# Inside of Tesla pack



## rochesterricer (Jan 5, 2011)

Good stuff


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## Karter2 (Nov 17, 2011)

Ahh !...
That blows a few myths any rumours out of the water !
Bulk charging, direct , not via the BMS .
No "cell level" BMS monitoring capability .
Etc


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## Hollie Maea (Dec 9, 2009)

Karter2 said:


> No "cell level" BMS monitoring capability .
> Etc


Yeah I laughed at some of the posts marveling that there was no BMS "beyond the 96 modules" or "at the cell level". Obviously they haven't thought very hard about how BMS works or what it does. The individual cells below the 96 module level are paralleled. That means that it is physically impossible for any of them to differ from the others in voltage even the slightest amount. If something happens to one of them to make it want to be a different voltage (I.e, a short), then the others automatically pour in current to it, the fuse pops, and that cell is no longer part of the pack.

The BMS for that module will see that a cell has dropped out, but it doesn't know nor care which one it is.


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## sholland (Jan 16, 2012)

Yeah, I had fun reading through the thread and seeing so much speculation that there is some kind of active charge shuttling or other magic going in the pack, only to see it finally sober up when they realize it's just a simple passive balancing BMS. No magic...


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## Karter2 (Nov 17, 2011)

Most importantly, it's reassuring to know that these packs can (with care) be broken down into smaller units ( 6S, 74P,....24v, 230Ahr ) for reconfiguration and reuse in different formats and voltage's.
Many, myself included, had until now thought that these packs were virtually impossible to break down for reuse without major effort reconnecting at cell level.


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## Hollie Maea (Dec 9, 2009)

sholland said:


> Yeah, I had fun reading through the thread and seeing so much speculation that there is some kind of active charge shuttling or other magic going in the pack, only to see it finally sober up when they realize it's just a simple passive balancing BMS. No magic...


I'm sure that if they don't have active BMS by now, they'll have it very soon.


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## sholland (Jan 16, 2012)

What makes you so sure? I like that they have so many cells in parallel, making the cells pretty much as matched as you could get, ESR and capacity wise. I can't see a need for anything other than really simple balancing.


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## Karter2 (Nov 17, 2011)

It sound like the Mod3 pack may be VERY different to existing Tesla packs in both size , complexity, and cell type.
IF..it is a smaller (30kWhr ?) pack made from the hinted 22700 cell format of 5+ Ahr, then that could be comprised of 15P groups or even less, instead of the current 74P groups, and balancing may become more important.
I certainly would expect that they will "improve" their BMS and other systems to suit these changes in pack design.


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## Hollie Maea (Dec 9, 2009)

sholland said:


> What makes you so sure? I like that they have so many cells in parallel, making the cells pretty much as matched as you could get, ESR and capacity wise. I can't see a need for anything other than really simple balancing.


Well that's more an argument for whether or not they need BMS at all, not what kind is best. And the answer is...usually they probably don't need it at all.

But sometimes they do...sometimes you get some imbalance. Especially when you have a few dozen thousand cars out on the road. And it's intolerable for any monkey business with the batteries to be visible to the customer.

So, in the instances where your BMS is actually having to do stuff, there are advantages to an active balancing system. The main advantage is that you can pull cells up in addition to pushing them down. That way you don't have to choose one side of the charge to balance on--you can stay balanced at all stages of the SOC. Especially with a big battery like Tesla has, if they are only balancing at the top, imbalances can start to creep in and multiply if an owner isn't in the habit of topping the SOC off on a regular basis.

Smaller companies are starting to implement active cell balancing, so it would be a little surprising if the biggest electric car company on the planet didn't also have it soon.


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## lithiumlogic (Aug 24, 2011)

To me it seems that the cells are spaced much closer together than in the earlier shots of a Tesla pack, which were from a RAV 4 EV. I'm wondering if that is why penetrations by road debris lead to a domino effect fire in the Telsa.. I know of at least one instance of road damage in a roadster and another in a Smart car which did not lead to fire.

They could have given up the completely flat cabin floor, for a minor transmission hump, and spaced them out a little better.


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## Karter2 (Nov 17, 2011)

Well the pack still has the cooling tubes woven between the cells.
Also. penetration damage effects would be all about random luck of where the penetration occurs.
Likely those Mod S pack fires could have been caused by the damage to the pack connection buss bars or other interconnects, causing high current shorts which even battery isolation could not prevent.
Questions remain unanswered as to whether these packs do actually have the fire retardant "intumescent goo" applied ?
.. its not apparent ?


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## lithiumlogic (Aug 24, 2011)

There is cell level fusing so even a shorted bus bar should result in immediate disconnection. The only type of short circuit that is possible is one within the cell itself, from mechanical disruption of the separator film. The consensus on TMC forum is that there is no intumescent goo.


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## Karter2 (Nov 17, 2011)

Each one of those cell fuses can carry 12+ amps at minimum according to 
Tesla forums, so each 74P module could dump 900+ amps into some random damage zone....much more for a short period. And at voltages up to 3-400v ?
Enough to melt a lot of plastic and generate some serious hot spots !??


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## lithiumlogic (Aug 24, 2011)

If a bus bar is shorted though, 99% of the time it is a low impedance short, so will blow the fuses instantly. I suppose it's possible for a high resistance short to make heat without blowing the fuses.

The pack is pretty well hardened against fire, there isn't that much to burn.. apart from batteries... the plastic content is a pretty tiny % of the mass, and there's that fire retardant mat above the cells. Even the cells themselves aren't that easy to set alight, they have a lot of thermal inertia, it's not like an extension cable running under my window, setting the curtains on fire (readily ignitable materials close at hand). 

The problem is the domino effect, once one cell ignites it seems to inevitably spread to the next. Tesla designed the roadster pack so that one cell can't set the whole pack on fire, and i'm sure the MS pack passed the same test, but the road debris event probably compromised several cells in the same place.

I also read somewhere that crush damage to the end caps of cylindrical cells is more likely to cause thermal runaway than being pinched in the middle.

Ultimately, once a cell goes into runaway, it's got a certain amount of thermal energy which it's going to give out. The trick of the pack design is to divert more of that energy into the environment than into neighbouring cells, and to maximise the ability of it's neighbours to get rid of the heat. It's where the computer modelling comes in.


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## Karter2 (Nov 17, 2011)

You could not rely on the pack thermal controls (cooling loops etc) to remain functional after a major impact/penetration damage.


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## sholland (Jan 16, 2012)

Hollie Maea said:


> Well that's more an argument for whether or not they need BMS at all, not what kind is best. And the answer is...usually they probably don't need it at all.
> 
> But sometimes they do...sometimes you get some imbalance. Especially when you have a few dozen thousand cars out on the road. And it's intolerable for any monkey business with the batteries to be visible to the customer.
> 
> ...


Please don't get me wrong, I am most certainly an advocate of active balancing, where it makes sense. 

BTW, I have 5A active cell balancing in my own car... The chips are out there if you know where to look.


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## gunnarhs (Apr 24, 2012)

evmetro said:


> This may have already been posted at some point, but I thought it might be cool to see. Here is a link just in case
> 
> http://www.teslamotorsclub.com/showthread.php/34934-Pics-Info-Inside-the-battery-pack


Well nice pictures. This seems a very simple system. 16 Blocks of each about 5 kWh (24V /210 Ah) which are connected in series to form a 80 kWh Block( 384V /210 Ah)
Each 5kWh Block seems to consists of about 70 (74?) parallel cells to form a chain with 6 such chains in series give the 24V /210 Ah (6x4V and 70 *3Ah).
Each 5kWh Block has its "own" BMS- Module with 7 inputs 
(nr. 0-6 on the board). 
So it seems that one parallel group of 70 (74?) cells (4 V, 210Ah) is considered as one "battery", 6 of them are in series and are watched by each (sub) BMS-module . 
This means that the Tesla BMS as total sees 96 batteries in series, 
(resulting from the 16 boards with each 6 "big" batteries).
There is no way that they can do cell balancing per cell level at one of those 7.000 cells. Forget active charging also. But it is a very simple way of construction and it is quite simple to see an error in one of the 96 modules and tracing it to one of the 6 submodules. And it is quite easy to bypass.
The question remains how they monitor the battery cell temperature.
What does Tesla demonstrate? 
Cutting edge technology ? No.
Clever usage of "old" concepts. Yes


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## pdove (Jan 9, 2012)

They don't have it simply because it doesn't work. No other reason.

The chip on Tesla board does have cell balancing capability Texas Instruments bq76PL536AQ1.

A friend of mine traced the circuit and it can shunt the cells across a 39.5 ohm resistance with the FETs on the board.

I still can't see how it works since the cells are on the level of miliohms all the current will still go through the cell.

The only possible thing to do is turn on the FETs to drain cells while the charger is off but it will only pull 100 mAmps 4.2 V / 40 ohms and this is across 74 cells in parallel so that would be less than 2 mill amps per cell I can see how this would balance a cell.
*
*


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## pdove (Jan 9, 2012)

gunnarhs said:


> This means that the Tesla BMS as total sees 96 batteries in series,
> (resulting from the 16 boards with each 6 "big" batteries).
> There is no way that they can do cell balancing per cell level at one of those 7.000 cells. Forget active charging also. But it is a very simple way of construction and it is quite simple to see an error in one of the 96 modules and tracing it to one of the 6 submodules. And it is quite easy to bypass.
> 
> ...


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## sholland (Jan 16, 2012)

pdove said:


> They don't have it simply because it doesn't work. No other reason.


I'm not sure what you are saying doesn't work... passive balancing with ~100mA on 74 parallel cells, or active balancing?

These are all very well matched cells. They don't need much balance current. 

All the current generation EV cars have passive balancing, and all in the range of ~50 - 100mA. There are only a few specialty vehicles that currently ship with active balancing. Active balancing is mostly used in packs where there are very large capacity cells (100mA wouldn't make a dent), significant impedance or capacity mismatch (read Chinese LiFePO4 cells), large temperature deltas (buses, trucks) or huge load or charge currents where any of these mismatched are exacerbated (performance vehicles, grid storage, telecom basestation, etc.).


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## pdove (Jan 9, 2012)

Well a little simple math 2.6 amphour cell.

Or in the case of the Tesla cell 74 in parallel 192 Ah cell.

lets round to 200 for ease of math.

200 divided by .1 it would take 2000 hours to drain the pack.

Lets say you just want to remove 1 % of imbalance that 2Ah.

That would take 20 hours. Ok say 0.1% imbalance well 2 hours.

How far will that get you down the road?

The whole idea makes no sense to me.


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## sholland (Jan 16, 2012)

Agreed, but it comes back to how much mismatch there is in the pack. Tesla (or any other OEM for that matter) won't tell anyone that number. 

All OEMs have put their money into quality cells, and skipping the complex BMS for now.


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## Karter2 (Nov 17, 2011)

pdove said:


> . A 100Ah CALB LiFePO4 cell has multiple cells inside of it all in parallel. ?


 I have seen the pictures,..
but has anyone accurately detailed how many parallel "cell units" there are, their capacity etc. for the 60, 100, 180 Ahr "bricks" ?


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## lithiumlogic (Aug 24, 2011)

Wait , are we talking about "shunting" or actual charge transfer?

Shunting means connecting a resistor across the cells you want to drain a bit, to bring their voltage in line with the rest of the series string. If this is the method they're using, it must be at a very slow rate as i don't see any heatsinks or any large resistors/power transistors that can dissipate much electrical energy as heat.

The more sophisticated scheme operates by transferring charge from the higher cells to the lower one. Since power is only being moved, rather than expended as heat, you don't need heatsinks etc.


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## sholland (Jan 16, 2012)

They are only shunting.


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## pdove (Jan 9, 2012)

Karter2 said:


> I have seen the pictures,..
> but has anyone accurately detailed how many parallel "cell units" there are, their capacity etc. for the 60, 100, 180 Ahr "bricks" ?


I believe there were 10 I have pictures somewhere.

There really not much to them. copper and aluminum foil with stuff smeared on them and plastic garbage bag separating them.. I believe Jack took one apart in one of his videos on EVTV as well. Maybe it wasn't a CALB but there's not much difference between manufacturers.


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## gunnarhs (Apr 24, 2012)

pdove said:


> We have the same issue even if cell balancing was possible. A 100Ah CALB LiFePO4 cell has multiple cells inside of it all in parallel. All of them do.
> Haven't you ever cut one open?


 I have not opened the ThunderSky/Wilson cells we have but I was told that they had about 10 smaller units (which indicated if you look at the picture) http://www.alliancerenewableenergy.com/100-Ah-TS-LFP100AHA.htm.
I do not consider my a specialist for batteries but I can assure you that it is more difficult to control 70 than 10-20 cells. The difference lies for example in the calculation of the internal resistance of a group parallel resistances.
But I agree that we have the same issue and we do the same thing to resolve it as Tesla by applying a passive BMS. We only have a simpler case than them. So the question is what is more reliable?


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## Hollie Maea (Dec 9, 2009)

gunnarhs said:


> There is no way that they can do cell balancing per cell level at one of those 7.000 cells


I can't believe how many people get this concept wrong. Paralleled cells are ALWAYS perfectly balanced, by definition, because they are shorted together.

From a balancing standpoint, a 96S76P pack is exactly the same as a 96S1P pack.


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## gunnarhs (Apr 24, 2012)

Hollie Maea said:


> I can't believe how many people get this concept wrong. Paralleled cells are ALWAYS perfectly balanced, by definition, because they are shorted together.
> 
> From a balancing standpoint, a 96S76P pack is exactly the same as a 96S1P pack.


 Are you talking about self-balancing then?
Not the same thing as using BMS which you have to do also for parallel as serial connection.
I may have used the wrong term but in my understanding balancing means both in sense of voltage and current loops within the pack. So if you have degraded batteries parallel together with good due to different internal resistance (which is inevitable) there are current loops there. 
Here more on that: 
http://www.mpoweruk.com/balancing.htm
And the golden rule:
"No matter what battery management techniques are used, the failure rate or cycle life of a multicell battery will always be worse than the quoted failure rate or cycle life of the single cells used to make up the battery."
Meaning more cells -> More failure rate
But I encourage you to make an experiment by using for example three chains of smaller batteries instead of one chain of bigger once.
Have seen that both with and without BMS


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## sholland (Jan 16, 2012)

Hollie Maea said:


> I can't believe how many people get this concept wrong. Paralleled cells are ALWAYS perfectly balanced, by definition, because they are shorted together.
> 
> From a balancing standpoint, a 96S76P pack is exactly the same as a 96S1P pack.


Well I assumed what was being discussed was balancing of series cells. Not sure why anyone would even consider balancing the parallel cells, for the reasons you mention.


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## gunnarhs (Apr 24, 2012)

sholland said:


> Well I assumed what was being discussed was balancing of series cells. Not sure why anyone would even consider balancing the parallel cells, for the reasons you mention.


Well to be able to balance the series cells you have to know the finite state of the parallel group (in terms of voltage and resistance) which makes up one serial "battery" 
You can rely to self balancing up to a certain limit for a small number of parallel cell units.
In our case this has worked well for up to 20 (200 Ah battery) in case of lithium cells.
The determining factors are internal resistances and total capacitance of the parallel group. Meaning that the (self or with help circuit) balance is reached in a certain time. 
However you still have to be able to monitor somehow the individual state as in the parallel group a shortcut of one cell can result a total damage of the "battery" (the more common case though is slowly increased resistance).
The Tesla concept implies the solution of being able to monitor each cell, both electrically and temperature - wise which would be an improvement to using LiFeYPO4 blocks (which are at least 4 parallel cells) as most converters are doing. However it seems (to me at least) that Tesla's implementation of just monitoring the 70 parallel cell as one serial looks even more crude. In my experience this would be a shitty concept except they have some additional cell(Temperature?) monitoring which I can not see.


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## sholland (Jan 16, 2012)

From looking at the tear down, they do not monitor individual parallel cells. If a cell does drop out of the parallel set, you would start to see voltage mismatch appearing under charge or load that would need to be balanced out. If a pack becomes difficult to keep in balance, that would indicate a certain number of cells have shorted and _removed _themselves, then the BMS will alert the higher authority controller, then presumably the factory.

I know of no one that monitors parallel cells.


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## gunnarhs (Apr 24, 2012)

sholland said:


> From looking at the tear down, they do not monitor individual parallel cells. If a cell does drop out of the parallel set, you would start to see voltage mismatch appearing under charge or load that would need to be balanced out. If a pack becomes difficult to keep in balance, that would indicate a certain number of cells have shorted and _removed _themselves, then the BMS will alert the higher authority controller, then presumably the factory.
> 
> I know of no one that monitors parallel cells.


And I know of no EV except Tesla that uses this amount of 70 (!) cells in parallel. In my experience 30 in parallel did not work well, neither in terms of charging nor balancing .If you have 10 cells in parallel changes of one cell are easily detected and countered by applying for example shunt resistance. But I doubt that it is as easy to detect a deviation of say 5 of 70 cells. I am not saying this affects performance much until one of the 5 shortcuts.

Correction: I know some drag racers which use big parallel packs of course  But they are not meant to last long


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## Hollie Maea (Dec 9, 2009)

gunnarhs said:


> And I know of no one except Tesla that uses this amount of 70 (!) cells in parallel. In my experience 30 in parallel did not work well, neither in terms of charging nor balancing .If you have 10 cells in parallel changes of one cell are easily detected and countered by applying for example shunt resistance. But I doubt that it is as easy to detect a deviation of say 5 of 70 cells. I am not saying this affects performance much until one of the 5 shortcuts.
> 
> Correction: I know some drag racers which use big parallel packs of course  But they are not meant to last long


The company I worked for made a 74P pack. It works just like a regular pack. Maybe you are thinking of parallel strings or something, otherwise what you are saying makes no sense.


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## gunnarhs (Apr 24, 2012)

Hollie Maea said:


> The company I worked for made a 74P pack. It works just like a regular pack. Maybe you are thinking of parallel strings or something, otherwise what you are saying makes no sense.


I am talking about connecting for example 3 LiFePO4 100 Ah units parallel and monitoring them as one battery.
I am just saying this did not work well with our normal BMS.
We had to downgrade to 200 Ah batteries and ended finally in getting a BMS from Australia where one circuit per battery was applied.
This of course did cost 2000 USD more but was worth it.
What company did you work for and for what application was the pack? Are you saying that they are having 74 p monitored like Tesla with a standard BMS-solution monitoring it one serial battery without any problems?


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## rochesterricer (Jan 5, 2011)

gunnarhs said:


> I am talking about connecting for example 3 LiFePO4 100 Ah units parallel and monitoring them as one battery.
> I am just saying this did not work well with our normal BMS.
> We had to downgrade to 200 Ah batteries and ended finally in getting a BMS from Australia where one circuit per battery was applied.
> This of course did cost 2000 USD more but was worth it.
> What company did you work for and for what application was the pack? Are you saying that they are having 74 p monitored like Tesla with a standard BMS-solution monitoring it one serial battery without any problems?


It seems as though you are talking about the Chinese batteries available to the EV community. From what I understand, they don't have the manufacturing consistency of the cells made by companies like Panasonic. The differences in IR in individual cells are likely greater in the stuff the DIY EV community works with.


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## sholland (Jan 16, 2012)

rochesterricer said:


> It seems as though you are talking about the Chinese batteries available to the EV community. From what I understand, they don't have the manufacturing consistency of the cells made by companies like Panasonic. The differences in IR in individual cells are likely greater in the stuff the DIY EV community works with.


Exactly. The cells any OEM is assembling a pack from are likely from the exact same lot, and even assembled into a pack directly from the battery assembly station. Tesla isn't there yet, but Nissan and others already do this. Once the Gigafactory is up, they could assemble packs directly off the cell production line.


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## gunnarhs (Apr 24, 2012)

rochesterricer said:


> It seems as though you are talking about the Chinese batteries available to the EV community. From what I understand, they don't have the manufacturing consistency of the cells made by companies like Panasonic. The differences in IR in individual cells are likely greater in the stuff the DIY EV community works with.


Yes that was what I thought first and in the first packs we ordered there was a failure rate up about 2 of 100. The other failures were due to BMS/charging. We finally the got a pack together which worked well but were financially exhausted.
At least it triggered a confidence for the government to buy some electric cars (they had been scared with first version of MIEV which was a dreadful car and the LI-battery was blamed).
But during that I had some look into the laptop-battery-business which uses"quality" batteries from for example Panasonic. I acquired a stack of newly failed laptops intending to get the batteries for testing. The ratio was 1/20 of laptops that had failed/diminished battery-packs, usually only one of the 6 failed. So the ratio seemed about the same there as in the Thunder-Sky (Wilson) batteries, meaning 1-2 of 100. That and some tests with different arrangement stopped me from the crazy idea creating a small pack from it as I did not see any advantage except the possibility of space arrangement and possible per-cell management but at exponential increased cost.


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## gunnarhs (Apr 24, 2012)

Good paper covering the subject, it gives you the formula for the time-constant , compares charge and discharge and different chemistry of batteries. Hope this helps a bit for understanding the problem
http://liionbms.com/php/wp_parallel_balance.php

As seen a big factor is the resistance, capacity and the number of the parallel cells. Also the importance of paralleling at the right State of charge.
74 does not look like a lucky number here


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## Karter2 (Nov 17, 2011)

gunnarhs said:


> I acquired a stack of newly failed laptops intending to get the batteries for testing. ...
> ..... I did not see any advantage except the possibility of space arrangement and possible per-cell management but at exponential increased cost.


 Using careful choice of 18650 cell ( not ex laptop) , there can be significant advantages in Energy density, power density, voltage "stiffness" and pack life,. 
Lifetime cost can also work out very competitive.
The disadvantage is complexity and assembly cost ..as can be seen from the Tesla pack, and hence why Tesla are developing larger capacity cell units.


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## gunnarhs (Apr 24, 2012)

Karter2 said:


> Using careful choice of 18650 cell ( not ex laptop) , there can be significant advantages in Energy density, power density, voltage "stiffness" and pack life,.
> Lifetime cost can also work out very competitive.
> The disadvantage is complexity and assembly cost ..as can be seen from the Tesla pack, and hence why Tesla are developing larger capacity cell units.


Yes it is for a big pack like they use, they can save about 10% weight.
For a 500 + kg. pack this is a lot.
In my case with new batteries (which I would have bought if going in that direction) , this would have been 10-15 kg for the small pack


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## kennybobby (Aug 10, 2012)

i guess it takes a big charger for all those little cells...


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## Hollie Maea (Dec 9, 2009)

gunnarhs said:


> Good paper covering the subject, it gives you the formula for the time-constant , compares charge and discharge and different chemistry of batteries. Hope this helps a bit for understanding the problem
> http://liionbms.com/php/wp_parallel_balance.php
> 
> As seen a big factor is the resistance, capacity and the number of the parallel cells. Also the importance of paralleling at the right State of charge.
> 74 does not look like a lucky number here


Our battery pack was built from LG cells. We did not test or sort the cells, but if you grabbed any random cell from the boxes, it was within one millivolt.

After assembling 6216 cells, the pick voltage was within ten millivolts of forecasted.

With good cells and good interconnects, there is zero problem doing this.


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## Hollie Maea (Dec 9, 2009)

gunnarhs said:


> What company did you work for and for what application was the pack? Are you saying that they are having 74 p monitored like Tesla with a standard BMS-solution monitoring it one serial battery without any problems?


The company is EVDrive, and the pack is for an electric Land Cruiser.

The BMS was just monitoring each voltage node, the 74 parallel groups truly act as one unit. You can see the BMS boards in the image below (I finally have clearance to show these images).

Incidentally, these are replacing, in the exact same box, a 24kWh Enerdel pack. Those are great modules, but we are putting in 69kWh in the same space, which goes to show how much greater the volumetric density is with these cells. If Tesla were using other cells, they would have the same crappy 100 mile range that everyone else has.


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## rochesterricer (Jan 5, 2011)

Good stuff Hollie. Can you disclose the total weight of that 69kWh battery box?


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## tomofreno (Mar 3, 2009)

Hollie Maea said:


> Our battery pack was built from LG cells. We did not test or sort the cells, but if you grabbed any random cell from the boxes, it was within one millivolt.
> 
> After assembling 6216 cells, the pick voltage was within ten millivolts of forecasted.
> 
> With good cells and good interconnects, there is zero problem doing this.


 Thanks for posting the photos. I suppose it may be proprietary, but how do you do the cell connections? Is there a spot welded wire to each cell which acts as a fuse as in the Tesla pack? What is the pack voltage? Provided the spot welding is very repeatable and high quality, and the cells are well-matched, the Tesla approach seems fairly bullet proof to me. Good application for a precise robotic welder though. I would guess Tesla has much of this automated for much greater quality and repeatability.


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## Hollie Maea (Dec 9, 2009)

rochesterricer said:


> Good stuff Hollie. Can you disclose the total weight of that 69kWh battery box?


We didn't get a final measurement of the pack in the box, but it was about 750 pounds.



tomofreno said:


> I suppose it may be proprietary, but how do you do the cell connections? Is there a spot welded wire to each cell which acts as a fuse as in the Tesla pack? What is the pack voltage?


Unfortunately I can't divulge the connection method, as that's the secret sauce. We don't use the wire fuse method Tesla uses though. IMO, I think our fuse has some advantages, but I might be a bit biased 

Nominal pack voltage is 310V (84S...that corresponds to the seven 12 unit BMS boards you see in the picture)


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## Hollie Maea (Dec 9, 2009)

Incidentally, I've been doing some tests on these cells, and I'm astounded at how much safer they are than they were a few years ago. These cells are not "power" cells--they are rated at 2C PEAK, which comes out to about 5.8 amps. Anyway, I took a damaged one and hooked it up for a six amp draw without cooling and left it there for 14 minutes, at which point it was depleted (it wasn't full at the beginning). That thing got up to 79 degrees, but didn't catch on fire.


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## Karter2 (Nov 17, 2011)

tomofreno said:


> s there a spot welded wire to each cell which acts as a fuse as in the Tesla pack? .


 The rumor is that Tesla are moving away from the "fuse wire" connects, and have patented a "flex PCB" system using foam pressure pads to make "dry" cell connects. (possibly with conducting paste ?) 
Both +ve and -ve contacts on the same end of the cell also.


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## gunnarhs (Apr 24, 2012)

Hollie Maea said:


> The company is EVDrive, and the pack is for an electric Land Cruiser.
> 
> The BMS was just monitoring each voltage node, the 74 parallel groups truly act as one unit. You can see the BMS boards in the image below (I finally have clearance to show these images).
> 
> Incidentally, these are replacing, in the exact same box, a 24kWh Enerdel pack. Those are great modules, but we are putting in 69kWh in the same space, which goes to show how much greater the volumetric density is with these cells. If Tesla were using other cells, they would have the same crappy 100 mile range that everyone else has.


Ok thanks for the great info this is very interesting to see. Obviously your LG-cells are a lot better than those we have been using from Wilson/Thundersky and the standard LI-cells I have been testing .We would not have the expertise in our team to put together the Battery packs so tight without damaging them. We were looking at the Enerdel - modules and had an offer from a distributor of them for a 25 kWh pack which. At that time the price was about minimum 20.000 USD. But as our National power company suffered from range anxiety after the MIEV experience they decided to buy a Thundersky pack which was in my opinion to big and had a crappy BMS with it (separately). We changed it to a decentral passive prototype BMS which applied one circuit per 200Ah battery (basicly shunting a group of parallel 20 cells in 36 serial battery pack) . This resulted a range of 200 km /1200 kg. car (160 km in worst condition) which was a lot more than the 100 km asked for. You can see some shnapshot of it here. Note that the "boxes" are for research purpose as the main purpose of the project was to test and monitor the batteries, it is not intended for final build.


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## gunnarhs (Apr 24, 2012)

Hollie Maea said:


> Incidentally, I've been doing some tests on these cells, and I'm astounded at how much safer they are than they were a few years ago. These cells are not "power" cells--they are rated at 2C PEAK, which comes out to about 5.8 amps. Anyway, I took a damaged one and hooked it up for a six amp draw without cooling and left it there for 14 minutes, at which point it was depleted (it wasn't full at the beginning). That thing got up to 79 degrees, but didn't catch on fire.


Ok how would it being managed in the pack shown on the picture, is there a thermal management or are the amps /watts restricted somewhere in the BMS?. I assume you must use also liquid cooling for the pack?
(I am not lurking for secrets here, well maybe a bit he,he) .
Tesla does not seem to hesitate to overspec their batteries in terms of rated C and power assuming they use the standard Panasonic version. For example their 60 kWh Tesla has the same power-specs as the 85kWh. Also they very recommend using their high speed chargers.
The other thing I would be interested in is the rough price for a 40 kWh module but I can contact EV-drive directly if that is better


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## Hollie Maea (Dec 9, 2009)

gunnarhs said:


> Ok how would it being managed in the pack shown on the picture, is there a thermal management or are the amps /watts restricted somewhere in the BMS?. I assume you must use also liquid cooling for the pack?
> (I am not lurking for secrets here, well maybe a bit he,he) .


No liquid cooling in this pack--the application is such that the speed is limited to about 25mph. So sustained periods of high current are impossible. I tested these cells without cooling at about 2 amps for 30 minutes. Temperature only got up to about 37. But with the size of the pack, 2 amps is 45 kWh, a power level that would be impossible to sustain for more than a minute or so.

In a higher power application, liquid cooling would be mandatory and higher power cells might be needed too.

You can contact evdrive directly if you are interested. Our prices aren't super low though, as we are a relatively small company and have engineering costs to amortize.

Edit: If temps did get too high, the BMS would indeed restrict power.


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## rochesterricer (Jan 5, 2011)

gunnarhs said:


> Tesla does not seem to hesitate to overspec their batteries in terms of rated C and power assuming they use the standard Panasonic version.


Tesla has stated they do not use the standard cells.


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## tomofreno (Mar 3, 2009)

Karter2 said:


> The rumor is that Tesla are moving away from the "fuse wire" connects, and have patented a "flex PCB" system using foam pressure pads to make "dry" cell connects. (possibly with conducting paste ?)
> Both +ve and -ve contacts on the same end of the cell also.


I was thinking of something like that - using flex circuits, as the EVdrive packages look as if they may use something like that. Pretty cheap to make all connections on flex circuit boards with integral fuses. Bit concerned with obtaining repeatable, low contact resistance with just pressure contact though. But individual cell currents are low, and some conductive paste would help. Liquid cooling would be a challenge.


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## Madmac (Mar 14, 2008)

gunnarhs
quote>>
And I know of no EV except Tesla that uses this amount of 70 (!) cells in parallel. In my experience 30 in parallel did not work well, neither in terms of charging nor balancing .If you have 10 cells in parallel changes of one cell are easily detected and countered by applying for example shunt resistance. But I doubt that it is as easy to detect a deviation of say 5 of 70 cells. I am not saying this affects performance much until one of the 5 shortcuts.
<<

Reading the attached file referred to above...... the section copied below sums up the advantage of putting large numbers of smaller cells in parallel.


http://www.mpoweruk.com/balancing.htm
quote>>


*Using small cells* connected in parallel to provide the same voltage and capacity as the larger cells results in many more interconnections, greater assembly costs and possibly more complex control electronics. Small, cylindrical, 2 or 3 AmpHour cells, such as the industry standard 18650 used in consumer electronics applications, are however made in volumes of hundreds of millions per year in much better controlled production facilities without manual intervention on highly automated equipment. The upside is that unit costs are consequently very low and reliability is much higher. When large numbers of cells are connected in a parallel block, the performance of the block will tend towards the process average of the component cells and the self balancing effect will tend to keep it there. The parallel blocks will still need to be connected in series to provide the higher battery voltage but the tolerance spread of the blocks in the series chain will be less than the tolerance spread of the alternative large capacity cells, leaving the cell balancing function with less work to do.
*Safety*On the safety front, the more reliable low capacity cells are much less likely to fail and if a failure does occur, the stored energy released by any cell is only one hundredth of the energy released by a 200 AmpHour cell. This lower energy release is much easier to contain and the likelihood of the event propagating through the pack is much reduced or eliminated. This is perhaps the most important advantage of designs using lower capacity cells.



 <<
I put together a small pack a few years ago (2009) using 18650 cells and they have remained in balance over that time. In the original design added an isolated DC to DC charger on the BMS board from the 12 volt rail to charge any one block of cells in each pack of 12 blocks (44 volt nominal per pack) using relays in case I had one block that was different capacity after extended use. In the time the pack has been working I have not needed to write code to support this function .....all blocks are in balance within 50mV during discharge. The shunt current is 250mA

A couple of previous posts here that have a few photos:-

http://www.diyelectriccar.com/forums/showpost.php?p=119080&postcount=6
http://www.diyelectriccar.com/forums/showpost.php?p=117397&postcount=3

The photo shows one of the 12 blocks with 54 18650 cells in parallel. In the pack they are arranged as two rows of 6 on top of each other behind the BMS PCB which also locates the power connections.



Very tidy and well packed Tesla charger design....anybody know the power rating?


George McDuff


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## Karter2 (Nov 17, 2011)

tomofreno said:


> I was thinking of something like that - using flex circuits, as the EVdrive packages look as if they may use something like that. Pretty cheap to make all connections on flex circuit boards with integral fuses. Bit concerned with obtaining repeatable, low contact resistance with just pressure contact though. But individual cell currents are low, and some conductive paste would help. Liquid cooling would be a challenge.


Tesla's proposed solution is to make both -ve and +ve contacts at the top of the cell , hence leaving the cell base "clean" for use as a flat contact onto a thermal sink .


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## kennybobby (Aug 10, 2012)

*Re: 10kW Charger Tesla*

Here is the label from the charger...


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## gunnarhs (Apr 24, 2012)

*Re: 10kW Charger Tesla*

Thanks to the community here for great responses!



Hollie Maea said:


> You can contact evdrive directly if you are interested. Our prices aren't super low though, as we are a relatively small company and have engineering costs to amortize.


 I will for sure sooner or later when I have the final specs, even if only to get a quote. Budget is always an issue of course.
Our first Taxi prototype will use Thundersky-cells as we know them now but it takes a lot of volume (weight is not an issue yet).



rochesterricer said:


> Tesla has stated they do not use the standard cells.


 They did in their (first?) Lotus Elise convertion, I have one battery from their pack  But the S-type picture looks different indeed. I have though doubt that Panasonic will on the long run do a special production line for the few ten millions of batteries. Remembering the EV1 story with the NiMH in the nineties.
The Gigafactory may prove me wrong, the name at least is realistic.



Madmac said:


> Reading the attached file referred to above...... the section copied below sums up the advantage of putting large numbers of smaller cells in parallel.
> [URL="http://www.mpoweruk.com/balancing.htm"]http://www.mpoweruk.com/balancing.htm [/URL]


 Hi George, nice to hear from you  I did quote this paper in previous post but maybe I misunderstood I was under the impression that the multiple cell concept was only recommended if each cell could be watched, otherwise the failure rate would be higher. The following paper here 
http://liionbms.com/php/wp_parallel_balance.php was better for me to understand (I prefer formulas over text  )
I did miss your DIY post with the 54P (no. 6) from 2009 when I was scanning the forum, read it now great stuff there. I will contact you on it for more info...



kennybobby said:


> Here is the label from the charger...


 Impressive, this is a big charger even if it is small size


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## gunnarhs (Apr 24, 2012)

I found this study based on the Tesla Roadster
http://www.pluginamerica.org/surveys/batteries/tesla-roadster/PIA-Roadster-Battery-Study.pdf

The result is interesting in two ways, it shows about 19% (partial or full)exchange of the battery packs. However if the pack made the 100.000 miles it seemed less degraded (80%) than proposed by Tesla (70%).

Also note that the (median) miles driven by the 123 owners is only about 34.000 km (21.000 miles).

It will be interesting to see the results for the S-type in 3 years which uses a different type of cell chemistry


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