The isolation is on the Slave modules. An opto isolator isolates the BMS monitoring IC from the Serial bus. This is why you need to feed the slave boards on each module 5V.

 

Looking at Tesla model s BMS remote modules there appears to be no isolation there

This is not correct.

The remote module (BMS slave board) inside each Model S battery module includes isolation. There is no need for additional isolation in the BMS master.

 

The battery module mounted circuit boards on a Tesla are called BMB's or Battery Monitoring Boards. The pack level controller is a BMS. BMB communications are always isolated since they communicate in a daisy chain and each module is at a different potential so some form of level shifting is required. Although isolation on the BMS side of the BMS/BMB interface is not required, it's general practice to do so for redundancy. We generally try and make sure single point failures don't cause HV safety issues. Similar to the idea of having 2 pack contactors. On a side note, the Model S BMB's have a designated daisy chain fault line that should be monitored by the BMS as well. It's another redundant means of protection the designers at Tesla felt necessary.

I have a BMS that I'm about to release that supports all Tesla battery modules S, X, 3 and the new M3 LFP packs most don't know about yet. My BMS has isolated communications to the all BMBs and monitors the fault line on the MS BMB's.

 

I've been working a little bit with the newer Battery Management ICs from Analog Devices and TI; I like their ring architecture with true galvanic isolation. Definitely a generation ahead in BMS design. Too much to take on for a hobbyist project right now though.

Look forward to what you are coming out with. After this I'm starting to think of doing Model s boards (cheaper than the ICs themselves at this point lol), hack together some of the SimpBMS code and create a new board with another opto isolator and lots of isolation space on the PCB.


Still don't like providing that 5V from the controller; might have to make something that provides 5V from a local module to keep further isolation.

 

Are you planning to Open Source the code for it to enable improvement, maintenance, and add features?

 

Wasn't planning on that exactly, but want to make that happen for the UI. The present processor is out of code space again, so adding a secondary "open source" processor or dual core micro is under serious consideration.

 

So nothing on the SimpBMS side of things? Probably a belt and suspenders type thing; but coming from the aerospace world I err on the side of extra safety. Even just having leads going into the high voltage box that aren't isolated on the controller side worries me. That's one physical wire failure away from a potential path from high voltage to low voltage.

 

How will you power up the main contactors in the pack if not from a LV wire into the HV enclosure?

Mitsubishi Cell Monitoring Boards (CMU) LV supplies. [edit The "HV and LV sides" on the schematic refer to where the component is located with respect to the 4mm isolation gap on the board]

 

How will you power up the main contactors in the pack if not from a LV wire into the HV enclosure?

Traditionally; yes.
You generally have that separated from the cells, not directly connected to a PCB with direct connection to the cells. The physical gap between sides is much larger at a contactor.

Two options to keep a full gap; pull power from a single module with a traditional switching DC/DC converter. Use optically or galvanical isolated commands from the VCU to control. Downside; this will lead to imbalance in the pack.

Other option; flyback converter from full pack. Same isolated comms.

 

Seems a bit weird that the 12V battery is labeled "HV side" and the supply off the HV battery module is the "LV side". The 12V system will be at HV potential relative to some modules.

Be careful with that, any of you trying to figure out that schematic.

 

The 12V has to be galvanically isolated from the modules for the contactors. As does anything going into or out of the passenger compartment.

You need a cut loop to depower the HV outside the battery box reliably.

DC-DC converters fail.

You need to be able to "jump start" the car from an external 12V source.

The automotive engineering brain trust on the planet has decided to use a separate 12V battery for when the HV battery has been shut down.

 

Oh, totally agree. That's the quickest simplest safest way; at least the best combination of those for contactor control.

I was just answering the question how you would keep the battery box 100% isolated. It can be done; just probably not cheaply. Redundancy would need to be present driving cost up significantly.

 

If you can figure out the microcontroller they programmed, might not be as horrific. I doubt they fabbed their own silicon.

 

This isn't a programmable microcontroller, just a line driver. We haven't been able to match its behaviour to any off the shelf product.