# Tesla BMS



## Sunking (Aug 10, 2009)

Anyone know for sure if Tesla BMS is Passive or Active.

I would think Active Mid Balance shuffling power from higher energy state Bricks to Lower state Bricks. At least I think that is what I have read before. I just don't see Tesla claiming to have the "_Most Advance BMS in the World._" Bleeding power off as lost waste heat is not advanced IMO. But then again Musk is full of himself. 

Anyone know or have links?


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## dcb (Dec 5, 2009)

I've read it was passive. A few folks have gotten excited thinking it was active then dissapointed to realize it was passive.

The balance current is trivial though, not quite worth the effort, especially with all those cells in parallel averaging things out and the temperature controls.


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## eldis (Sep 3, 2013)

I've seen those boards a year or so ago, and they are using standard and proven passive balancing, handled by a BQ76PL536A from TI (I'm using the same chip in my BMS system). There is no power anyway that would be worth saving. I don't know how people came up with the idea of high currents being dissipated in the BMS. 

You can see the size of the balancing resistors to get an idea about losses:










Everytime I see some commercial semi-DIY crappy BMS with huge balancing resistors on board, I think to myself "okay, another poor battery pack is going to die thanks to these"


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## Sunking (Aug 10, 2009)

eldis said:


> I've seen those boards a year or so ago, and they are using standard and proven passive balancing, handled by a BQ76PL536A from TI


Interesting. At what level is the balancing done? I assume what Tesla calls Brick Level? If that is the case not sure I see the point as the power is so small could take a very long time. 

Only reason I ask is because I have heard both ways with no authoritative answer. However if the Tesla board uses the TI chip is convincing.


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## eldis (Sep 3, 2013)

tomofreno said:


> Boy, that sure is the pot calling the kettle black!


Nice saying, never heard it before! 

As a matter of fact, this indeed is (one of) the best BMS systems in the world. The boards have a timestamp of 2012, but if I would be redesigning those now in 2015, I wouldn't change a thing. Also the amount of Tesla's on the road without major battery failures sends a clear message - he was not lying.


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## tomofreno (Mar 3, 2009)

eldis said:


> Nice saying, never heard it before!
> 
> As a matter of fact, this indeed is (one of) the best BMS systems in the world. The boards have a timestamp of 2012, but if I would be redesigning those now in 2015, I wouldn't change a thing. Also the amount of Tesla's on the road without major battery failures sends a clear message - he was not lying.


 I thought it was, but then thought someone might think it had racial connotations so I deleted it. Too bad, as I don't know another one that conveys the idea so well.


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## eldis (Sep 3, 2013)

Sunking said:


> Interesting. At what level is the balancing done? I assume what Tesla calls Brick Level? If that is the case not sure I see the point as the power is so small could take a very long time.
> 
> Only reason I ask is because I have heard both ways with no authoritative answer. However if the Tesla board uses the TI chip is convincing.


In that case this is the definitive answer to their BMS "mystery". The balancing is done on a parallel group basis. Even if there would be a slight misbalance, even a small resistor is able to level it out. That's how little today's batteries drift apart. Of course we are talking about high quality cells with thermal management. Chinese LiPos glued somewhere close to the motor in an RC airplane will be a different story.


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## Sunking (Aug 10, 2009)

tomofreno said:


> I thought it was, but then thought someone might think it had racial connotations so I deleted it. .


I have no problem with you being a Racist and making a Freudian Slip. Happens to everyone from time to time. Everyone in the world is prejudice. You know I am Black, and I don't like crackers. So we are even.

Now aren't you proud of yourself starting this? I asked valid question and you turn it into racial slurs.


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## Tomdb (Jan 28, 2013)

To get a pack this big out of balance would be hard, because the pack never goes below its knee. At least not without all sorts of warnings going off in the car, maybe even the car will just refuse to work below a certain cell level.

Besides regulating it, think about the statistics of the amount of cells used. This would even out all the variations that are present among cells. Also quality control for the production of the 18650's are in a whole other league then the Iron phosphate bricks from china.


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## Sunking (Aug 10, 2009)

Tomdb said:


> To get a pack this big out of balance would be hard, because the pack never goes below its knee.


That is what had me questioning what I have heard. I have heard both ways but Active seemed more appropriate.

So would it be correct to say Telsa is using Mid Balance with Passive Balance?


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## Tomdb (Jan 28, 2013)

No, they are most likely using top balancing. Most likely, and most appropriate for a "normal" car. Because when will cars stand still for a longer time, at the charger ofcourse. So balancing with a full pack makes most sense for a commercial product.


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## dcb (Dec 5, 2009)

well, it doesn't look like a "shunt" sized resistor, so I'm not inclined to think it is necessarily "top" balanced, as in something convenient to do while charging. And you can't bring up cells using bleeding alone.

If it never hits the knee(s), then it is middle balanced, as in somewhere in the middle. I suppose one could charge/discharge a "new" tesla and see where the cell voltages align.

edit: this link seems to indicate that you need to occasionally do a %100 charge/balance, not sure how accurate it is or anything, but there it is:
http://www.teslarati.com/top-5-tips-to-maintaining-ev-battery/#dY3irRL4TQqByA23.99 , it seem quite possible that the author may not actually know what they are talking about in this regard though.


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## Sunking (Aug 10, 2009)

Tomdb said:


> No, they are most likely using top balancing. .


Thanks Tom but I don't think Tesla Top Balances as I am pretty certain they never take the batteries to the top or bottom. I do not think any commercial EV's takes their batteries to 100%. I know Chevy Volt runs 80/20, and Nissan is similar. That is the only way they can offer such long warranty periods.

Personally I do not think any of the EV manufactures do not have to do a lot of balancing to begin with. They buy much higher quality cells then we can and buy them by 100's of thousands so they can match cells up and sort them out. That would allow them to either Top or Bottom balance initially. Since they do not drift much if at all, no reason to continuously balance, just run them in the Middle. At least that is what I seem understanding and it makes sense. 

I was just under the impression they used Active balancing rather than Passive. Figured someone here would know. All of them have been around long enough to have some salvaged and someone would Reverse Engineer. Or defectors from the EV manufactures would leak.


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## PStechPaul (May 1, 2012)

As to the PKB reference, the Wiki has an interesting twist on the saying, and similar sayings dating back to BCE:

http://en.wikipedia.org/wiki/The_pot_calling_the_kettle_black

It is not a racial slur.


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## Ampster (Oct 6, 2012)

I don't know for sure but I see balancing wires on those oem packs. Sure, those wires are also used to monitor the packs. I am not sure it matters if the balancing is done at the top or somewhat below. 
For example, I have a stationary pack used to enhance the solar panel system and my grid tie inverter. I never take those cells to the top. I charge the cells to 3.50v but the balancing begins at 3.35. At first the balancing was being done all the time but now it only happens when the charger is charging. My pack feeds the inverter from 14:00 to 20:00 when my TOU rates are the highest. I charge from 22:00 to 08:00 when rates are low. I use an Orion BMS and that is just one example of the flexibility that probably exists in OEM Battery Management Systems.


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## rochesterricer (Jan 5, 2011)

What? Since when is "pot calling the kettle black" racist, or even a reference to race at all? Do we perhaps have some rather young folks making assumptions about a very old expression they may not be familiar with?


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