# Two Tesla Modules Ignite, DIY Project



## boekel (Nov 10, 2010)

MattsAwesomeStuff said:


> I don't think that the issue was a lack of BMS. I suppose cells might have been out of balance, and, well, of course if you were monitoring the voltage of every cell it would have shut off, but, that's a bit misleading. You could charge a pack just fine in bulk.


Think again... a bms would have stopped the charger before overcharging would occur.

only other possibility is debris on the module, shorting a cell's positive and negative terminal. But seeing how fast the other cells started popping, I'm 99% sure either the whole battery, or one cell group was way overcharged.


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## brian_ (Feb 7, 2017)

MattsAwesomeStuff said:


> I don't think that the issue was a lack of BMS. I suppose cells might have been out of balance, and, well, of course if you were monitoring the voltage of every cell it would have shut off, but, that's a bit misleading. You could charge a pack just fine in bulk.
> 
> I think they just got overcharged.


If they were overcharged, a BMS would have solved the problem:


boekel said:


> Think again... a bms would have stopped the charger before overcharging would occur.


"BMS" means "battery management system", not "battery balancer". Whatever a BMS might do for balancing and state-of-charge calculations, the most basic functionality is to cut off charging and discharging at safe limits.


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## MattsAwesomeStuff (Aug 10, 2017)

boekel said:


> Think again... a bms would have stopped the charger before overcharging would occur.


Obviously. I said that. You quoted it. "and, well, of course if you were monitoring the voltage of every cell it would have shut off".

To be more clear, what I was saying is that, the problem wasn't that there was no BMS. A BMS would have prevented the problem from having a disastrous outcome, but one being absent is not an issue itself. The problem is that the batteries were being given 4.8-5v per cell.

Similarly, a cooling system might have prevented catastrophe... maybe. I suppose if a cooling system is powerful enough you can leach heat out of the batteries faster than they can make it, even when being given 15% overvoltage. I'm not sure what's happening, chemically, inside the battery at that point. They might still pop, cold.

It's like saying, in a runaway diesel, the issue was that the vehicle didn't have airbags or strong enough brakes. True, but, the actual problem was that there was a runaway diesel.

The problem here (presumably, if they hooked up the old 48v lead acid charger) isn't insufficient ancillary systems, the problem is giving the batteries too high a voltage.


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## brian_ (Feb 7, 2017)

MattsAwesomeStuff said:


> To be more clear, what I was saying is that, the problem wasn't that there was no BMS. A BMS would have prevented the problem from having a disastrous outcome, but one being absent is not an issue itself. The problem is that the batteries were being given 4.8-5v per cell.


But with a properly operating BMS, the potentially high supplied voltage is not a problem. The charger output would never reach its maximum level, no cell would ever see 4.8-5 volts, and no cell would be overcharged with a BMS; it is real solution, not just a disaster-mitigation system.


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## MattsAwesomeStuff (Aug 10, 2017)

brian_ said:


> The charger output would never reach its maximum level


Yeah, I guess I see your point. Seems like playing with fire though. I mean, a bench supply would have also (presumably, we don't actually know that the charger in use was the original one) prevented this and also charged them to the correct voltage.

In that sense, you could hook up a 240v charger to the pack, and expect the BMS to terminate at 50.4v, but, seems a little bit like walking on the knife edge.


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## brian_ (Feb 7, 2017)

MattsAwesomeStuff said:


> I mean, a bench supply would have also (presumably, we don't actually know that the charger in use was the original one) prevented this and also charged them to the correct voltage.


Not quite, because a BMS monitors the voltage across each cell level, while an external regulated supply can only see the overall pack voltage.



MattsAwesomeStuff said:


> In that sense, you could hook up a 240v charger to the pack, and expect the BMS to terminate at 50.4v, but, seems a little bit like walking on the knife edge.


Any charger/power supply should be current-limited as well, and something needs to ensure that charging current is reasonable... although acceptable charge current is high for Tesla modules (hundreds of amps?) compared to what a random power supply that cheapskate amateurs might have lying around.


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## MattsAwesomeStuff (Aug 10, 2017)

brian_ said:


> Not quite, because a BMS monitors the voltage across each cell level, while an external regulated supply can only see the overall pack voltage.


Well obviously, I get what a BMS is and does, but, there are guys who manually balance their cells once a season and otherwise have no BMS at all, and no issue for years. Bulk charging can be just fine.

I mean, I suppose the problem here could have been a balancing issue, but, more likely was just straight overcharging the whole pack.

It's I suppose a silly hypothetical, obviously you're best off with both (I suppose not obviously, some are strong proponents that BMSs are a dead pack waiting to happen), but, if you had to pick one or the other, or blame the absence of one versus another, I'd pick a charger with the correct max voltage and rely on it versus relying on a BMS to keep a higher voltage from murdering a pack. One's only an issue if there's significant balancing issues, the other is certainly a significant issue every single time.


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## boekel (Nov 10, 2010)

MattsAwesomeStuff said:


> Well obviously, I get what a BMS is and does, but, there are guys who manually balance their cells once a season and otherwise have no BMS at all, and no issue for years. Bulk charging can be just fine.


No it's not. yes it can be fine for seasons...but no guarantee, and most people end up killing cells, or packs.

But...this practice usually was with LFP cells, they are a bit more difficult to ignite.

Also this was when some available bms systems did more wrong then good.



> I mean, I suppose the problem here could have been a balancing issue, but, more likely was just straight overcharging the whole pack.


That's basically has the same effect, though it is likely that the whole pack was overcharged.
Another possible issue is the way Rich handles the modules...lots of physical abuse, and coolant everywhere... Leak coolant into the modules and they will get out of balance...seen it happen... I won't buy separate modules from car wreckers...



> It's I suppose a silly hypothetical, obviously you're best off with both (I suppose not obviously, some are strong proponents that BMSs are a dead pack waiting to happen), but, if you had to pick one or the other, or blame the absence of one versus another, I'd pick a charger with the correct max voltage and rely on it versus relying on a BMS to keep a higher voltage from murdering a pack. One's only an issue if there's significant balancing issues, the other is certainly a significant issue every single time.


Better think again...when using LFP cells (and some others might also be more forgiving / harder to get to burn) this might hold -some- truth (5 years ago...)
But when using Tesla modules, or other OEM modules that are highly flamable...

Please reconsider these thoughts...and don't promote people to use these ideas...


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## Jimbo69ny (Feb 13, 2018)

Just like that guy with the Jeep that burnt down his house. The site has been wiped of the story but he wasn’t using a BMS on these modules either. Proof that you NEED TO USE A BMS! Don’t listen to Jehu. Lol


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## Duncan (Dec 8, 2008)

Jimbo69ny said:


> Just like that guy with the Jeep that burnt down his house. The site has been wiped of the story but he wasn’t using a BMS on these modules either. Proof that you NEED TO USE A BMS! Don’t listen to Jehu. Lol


Nope
You don't need a BMS if you are sensible 
If you are not sensible then a BMS will probably not help anyway


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## electro wrks (Mar 5, 2012)

Sorry Duncan, sensiblity is just not in the DNA of a lot of people, Also, think about it with these Tesla modules. They get old and start to drift(as eventually we all do) on the voltages. You get a little lazy about manually monitoring voltages and temperatures. You forget just one time and leave the charger on just a little too long: KABOOM!

Judging by the Tesla module failure incidents, it IS most likely going to be a KABOOM. Not a swell, stink and sizzle failure like with your Volt cells, and other safer cells. I really can't believe if you had Tesla modules in your EV, that you wouldn't be using a good BMS.


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## Duncan (Dec 8, 2008)

Hi electro wrks

Disagree
Every failure I have heard of (other than total loonies like the video) has been a two stage failure
(1) cells die
(2) charge up as usual - leading to an overcharge

What is needed is a warning that a cell has died - I use the Lee Hart Batt Bridge

I also charge and discharge conservatively

if I could use the Tesla (or Chevy) BMS I would but the aftermarket BMS's appear to simply CAUSE problems


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## Kevin Sharpe (Jul 4, 2011)

"Tesla Battery Fire, What Went Wrong?"


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## frodus (Apr 12, 2008)

Kevin Sharpe said:


> "Tesla Battery Fire, What Went Wrong?"



Good second video Kevin.

It was a single cell that appeared to overheat FIRST. This could be because of it being unbalanced more than another and hit a higher peak voltage first, or that it has a higher IR than another causing overheating. But it was the one that started the chain reaction. Not only that, but at the top of the charge, the cell has the most energy in it, so the most potential to be volatile.

That cell was the instigator. The rest was all thermal runaway.

IMHO They should have immediately sprayed water, and kept spraying the pack with water to cool it down. From the video, it doesn't look like they did a lot during the fire. 

We had to do this with a Motoczysz bike with Kokam cells when they overheated during a race at Portland International Raceway for the TTXGP. It saved the bike, and kept the other cells from igniting.


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## electro wrks (Mar 5, 2012)

Duncan said:


> Hi electro wrks
> 
> Disagree
> Every failure I have heard of (other than total loonies like the video) has been a two stage failure
> ...



Could you please show us how you would use Lee's Batt Bridge with Tesla modules? Maybe a schematic?


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## MattsAwesomeStuff (Aug 10, 2017)

frodus said:


> It was a single cell that appeared to overheat FIRST. This could be because of it being unbalanced more than another and hit a higher peak voltage first, or that it has a higher IR than another causing overheating. But it was the one that started the chain reaction.


I disagree with what I think you're concluding with that assessment.

If you put 800 of anything flammable in a box, and then apply heat, it will always be 1 of them that ignites first. They won't synchronize and all burst into flame at the same moment.

If you take 800 strands of spaghetti and bend them, 1 of them will snap first. If you keep bending, soon after, more and more strands will break. This does not mean that the first strand of spaghetti "set off a chain reaction", as if all the other strands were still okay at that point. They were all already on verge of snapping.

If you rolled 800 dice twice, the few that rolled a 6 both times doesn't mean those dice were out of balance. Some are just going to do that. You could argue that it's not "randomly", and, sure it comes down to microscopic differences in the dice and the way they were thrown, but, it's random.

In fact, the cell that first exploded had many other cells in parallel with it. That whole parallel group, by virtue of being in parallel, were balanced with each other, yet they didn't light like a 21-gun salute.

In terms of the battery, it would be misleading to conclude that the cell exploded because it was imbalanced and then its fire caused its neighbors to overheat. I would find that to be the case if the rate that the fire propagated seemed proportional to the heat a cell gained from its ignited neighbor, but that doesn't appear to be what happened here. 

You can hold a blowtorch against an 18650 for a long time before you can get it to ignite. That's what you'd expect if it was just one cell that was imbalanced. That the cells burst so quickly in succession, to me, indicates any one of them could've been the one that popped first.

I suspect that *all* cells were right on the very brink of igniting, *all* were simultaneously overcharged, and that the only thing the first cell popping did was provide a small bit of extra heat to tip the rest over the edge.

That is, the fire was not caused by a chain reaction of the cells being out of balance, and would not have been prevented or even delayed by more than a few seconds had the cells been perfectly balanced. Even 800 cells with perfectly equal voltage, one would have ignited first. The fire was caused by all the cells in the pack being massively and cohesively overcharged until one finally lit.

The takeaway should be "Don't massively overcharge your battery pack", of which this result was guaranteed, every single time, not "This is what happens when you have imbalanced cells", which is something that might be an issue 1/1000 times.


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## frodus (Apr 12, 2008)

Well, you're wrong with what you think I'm concluding with my assessment. I don't see where I said the others weren't on the verge of igniting. 
It just so happens, we're mostly in agreement!

In summary:

One (or more) hit it's thermal limit before the others. *That is fact. *. The video supports this. It's hard to tell if one or more simultaneously ignited, but it was definitely within the same area on one of the packs.

You and I both seem to surmise that it did so because it was overcharged. That is reasonable and prudent to assume. 

You and I both seem to surmise that if that one didn't hit thermal limit due to an overcharging situation, a different one likely would have. That is also reasonable and prudent to assume.

What I do think, is that it all starts with one (or more) going into thermal limit first. From my experience, one cell going thermal can start snowball effect that heats up the next cell (on the edge or not) and then the next, and the next. That is the basic idea of thermal runaway. 

Long and short, it is likely that all cells got overcharged, and this one hit its thermal limits first, which caused a chain reaction.


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## Duncan (Dec 8, 2008)

Hi electro wrks

All you need to use a Batt Bridge is a centre tapping - and it does not NEED to be "center"

What you are doing is comparing the two halves - or quarters with each other
When I had four strings of Headways I used four Batt Bridges

The "comparison" is ongoing and if the two halves are not the same than a lovely red LED will light

This means that you are comparing the "halves" under load - not just under no-load conditions
If you "lose" a cell it will show up under load - and then you will need to get in there with your meter to see what has gone wrong


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## SWF (Nov 23, 2007)

frodus said:


> You and I both seem to surmise that it did so because it was overcharged.


Based on the second video describing how they just used a charger they had on hand, I also agree that overcharging was the main issue. Technically, all the paralleled cells in a cell group in the module would have been at exactly the same voltage, and perhaps all the cells in the group were getting hot, but slight differences in each cell in the group and their point of pressure release and ignition would result in one individual cell failing first.

A charger with an appropriate lower voltage may have prevented the problem, if you assume all the cell groups in the modules were balanced with each other. But a BMS that shuts down the charger as soon as any cell group reaches the cutoff voltage would also have prevented the problem regardless of the characteristics of the charger and any imbalance between the cell groups.


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## MattsAwesomeStuff (Aug 10, 2017)

> if that one didn't hit thermal limit due to an overcharging situation, a different one likely would have.


Ahh, my bad. It seemed like you were stressing the opposite, since you mentioned balancing and emphasized FIRST.

Yep, we agree on all fronts then.


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## frodus (Apr 12, 2008)

SWF said:


> Based on the second video describing how they just used a charger they had on hand, I also agree that overcharging was the main issue. Technically, all the paralleled cells in a cell group in the module would have been at exactly the same voltage, and perhaps all the cells in the group were getting hot, but slight differences in each cell in the group and their point of pressure release and ignition would result in one individual cell failing first.
> 
> A charger with an appropriate lower voltage may have prevented the problem, if you assume all the cell groups in the modules were balanced with each other. But a BMS that shuts down the charger as soon as any cell group reaches the cutoff voltage would also have prevented the problem regardless of the characteristics of the charger and any imbalance between the cell groups.


Exactly! Monitoring the parallel group voltages and shutting the charger off when one (or more) get to a threshold, would have likely prevented this from occurring.


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## Jimbo69ny (Feb 13, 2018)

MattsAwesomeStuff said:


> Ahh, my bad. It seemed like you were stressing the opposite, since you mentioned balancing and emphasized FIRST.
> 
> Yep, we agree on all fronts then.



Holy crap! Matt acknowledged that he misunderstood and then agreed! There is a first time for everything!


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## electro wrks (Mar 5, 2012)

Duncan said:


> Hi electro wrks
> 
> All you need to use a Batt Bridge is a centre tapping - and it does not NEED to be "center"
> 
> ...



So a Batt Bridge would be attached to each of the 6 paralleled groups of cells(70+cells per group, as I recall) in the Tesla module? Are you sure this will safely replace the OEM or a good aftermarket BMS?


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## Duncan (Dec 8, 2008)

The OEM BMS? - no I expect that would be better

An aftermarket BMS? - Those are the things that have killed at least ten times as many batteries as people like me not using BMS's have done


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## aquabiologist (Sep 8, 2017)

Duncan said:


> The OEM BMS? - no I expect that would be better
> 
> An aftermarket BMS? - Those are the things that have killed at least ten times as many batteries as people like me not using BMS's have done




Could you give some insight on how a bms typically kill batteries or what issues aftermarket bms have? 


Gesendet von iPhone mit Tapatalk


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## Duncan (Dec 8, 2008)

aquabiologist said:


> Could you give some insight on how a bms typically kill batteries or what issues aftermarket bms have?
> 
> 
> Gesendet von iPhone mit Tapatalk


Historically aftermarket BMS's have had a number of problems

(1) - just failing and draining some cells
(2) - working as designed but unbalancing your battery by taking more out of some cells than others
(3) - You leave your car for a couple of weeks and the BMS drains the battery down to the failure point

OEM batteries appear to be a lot better but the aftermarket cells appear to have a failure rate about 3% - 3% of the cells just die - there is nothing a BMS can do about that 

Balancing - one of the functions of a BMS is to "balance" your cells
The only problem is that you cannot balance the cells except at the very top of the charge
BUT if you regularly go to the top of the charge you reduce battery life


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## electro wrks (Mar 5, 2012)

Duncan said:


> Historically aftermarket BMS's have had a number of problems
> 
> (1) - just failing and draining some cells
> (2) - working as designed but unbalancing your battery by taking more out of some cells than others
> ...



OK, you've convinced me (for now). Where would I hook up this: http://www.evdl.org/pages/battbridge.html to a pack of say, 6 Tesla modules?


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## Duncan (Dec 8, 2008)

electro wrks said:


> OK, you've convinced me (for now). Where would I hook up this: http://www.evdl.org/pages/battbridge.html to a pack of say, 6 Tesla modules?


That would depend on how you were using the modules and how much protection that you needed

With my Volt Pack I am simply comparing the two halves of the pack

When I was using Headway cells I used four strings and each string had a seperate center connection along with the shared positive and negative

This easily detected when one of the Headways died so that I could disconnect it's string

If I knew how to make the Tesla BMS work I would do that!

But if not then all the Batt Bridge needs is some tapps - so that you can compare one part of the module to another part

The process is
(1) Manually check cell level voltage at minimum charge
(2) Manually check cell level voltage at maximum charge

So you start off with a known situation

Then you use the car - do not go beyond the levels you checked and have the Batt Bridge to tell you if the parts of the modules are still the same as each other

At intervals repeat the manual check


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## electro wrks (Mar 5, 2012)

And...We're off to the BMS debate races! Duncan I like simple analog solutions I'm sure as much as you do. However, with the Tesla modules and their increased inherent danger over most other battery systems, I think more than the Bat Bridge(BB) is need to monitor and manage them. 

For example, does the BB somehow tell if there is a temperature increase in the module prior to a failure like a OEM or a good aftermarket BMS? Maybe, in a situation when the vehicle is charging unattended?

Also, the manual checking and monitoring required with the BB concerns me RE the practicality for everyday use. Is your ride a everyday driver? Are you the only one that uses it. I'm thinking the BB could work in are personal vehicles, driven intermittently and only by us. But in vehicles driven everyday and by other people that maybe are not well versed in how the BB system works, there could be problems.

I know of people who drive their ICE cars until the oil pressure warning light comes on before they add oil to the engine. I've heard of others who have tried to drive to a service station or home, miles away, after their oil pressure light has come on. Do we want these kinds of people driving BB equipped EVs, and more importantly, would it be safe for them to do so?

What about including both systems in an DIY EV? It would be something like having a oil pressure gauge or oil level gauge AND a oil pressure warning light in an ICE vehicle. Not the best comparison, but you get the idea.


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## Tony Bogs (Apr 12, 2014)

Exactly, everyone has his or her ideas about who much safety is needed. 

I'm staying clear of Tesla modules in DIY projects. 

People here generally put too much trust in a BMS. It's just a piece of hardware, that is just as likely to fail as any other piece of hardware. It becomes (exponentially) more unreliable as the complexity increases.

So I'm with Duncan on this: keep it simple in a DIY project and use the safe(r)(st) chemistries. 



Some chemistries do need more than a basic BB, especially during unattended charging. I think electro wrks has a point there.


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