ev.com/mcu.html might work

As their satellite uses the same chip https://www.thunderstruck-ev.com/bms-satellite.html

Best to reach out to Thunderstruck

[Daveturpin]

I will do that, thank you. I will post additional information here for others when I get it and if I get the OEM satellite BMS working. (I realize that I have been using the antiquated "slave" terminology; apologies)

[Daveturpin]

Looks like the DC1941D demo card is what I need to test and log these isoSPI signals. $88.

Thunderstruck said that the MCU can “theoretically” run the satellite BMS modules but recommended against it. (Even if it would work I’m sure they’d say that for liability reasons)

I’ll maybe get it anyway and then use my new demo board to explore further…

[Daveturpin]

Okay I’ve learned that the DC1941D will probably work but you also need either the DC590B usb interface or the Linguino interface for it. Picked up the former on eBay and trying to get the SPI wires right to see if I can get data.

Also the connector to the SPI interface: found it on Digikey:

https://www.digikey.com/en/products/det ... 1/10478170

Here is my setup:

Computer connects to the DC590B demo card. This can interface with a bunch of stuff (including a gas gauge driver I also have) but I am connected to the DC1941D. The RJ45 uses ONLY pins 1 and 2, which I connected to a connector so I can experiment with the final connection to the Mercedes BMS satellite. Now I don’t know which is SPI in and which is SPI out, and of those choices and don’t know which is IM and which is IP… so 4 possible configurations to test.

I still don’t know if the MCU will work; don’t want to drop $480 just to test it, at least without more data.

[Daveturpin]

More progress! So, long story short. The "Quickeval" software and DC590 board does NOT work with the LTC6813 18-cell monitor.

So I tried using an Arduino Uno to read the cell voltages directly. That doesn't work, without building a separate LTC6813 circuit for isoSPI to SPI conversion.

Finally I broke down and bought the Linduino One (DC2026C), which interfaces with the LTC6820 isoSPI board.

Took a little setting up. The LTSketchbook available on GitHub doesn't seem to have the right part number, but I found the right code and installed it, and using the above setup, was able to talk to the modules!

And I immediately freaked out. I checked all 5 modules, and 2 of 5 are reporting less than 3 volts per cell (bad!)

But then I checked the MODULE voltage, and each are at more than 64 volts... Then I realized each of those "bad" cells are showing EXACTLY 2.7306v. I checked the documentation and this corresponds to a return of 0x00. And then I further realized those two modules are "left handed" modules. Which means I am probably reading the wrong "side" of the isoSPI...

So... Phew. I believe that I have a working way to communicate with the satellite BMS cards on the modules, and I believe that all my modules are good (and apparently at about 50% SOC?)

Also big takeaway: If I can communicate then the Mercedes modules are not using some sort of proprietary control suite. So... PERHAPS I will be able to use this to act as the Master BMS with some additional code (I found some SAE team code that might work) Or PERHAPS I can use the Thunderstruck BMS off-the-shelf?

[tom91]

So you are finding so far what I said is true.

Good to know Mercedes have not opted to try and lock down their BMS for no real reason what so ever.

[johnlr]

Another option to communicate with ISOspi modules is Arduino shield DC2617A from Analog Devices (they are in short supply though), this will plug into an Arduino board (Uno a bit slow?, Uno R4, Duo) this provides CAN bus and ISOspi. I have hacked the Simpbms code to work with Uno R4 and LTC6812 based Polestar 2 satellite modules, could be adapted to work with LTC6813 satellite modules. It would be nice to see a ISOspi version of EVS-BMS(new Simpbms) hardware, this would make it simple to use ISOspi based BMS modules.

[tom91]

It would be nice to see a ISOspi version of EVS-BMS(new Simpbms) hardware, this would make it simple to use ISOspi based BMS modules.

The demand of this has been low, I have designed the EVS-BMS with an option board. I will review if ISOspi with the LTC6820 is possible, did not look into when doing the design due to the chip being on impossible lead times.

I have also ran the DC2617A, but with a teensy 3.2 (simpBMS) and the polestar modules.

[Daveturpin]

So you are finding so far what I said is true.

Good to know Mercedes have not opted to try and lock down their BMS for no real reason what so ever.

Yes, and I am very appreciative of the help you are giving me. When I talked to Thunderstuck, though, they recommended against using their MCU without also using their satellites. (Which are expensive and would require me rewiring EVERYTHING which seems both counterproductive and hazardous)

Everything sinse then has been me learning what you already know! Last month I didn't even know what isoSPI was (or that Arduino uses SPI; I have used Arduinos for ages and didn't know this).

I haven't used the LTC6812 but from what I understand it is pretty similar to the '11 and '13. The '11 Does 8 cells, the '12 15, and the '13 18.

[JonasErnst]

Hey, i plan on using the exact same modules, did you get the BMS Interface working? Maybe you can post your code Snippets here so we can work on getting it running together