# causes for LiFePO4 venting and 0v?



## Duncan (Dec 8, 2008)

Hi
I assume you don't have a BMS (neither do I) - but I would recommend one of these

http://www.evdl.org/pages/battbridge.html

Cheap and cheerful - but tells you if a cell has died

I'm using Headways - (the obsolete 16Ah ones) and I have had a number of them "just die" like yours
(Headway were very good and replaced them for me)

They just died - seemed to become an internal short - no fuss just dead


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## Duncan (Dec 8, 2008)

scott glen said:


> loosing a CALB cell is as normal as buying CALB in the first place. Not just CALB but all Phosphate. Now you know why American car manufacturers don't use Chinese cells, Japanese LG is the popular one making them for the Volt and Leaf. Dump them before you get stranded. A BMS doesn't help a cell that takes a dump.



You lose a cell - that does not strand you anywhere - just replace it when you get around to it

Yes I would love to have gold plated cells - but I would also like an Aston Martin ...


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## Sunking (Aug 10, 2009)

dtbaker said:


> ANY idea on what might have killed one cell?


I am guessing you Top Balance with Vampire boards, and depend on the controllers to monitor total pack voltage rather than individual cell voltages. 

What you you describe is common on Top Balanced systems. No two cells have the same capacity. Capacity from one cell to another can vary some 15%. So when Top Balanced only voltage is balanced and not capacity.

So on the discharge cycle it is very possible to drive the weak cells to reverse polarity and/or zero volts. If you monitor pack voltage vs cell voltages you will not catch it. 



dtbaker said:


> How it still runs with the cell completely at 0v? is it likely shorted internally?


Exactly. Over discharged cells fail shorted. One of the unique characteristics of lithium batteries.


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## onegreenev (May 18, 2012)

They can get seriously hot and kill surrounding cells. A Batt Bridge can help find a bad cell. Bottom balance and don't let your friends drive your car unless they fully understand the reasons to stop vs continuing to drive. The safeties are there for a reason.


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## pm_dawn (Sep 14, 2009)

Hi !
I have lost two TS 160 cells the same way.
One of the actually got so bloated and hot it melted the cell casing halfway up the cell and spewed its solvent out in my battery box, all my insulating foam melted from the fumes.
The melted cell kind of welded itself to the cell next to it.

The first cell that died shorted never gave it up that violently.

But yes they both turned into busbars.

I think that the last one that vented big time actually died during charging.

But it could very well be that it had already died from overdischarge and then was slowly heated and recharged to the boiling point.......


Regards
/Per


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## dtbaker (Jan 5, 2008)

I do suspect overdischarge killed the cell.... I knew that was a risk with top-balance, and no BMS. I did expect the LV setting of the zilla to catch it since I had it set fairly conservatively.... 

the intent was to 'limp home' if the pack voltage fell below an avg of 2.0vpc.... but the person that borrowed the car was not terribly aware of the consequences, and 'limped' a long way at least once.

the good news is that it must have failed under relatively low current while 'limping' since it vented 'just a little' and does not appear swollen or melted.


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## Sunking (Aug 10, 2009)

dtbaker said:


> I did expect the LV setting of the zilla to catch it since I had it set fairly conservatively....
> 
> the intent was to 'limp home' if the pack voltage fell below an avg of 2.0vpc....


Not trying to be a Jerk, but 2 vpc is anything but conservative. Exact opposite is very Liberal and asking to get screwed over. 

I do not know how many cells you are talking about (pack voltage) but for grins let's say it is 45S or 144 volts. 2.5 vpc is the Do Not Cross Line. If you only used the Zilla Controller for LVD that is *112.5* volts. Well my friend 2.5 vpc is slippery slope to look over the edge. For example say you have one or two weak cells of 120 AH, and all the rest are spec 130+ AH. Very realistic you can drive those two cells to reversal at 0 Volts as all the other cells are getting low, but are still 3 volts or slightly higher. 43 cells x 3 volts = 129 volts and God Zilla Controller still thinks everything is Hunky Dory, when in fact you just destroyed one or two two cells. Exactly what happened to you my friend. 

Only possible way to use the controller LVD is if you Bottom Balance. Top Balance only balances voltages, not capacity. Bottom Balance balances capacity and voltage @ 0% Capacity. Done that way all cells arrive at 2.5 volts at the same time and God Zilla can then save your batteries. 

I only have a 48 volt NEV, and bottom balance the pack. I have my controller LVD set at 2.9 vpc or 46.4 volts. I also have a Smart Battery monitor, an Orion Jr looking at cell voltages. The Oroin will initiate a disconnect if any cell dips below 2.5 volts for more than 15 seconds. The motor controller will operate at 46.4 volts and should trip before the Orion does. Almost no chance in you know where will I ever over discharge. 

On the charge side I only charge to 56 volts. The Orion monitors cell voltages and if one goes above 3.6 volts will terminate charge. That has never happened yet as mu cells stay in balance. I have two cells that get close to 3.6 volts. The two weaker of the bunch. My last thought for you. There is no meaningful power below 2.9 volts, so why mess around that cliff?


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## dtbaker (Jan 5, 2008)

Sunking said:


> Not trying to be a Jerk, but 2 vpc is anything but conservative. .... 2.5 vpc is the Do Not Cross Line.



I disagree..... in part. 

I agree that anything below 2.5vpc is bad news on a steep part of the curve if the system is not under load. In fact my personal danger line was when and if I ever saw the pack voltage drop below 3.0vpc when resting. BUT, I was finding that on cold winter days, under heavy acceleration, it was not uncommon for the cells to sag to about 2.0 vpc; and then recover as soon as load reduced. I didn't want the controller to clamp off my amps at the stoplights on cold days, so I set it down to the lowest expected sag.

So I would qualify my thought process in setting the LV on the controller at 2.0vpc is not all that aggressive as long as the no-load recovered to 3.0vpc. The problem was that the borrower of the car was unaware of that second part, which required taking a look at the pack voltage when at the occasional stoplight.

The lesson learned is that using the LV setting on the controller without an additional look at the pack voltage recovery at low loads was a mistake that cost me a cell....


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## Sunking (Aug 10, 2009)

dtbaker said:


> I disagree..... in part.


All I can say is if you had your Controller LVD set to say even 2.5 vpc we would not be having this conversation right now or even a thread.


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## dtbaker (Jan 5, 2008)

Sunking said:


> All I can say is if you had your Controller LVD set to say even 2.5 vpc we would not be having this conversation right now or even a thread.



thats *probably* true.... but I would have also had the controller clamp down on max current needlessly most of the time on cold days. Now, we all know more because of this thread. 

yes, setting the controller LV warning/clamp below lowest sag is a little risky if 'resting' pack voltage is not taken into consideration; but it also retains full performance under normal conditions.

I think its productive to HAVE discussions so people can make informed choices, know the consequences, and adapt to their own priorities.


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## Sunking (Aug 10, 2009)

dtbaker said:


> I think its productive to HAVE discussions so people can make informed choices, know the consequences, and adapt to their own priorities.


No argument here on my end. Being an engineer and couple of years thinking, toying around, and research I came to the conclusion Bottom Balance is just the way to go. Lot less expensive and complicated to implement, and pretty much eliminates the risk of over and under charging. You don't have to give up capacity and gain quite a significant gain in cycle life staying away from 100 and 0% SOC voltages. For me it is smart money.


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## arber333 (Dec 13, 2010)

Let me interrupt with my 2c. 

One day when i was normally driving and climbing a steep highway hill at 100km/h one of my 200Ah cell sagged to cca 2,7V while all others were stable at 3V. Huh? I let off the pedal and relaxed the drive and it returned with the rest... BUT if i throttled again i saw the same sag. I said, Well OK i will take the cell out and measure the cell tomorrow (sunday). I put the car to charge and myself to bed. 
Next day it smelled like nail polish in the house!!! I ran to garage and one of the cells in forward box was venting and was fused to surrounding cells and they were HOT! It took two buckets of water to cool them. While i took apart the cells i measured voltage, DOH! 4V each. It seems while charging one cell went short and all the rest saw too high voltage. Poor BMS hadnt had a chance. 

My mother says she heard BMS contactor click while she was walking a dog. I guess it was trying to save batteries to no effect. Charger was off but voltage kept rising as it seems the shorted cell bloated outwards and touched metal casing! That put half pack in short with the other half. All this wouldnt mean anything but i found out later there was also short in water heater coil putting + line in 100R contact with chassis. 
That did it and poor cells were cooked. I lost 2/3s of 42 cells.
And i had digital BMS with failsafe charger turnoff. 

In my business we call this "cheese hole" configuration. All the holes in cheese slices were aligned and damage fell trough.

A


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## EVfun (Mar 14, 2010)

I'm a little confused. If the cells where at 4 volts something was (or was very recently) pumping amps into them, usually that means charger wasn't off. 

This is why I'm top balanced (no BMS in my case.) The loss of one cell won't drive up the voltage in any other cell enough to worry about. They all go up together at the end to 137 volts for my 39 cells. The loss of one cell changes the end of charge voltage from 3.51 volts to 3.61 volts. Even with some scatter it is unlikely any cell would hit 3.7 volts. 

Likewise, the loss of one cell in your pack, if the BMS top balanced it, should have been unable to drive the other 41 into a bad place. Even if you where targeting 153 volts for the hold voltage no cell should have gone over 3.8 volts because one failed. You can take the cells to 4.0 volts without short term problems (it almost certainly shortens life to do so regularly.)

For future work I would carefully check the charger as it may have failed to limit charging voltage. If you run a BMS I would also set it up so that it opens and leaves open the relay or contactor that controls the charger. It sounds like you where very close to having a fire.


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## Sunking (Aug 10, 2009)

evlithium said:


> a BMS is necessary for the CALB battery pack.


Nonsense.No BMS is needed. That is what salesman and manufactures want you to believe.


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## Hollie Maea (Dec 9, 2009)

Sunking said:


> Nonsense.No BMS is needed. That is what salesman and manufactures want you to believe.


Well, he IS the salesman. His posts are all thinly disguised advertisements.


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## LiFePO4 (Jan 30, 2013)

I don't reply much but given cell variability, cost, etc for LiFePO4, why on earth would you NOT use a BMS to protect your investment? And this is independent of how you balance, charge, whatever.

Flame away....


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## dougingraham (Jul 26, 2011)

LiFePO4 said:


> I don't reply much but given cell variability, cost, etc for LiFePO4, why on earth would you NOT use a BMS to protect your investment? And this is independent of how you balance, charge, whatever.


The key phrase is "protect your investment." And the BMS does not do this. About all it can do is tell you which cell is going bad. It can't stop it from happening. It can just tell you that it is happening. A LiFePo4 battery is a simple and reliable device. Apart from the connections at the terminals there is nothing that can go wrong as long as you don't over charge or over discharge. And you don't need a BMS to prevent either of those. The addition of a BMS is the source of cell imbalance and once you add one you are required to fix the imbalance with some sort of mechanism to rebalance the pack. The BMS typically does this with a shunt balancer. Get rid of the BMS and you no longer need the daily balancing.

There are a lot of assumptions about LiFePo4 batteries based on knowledge of other battery types and if those assumptions were true you could benefit from a BMS. For the most part the assumptions are not true.

Spend the money saved on not buying a BMS on a few extra cells and you are far ahead over the long run.


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## dtbaker (Jan 5, 2008)

dougingraham said:


> Get rid of the BMS and you no longer need the daily balancing.



this is part of why I have not used a BMS on either vehicle. I firmly belive that a good initial balance, and occasional checks are all that's needed as Li doesn't seem to 'drift' much. Most of the BMS/balance systems I have followed seem to create as many problems as they prevent with multiple points of failure..... and at significant expense.

but I really don't want this thread to turn into BMS/no-BMS religious war. I would prefer if we stuck to CAUSES for cell death.

I this case, I'm pretty sure it was the predictable result of inadequete driver awareness ignoring gauges and readouts, and driving the car too far in 'limp' mode, killing the cell that was at the bottom of the pack in capacity.


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## Sunking (Aug 10, 2009)

LiFePO4 said:


> I don't reply much but given cell variability, cost, etc for LiFePO4, why on earth would you NOT use a BMS to protect your investment?


The BMS is not protection. it is the root cause of failures. They are based on Lead Acid mentality taking every cell to 100%, and do not prevent over discharge of single cells. Manufactures want the public to believe they need a BMS, that is how they much big bucks selling you a BMS and replacing destroyed cells.


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## GerhardRP (Nov 17, 2009)

dtbaker said:


> but I really don't want this thread to turn into BMS/no-BMS religious war. I would prefer if we stuck to CAUSES for cell death.


Good sentiments...

1) The OP reports loaning the car to a non-aware friend who ran the car down a couple of times. He smelled something and then TWO DAYS LATER tested the batteries. Luckily found something dead, not burning. There is no report of the balance status or management system.
2) In post #7 there is a report of the loss of two cells, probably during charging. There is no report of balance status or management system.
3) in post #14 a BMS [not identified] reports a problem which is ignored and the car is charged and nearly burned down. [What is the BMS?] and why was it unable to recognize that there is a fatal error?
Gerhard


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## PStechPaul (May 1, 2012)

Interesting thread. I am fairly well sold on bottom balancing for most purposes, and it should cause a very rapid drop in voltage under load that should be detected by the controller and cause a shut-down. There may be overrides to enable a limp-home mode by a savvy driver, but it should require a special code of some sort to avoid damage from someone who does not understand. It seems that it was fortunate that the failed cell in this case shorted solidly and allowed continued operation at reduced voltage, but more catastrophic event s could have occurred. It seems best to avoid conditions that border on the top and bottom 5% of capacity and getting a reliable 90% of actual capacity is reasonable and safe.


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## eTrike (Apr 21, 2014)

PStechPaul said:


> It seems best to avoid conditions that border on the top and bottom 5% of capacity and getting a reliable 90% of actual capacity is reasonable and safe.


This is excellent advice. This solves the issues of straying voltages at extremes, as that is where you will notice relatively large voltage differences between cells and thus avoid these potentially damaging circumstances. The flat discharge curve of LiFePO4 is a brilliant example to comprehend capacity vs. voltage and cell limits, without much need for more than a voltmeter and a calculator. Both high and low voltage are stressful, so by avoiding the outer limits you reap much greater rewards in longevity, as you can see in any whitepaper on the subject. 

CALB 130AH seem to be rated at 0.3C and 80% DOD to 2000 cycles. If you were to treat your cells like this, they might last that long, but you'd be limited to about 40 amps and 100AH of capacity. This would be well suited to longer range at the cost of lower speed driving.
Avoiding the top 10% and the bottom 10%, or the bottom 12% and the top 8% of capacity and your batteries will reward you with a much longer lifespan, even if you abuse them with high C rates or temperatures(naturally these should also be avoided to prolong lifespan).

This leads me to Low voltage/high voltage cutoff (LVC/HVC). The problem the OP experienced has already been surely nailed, as over-discharging the pack killed the weakest cell. 2.0V is *way too low*,(CALB recommends 2.5V Min)-- as LiFePO4 drops off a cliff after ~3.15V, with very little capacity between 3.00 and 2.5V. Looking at the whitesheet for discharge at 1C(130A) for CALB 130 you see a similar pattern, with ~10% capacity remaining at 3.0V, ~7.7% at 2.9V, ~3.6% at 2.8V, whereas 3.0-3.1V holds 21% capacity, and 3.1-3.2V holds ~68% capacity. Above 3.2V is the remaining 4% to peak charge. That last +4/-8% IS NOT WORTH IT vs the longevity of your pack, and from experience you and I can say that that *one* last mile is not worth it without an intelligent cell-level protection system (BMS). 

I use A123s, so the characteristics are similar to your CALB cells, but comparable for all LiFePO4. My A123s are rated at over 2000 Cycles at 1C charge and 2C discharge at 100% DOD, so moderate constraints on their ratings yield impressive results (I've seen as much as 10,000 cycles). Venture a guess how many cycles one over discharged cell lasted? I *just* had to get home... I didn't make it, and killed one cell. Better than the whole pack, but I learned my LVC lesson.

In my opinion, LiFePO4 cells should not be discharged below 3.0V, 2.9 absolute minimum under 1-2C load. 3.45V is plenty charged, but overcharging is usually not as harmful. Above 3.65V should be avoided, but if you find your cells above that, just discharge them(drive the car a short distance, run the A/C, use a lightbulb, whatever you can do to remove the charge). There will be very little energy (wh) to go from 4.0v-3.6V from LiFePO4, but if allowed to sit at high voltage it will discharge that excess energy as damaging heat. 


OP mentioned charging in cold temps. Cold temps affects the capacity of LiFePO4 significantly, so your experience with cells sagging to 2.0Vpc under load while cold but not depleted is understandable, but worrisome that cells would be allowed to go that low. 
Temperature differences between cells can cause imbalance issues, so if cold is affecting your pack that much, you will want to consider insulating and possibly heating your pack in the winter. Once temps are above 60F you shouldn't notice a difference, but capacity/range will suffer noticeably below ~40F. Sitting at low temperatures will not negatively affect their lifespan as high temperatures will (cold helps for storage!), but abusing cold batteries is not a good practice. Best to design a system to keep them warm, or draw moderately until they have had a chance to warm up. On my old abused lead-acid setup, I'd take a quick fast trip to really crank and drain the batts first thing in the morning, then set them on a charge to top off and warm a bit more in the process before the actual commute. A garage would have been nice 

Regarding BMS, etc., perhaps the best BMS is vigilance. You can either install a system for less than $200 which gives you voltage data with basic alarms, or install a $1000+ system that might do several neat things for you, or just manually check periodically once you know how your pack tends to behave. Any system can fail, but the good ones are efficient and reliable, regardless of price or method. Just as you would check fluids, etc, similar attention should be paid to batteries no matter which system you use. Ignore and pray doesn't usually work well, unless you use low C rates, reasonable LVC/HVC, quality parts, etc. "Batteries don't die, they are murdered." (source?)

Regarding BALANCING, every major manufacturer and paper I've seen supports top balancing for a myriad of reasons, if only when needed.


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## Sunking (Aug 10, 2009)

eTrike said:


> Regarding BALANCING, every major manufacturer and paper I've seen supports top balancing for a myriad of reasons, if only when needed.


I would too if I were a manufacture or distributor. Gotta keep-em coming back for more product.


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## wb9k (Apr 9, 2015)

Wow...some interesting ideas in this thread. 

I'll throw in my 2 cents here (and as warranty lead at A123, I have some experience to back all of this up). I would agree that the OP's car had a cell overdischarged (and thus very quickly destroyed), enabled by two things:

1. The BMS (and not using some type of battery management system--whatever that means to you--in an electric car is just begging for trouble) was only capable of setting limp mode when any cell fell to under 2.0 Volts under load (I think this is a perfectly reasonable limit for a typical EV load, BTW...I can explain why more later). It was not capable of shutting the car down completely if a cell dropped to a truly dangerously low level, which, as others have pointed out, is not going to be far behind the trip point at all.

2. The car was being driven by somebody who had not been trained in how to function as the human side of this particular "BMS". Thus, they drove for extended periods of time in limp mode without realizing what this might do and the car did not stop them at any point. One time "they drove for a really long time in limp mode". You can bet this is when the weakest cell was overdischarged--probably driven negative--for long enough to destroy the cell and it later developed an internal short and vented. That smell is VOC's evaporating off the electrolyte. 

Let me clarify my position on BMS's, since this seems to be a religious issue around here...it is at ES as well, but maybe a little less so and in generally more justifiable circumstances. I have lots of little LFP (A123) packs that I use around the house for myriad purposes. I run my ham radio station on 12V packs, starter batteries that have decent BMSs built in. I run my audio system on LFP packs, and I do not use a "BMS" on those--I use certain tools to check up on them regularly to make sure they have not somehow strayed from the expected. When charging, I set max voltage to just below 100% SOC and don't worry about balancing but maybe once or twice a year. Same with the pack in my lawn mower, and in the 48V scooter I used to have. These are all packs that either have sense harnesses installed so that I can quickly slip on a cellog and know the OCV of all cells, OR all sense points can be readily clipped to for measurement and balancing when needed. All packs need balancing sooner or later. Flying partially blind is risky, but manageable by some people. The OP here might never have had a problem if he had not let somebody else drive his car without understanding how the battery protection was set up to work. So maybe some of us can get away with not using a BMS all the time. But the larger and more complex a pack gets, the easier it is for it to get away from even a trained user. Imagine selling thousands of cars like this on the open market. It's a formula for customers that are not just pissed off, but sometimes dead. 

A bad BMS might be worse than no BMS, but no BMS sucks ass compared to a really good BMS. It's not even an option for production vehicles to not have a BMS. Vilifying them across the board makes no sense and is indeed counterproductive. I'm not a believer in bottom balancing either...and I don't even sell BMS's. 

Cold was mentioned as a potential problem. The biggest risk to LFP in cold is charging at too high a rate at too low a temperature. A cold cell can only accept charge at a reduced rate. Forcing more charge current through under this condition causes Li to plate onto the cathodes very quickly, resulting in capacity loss. It is not likely to result in the failure mode seen here, which is classic overdischarge stuff.


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## dtbaker (Jan 5, 2008)

eTrike said:


> CALB 130AH seem to be rated at 0.3C and 80% DOD to 2000 cycles.


wrong by 10x.
They are rated for 3C continuous discharge.




eTrike said:


> This leads me to Low voltage/high voltage cutoff (LVC/HVC). The problem the OP experienced has already been surely nailed, as over-discharging the pack killed the weakest cell. 2.0V is *way too low*,


the 2.0 vpc was intended to be a pretty decently safe LVC that would start to occur only under heavy load/sag from stoplights.... warning the educated user that they needed to stop RIGHT AWAY and charge. Problem in my case was that the driver was un-educated.... and squeezed out more driving in 'limp home' mode ( at low 1C or less output ) and fell off the cliff, killing the lowest cell.


"Batteries don't die, they are murdered." (source?)

The REALLY interesting thing is that the one cell basically fused internally, vented a *little*, and the pack continued to operate 'normally'. My only real clue when I got the car back was a really odd smell after a few heavy accelerations. Presumably the dead cell resistance warmed up the guts, and it vented a little more, creating the odd smell, but seemed to work just fine! I only found the cell (in my rearmost covered spare tire well) after I decided to find the source of the smell and was checking every cell voltage by hand.

I have since cabled around the dead cell, and am running on 47x cells instead of 48x.


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## wb9k (Apr 9, 2015)

And I think 2.0 V is a perfectly valid number for doing what you describe. The problem came later when an untrained driver didn't know how to interpret and respond to what was happening. When LFP gets overdischarged, the copper anodes begin to dissolve, Cu going into solution in the electrolyte. When you attempt to charge the cell back up, the copper forms dendrites that eventually puncture the separator, causing a dead short inside the cell. After the chemical energy inside is exhausted (and this could have ended much worse for you, so don't take your final result as the only possible outcome) you more or less have a piece of wire where there once was a cell. You can cut it out and put a jumper in its place. Be aware though, that adjacent cells likely sustained some damage from elevated heat. Other cells may have also been overdischarged, just not as severely as the one that went up. How is the capacity now compared to before? Any cells with excessive sag under load or rise during charging?


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## eTrike (Apr 21, 2014)

dtbaker said:


> wrong by 10x.
> They are rated for 3C continuous discharge.
> 
> the 2.0 vpc was intended to be a pretty decently safe LVC that would start to occur only under heavy load/sag from stoplights...


Apologies, I should have offered my source: http://evolveelectrics.com/PDF/CALB/CALB_130AH.pdf
"Life Cycle @ 0.3C, 80% DOD"
This mirrors the manufacturer's data: http://www.calb.cn/UpLoadFiles/product.pdf


My point on the LVC can be seen in the chart, which maxes at 1C. Extrapolating a bit for 2-3C, you'll see that it would never hit 2.0V until it was risking damage(excepting the cold instance). My point was that it was far too low for an LVC. If the manufacturer recommends 2.5V min, or 80% DOD for longevity, it is your choice on what to which advice to follow, because those two are not the same point of discharge. If you set it 2.5V or higher, you greatly improve the safety buffer and should still be able to feather the throttle and limp as needed. 

The magic is that your voltage should drop like a rock near the end, so it should be very easy to see where the tipping point is. For me, it is 3.0V/Cell at 1C, even on A123s. I can go farther, but not much. Same with above 3.4V. There is very little capacity difference from there to 3.65. 
For you, on 47S, 3.2= 150.4V, 3.0V=141 V, 2.9=136.3 2.5=117.5V, and that last 24V drop off will happen quickly. I'd be curious to know what range you have left below 136V when pulling 1C or less. You're probably missing ~4% with one pack gone?


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## mizlplix (May 1, 2011)

All of these "war stories" is the reason I decided to put 3/8" plywood liners in my battery boxes. (That and a little insulation from the nice Arizona summer road heat.) The winters are not that bad where I live. It sometimes gets to 30 F......LOL And the plywood keeps some heat in the pack while driving. 

The shorting-to-the-battery-box story is also a concern, especially if roads are really rough. 

Yes I top balance. My charge stops at 3.49 VPC. I discharge to 3.1 VPC.
I might leave some cell capacity on the table, but 90% and a long life is a good trade off. 

I have 38-130AHA cells. I drive 48 Miles at the max. 

I still check cell balance annually. Yes, I still get some slight drift. I usually top up 4-5 cells who are .017 Volt lower than the average. 

Observation: These Calb 130 AH cells sag a lot. Even when freshly off the charger, a 8 second hard acceleration will set off the 2.5 volt alarm. This gets really bad as the pack gets near my max mileage, but it is a good reminder to be easy with the foot.....

Miz


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## wb9k (Apr 9, 2015)

Yes, shorting to the box can happen. Metal battery boxes are a bad idea. A123 stopped using them several years ago in favor of fiber reinforced plastics. For packs with voltage upwards of 600V+, a metal container can be exceedingly dangerous, especially in a DIY installation. 

I should clarify my LVC position here...A123 cells are actually pretty hard to kill with overdischarge. A cell that never spends more than a few seconds below 0.5 Volts (without ever being pushed negative) is going to survive just fine. You can take these cells out of extended storage, and if they're 0.5V or better OCV, simply charge and use. This is technically in "negative SOC" territory, but it doesn't matter. As I've said, you can't discharge this deep regularly without paying a calendar life penalty, but occasional discharge to this depth is almost harmless. It won't destroy the cell. Maybe CALB is different in this regard, I don't really know. We have customers that use loaded LVC's as low as 1.5 Volts without problem. 

Keep in mind that sag is relative to load. Limp mode at low SOC is a good idea, just make sure you don't push your luck with distance. Better yet is a secondary cutoff that opens contactors before a cell gets destroyed. 

dtbaker, if you have access to a thermal camera, I strongly recommend observing your surviving cells during charge, or immediately after driving with one. Any other cells that have been damaged will display elevated temperature over the others. Any cells getting inordinately warm should be removed and replaced now, before they develop an internal short. If you don't have thermal camera access, watch for cells that have developed problems holding charge or excessive voltage excursion under load or charge. I would look for this stuff now, not later. You have a potentially dangerous situation lurking.

For fire suppression, I recommend Halon/Halotron. The cooling effect they have is at least as valuable in a battery fire as the oxygen removal effect. You can arrest thermal runaway with Halon if you catch the event in time. Not so with most other extinguishing media.


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## dtbaker (Jan 5, 2008)

mizlplix said:


> I still check cell balance annually. Yes, I still get some slight drift. I usually top up 4-5 cells who are .017 Volt lower than the average.


...same here. I've extended my manual re-balance to 5000 miles or annually. I see very little relative drift between cells.




mizlplix said:


> Observation: These Calb 130 AH cells sag a lot. Even when freshly off the charger, a 8 second hard acceleration will set off the 2.5 volt alarm. This gets really bad as the pack gets near my max mileage, but it is a good reminder to be easy with the foot.....
> 
> Miz


also the case with mine... which was why I set my LV down to 2.0. The Miata is 'intended' for fun, so I leave the Zilla wide open. I can't help the occasional full throttle getaway.  It only lasts 5-10 seconds. But yes, toward the end of a charge I take it easy, and rarely go more than 50% DOD on any given day.


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## arber333 (Dec 13, 2010)

dtbaker said:


> The Miata is 'intended' for fun, so I leave the Zilla wide open. I can't help the occasional full throttle getaway.  It only lasts 5-10 seconds. But yes, toward the end of a charge I take it easy, and rarely go more than 50% DOD on any given day.


Yup. My MX3 is the same. I saw 20% voltage drop with normal acceleration, but with heavy foot i got to 2.8V quickly. When i drove on highway ramp i reached 100km/h quickly but i was sheading 700A and OV LED was blinking. But when i released throttle at 100km/h cells climbed back to 3.2V... Only last time i saw prolonged drop in one cell despite throttle release. It seems that one was damaged anyway...

Now i got all LiPo cells and i have them wired in paralel while i wait for 300V controller. I must say they are great cells. Regen is 50% better than LiFe and cells respond quicker to acceleration. 
I charge them only to 4V and discharge to 3.2V, since any more would mean certain death of cells.

Also i found out cells change their Ri at the end of charge and voltage can rise over 4V in ten seconds! So i set my BMS to reduce charge to 1/10th of C when at 4V.

A


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## mizlplix (May 1, 2011)

And YES, I use those "vampire boards". (The only "Vampiric" action is to my 12VDC aux. battery.) It will go dead in 2 weeks of sitting, operating 3 always on systems.

The BMS system cost me $475. After a year of chasing my cells all over by hand, I gave in. Now I rarely give the car a thought. I just get in and drive.

I have a "watchdog" on every cell for over/under charging. I can set the BMS to terminate the charge OR the Elcon, at my choice. I shut off at 3.49 VPC. it drops to 3.38 VPC over night. 

Given my driving area and habits, I can not tell any difference than when I stopped charging at 3.60 VPC and settled to 3.58 VPC. (When I started driving it immediately dropped to 3.41 VPC. Indicating to me that there is very little capacity up there.)

Regards,
Miz


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## skooler (Mar 26, 2011)

Hi All, 

I have been doing one hell of a lot of work recently on liFePO4. All I will say is I have now installed several thousand Sinopoly cells using the bottom balance system and have *never* lost a cell in normal use.

I have lost cells to impact (crash) and when testing top balancing.

When dealing with single cells with no series links, overdischarge does not cause any major issues until you get below about 2v.

The issues come when you have other series cells which have more 'charge' than the lowest cell and when overdischarged, it can go into 'reversal' (voltage turns negative). This will happen when you top balance or use a conventional bms - the cells are out of balance when near flat.

Its all well and good saying the cells were at 2.5vpc but when top balancing this is an average rather than an actual. I suspect the truth is the highest and lowest cell voltages were not known.

What killed the cell? I suspect a combination of overdischarge and maybe reversal causing an internal short (hence 0v). I almost gaurantee that this was your lowest capacity cell and hence the first to become empty.


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## wb9k (Apr 9, 2015)

I'm interested to know how folks' bottom balancing schemes work...surely there are multiple flavors, but I've never been convinced that the approach can work satisfactorily. IMO, the answer to all of these problems is not bottom balancing, and it sure as hell isn't having a pack with no controls at all. The real answer is a decent control system that monitors the voltage of all cell groups in the pack and forces a stop to discharge whenever necessary. A proper BMS does a lot more than just balance cells and manage HVC.


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## skooler (Mar 26, 2011)

wb9k said:


> I'm interested to know how folks' bottom balancing schemes work...surely there are multiple flavors, but I've never been convinced that the approach can work satisfactorily. IMO, the answer to all of these problems is not bottom balancing, and it sure as hell isn't having a pack with no controls at all. The real answer is a decent control system that monitors the voltage of all cell groups in the pack and forces a stop to discharge whenever necessary. A proper BMS does a lot more than just balance cells and manage HVC.


Lets not make this thread a bms vs top balancing vs bottom balancing debate.

The bottom balancing method I have now used on dozens of packs is on the below link.
http://www.diyelectriccar.com/forums/showthread.php?t=85458


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## wb9k (Apr 9, 2015)

I'll check out the thread, thanks for the link. However, I see nothing inappropriate in pointing out that a proper control system would have prevented the OP's cell failure from ever having occurred.


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## Sunking (Aug 10, 2009)

wb9k said:


> I'm interested to know how folks' bottom balancing schemes work...surely there are multiple flavors, but I've never been convinced that the approach can work satisfactorily.


Just like Top Balance there are different flavors. I can tell you how I do it with good success. 

Starts from the day the cells arrive. Wire all of them in parallel and discharge them to 2.5 volts and hold them there for a day. Then assemble them. I use an Orion Jr control unit with Top Balance turned off. I use it to monitor all 16 of my cells with two programmable outputs. 

One output goes to the very simple CC charger to turn it off when any cell reaches 3.5 volts. FWIW it is always the same cell #3, the weakest in the string. The charger is set to provide 35 amps and will shut itself off if the output voltage reaches 57.6 volts as a second fail safe. 

The other output goes to a LVD if any cell goes below 2.5 volts for more than 15 seconds. Has never happened. 

Last line of defense is the motor controller will operate the LVD if the pack voltage reaches 46 volts. Has never happened except once when I forced it that low during test. 

This keeps my pack operating between 10 to 90% SOC. It would take a catastrophic failure to over discharge or over charge the battery.


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## aeroscott (Jan 5, 2008)

skooler said:


> Lets not make this thread a bms vs top balancing vs bottom balancing debate.
> 
> The bottom balancing method I have now used on dozens of packs is on the below link.
> http://www.diyelectriccar.com/forums/showthread.php?t=85458



BMS's have been Known to fail causing loss of not only the battery packs but the whole car due to fire.

I was jumper charging (pack to pack) last night A123 cells in 48 volt configuration . High pack at 47v. to low pack at 42v. with a set of old rusty jumper cables . The resistances are so low in these cells that they pulled 150 amps. This only lasted a minute then slowed 75 amps then to 35 amps then to 5 amps.
Any BMS must be able to deal with these large currents and heat . Some day we will have great BMS systems when this current/heat can be controlled .


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## wb9k (Apr 9, 2015)

Sunking said:


> Just like Top Balance there are different flavors. I can tell you how I do it with good success.
> 
> Starts from the day the cells arrive. Wire all of them in parallel and discharge them to 2.5 volts and hold them there for a day. Then assemble them. I use an Orion Jr control unit with Top Balance turned off. I use it to monitor all 16 of my cells with two programmable outputs.
> 
> ...


OK, that all sounds fine on paper, but how do you cope with drift over time? How do you rebalance the pack when necessary?


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## wb9k (Apr 9, 2015)

aeroscott said:


> BMS's have been Known to fail causing loss of not only the battery packs but the whole car due to fire.
> 
> I was jumper charging (pack to pack) last night A123 cells in 48 volt configuration . High pack at 47v. to low pack at 42v. with a set of old rusty jumper cables . The resistances are so low in these cells that they pulled 150 amps. This only lasted a minute then slowed 75 amps then to 35 amps then to 5 amps.
> Any BMS must be able to deal with these large currents and heat . Some day we will have great BMS systems when this current/heat can be controlled .


I understand that electronics failures are problematic. So are abused cells. Trading one problem for another is not a solution. The fallacy I see in your description here is that you seem to think that all BMS's use FETs to pass current in and out of the battery--I can think of no other reason for a BMS to "not be able to cope with the current". Don't use FET's in that system, use contactors. Problem solved. Those systems exist today, BTW. Every OEM EV out there does it. 

I have to point out that these solutions (no-BMS, bottom balancing) can be made to work, sure. But these are not solutions for the mass market. Cars for the masses MUST be designed to operate more or less "brain free" on the driver's part. The OP in this thread illustrates exactly why that is so. There is no way that systems like this represent the future for EV's--they simply aren't robust enough for average users in the real world. No OEM would ever entertain even the thought of designing a vehicle like this, for a multitude of reasons.


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## aeroscott (Jan 5, 2008)

Earlier the in the day I fell asleep with the inverter on but no load, maybe 1/2 amp at 40 volts at shutdown. Checked each cell and lowest was 2.85 volts highest was 3.00 volts . I could smell some venting. Bottom balanced with power supply to 3.00 volts then the pack to pack charge.


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## aeroscott (Jan 5, 2008)

And when the contactors fail or weld together. Not a easy problem .


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## skooler (Mar 26, 2011)

aeroscott said:


> Earlier the in the day I fell asleep with the inverter on but no load, maybe 1/2 amp at 40 volts at shutdown. Checked each cell and lowest was 2.85 volts highest was 3.00 volts . I could smell some venting. Bottom balanced with power supply to 3.00 volts then the pack to pack charge.


 I assume this is LiFePO4 you are talking about?

If so it will not be 'venting' at 2.85v.

bottom balancing to 3v is fairly pointless -to much variation in SOC.


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## wb9k (Apr 9, 2015)

aeroscott said:


> And when the contactors fail or weld together. Not a easy problem .


Won't happen in a properly designed system. Use a precharge circuit and contactors rated for the current, don't switch under load. Also, put contactors on both + and - legs to the battery. In an emergency power off (EPO), where switching under load may occur out of necessity, open both at once. You might weld one, but you won't weld both.


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## wb9k (Apr 9, 2015)

skooler said:


> I assume this is LiFePO4 you are talking about?
> 
> If so it will not be 'venting' at 2.85v.
> 
> bottom balancing to 3v is fairly pointless -to much variation in SOC.


Agreed. Balancing (of any sort) must happen at the outermost "tails" of the curve, or it's just too imprecise. If you're venting at this voltage, then you have probably been overdischarged somewhere at some time along the way, or a cell has sustained serious damage some other way.


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## aeroscott (Jan 5, 2008)

Jack of EVTV says the cells are more reliable then the electronics .


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## wb9k (Apr 9, 2015)

aeroscott said:


> Jack of EVTV says the cells are more reliable then the electronics .


Good for him. Neither are foolproof, so it's a moot point. Good electronics systems have diagnostic self-checks that regularly vet the continued operation of the electronics.


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## aeroscott (Jan 5, 2008)

skooler said:


> I assume this is LiFePO4 you are talking about?
> 
> If so it will not be 'venting' at 2.85v.
> 
> bottom balancing to 3v is fairly pointless -to much variation in SOC.



Chevy Spark cells (A123)

A few minutes at 5 amps did it.


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## aeroscott (Jan 5, 2008)

wb9k said:


> Good for him. Neither are foolproof, so it's a moot point. Good electronics systems have diagnostic self-checks that regularly vet the continued operation of the electronics.




I like your passion , but how long have you been into ev's .


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## wb9k (Apr 9, 2015)

aeroscott said:


> Chevy Spark cells (A123)
> 
> A few minutes at 5 amps did it.


Sounds like something may be very wrong. Are you sure you smelled electrolyte? Was it a strong smell, or did you have to get right up on the module to smell it? The smell of the welded aluminum bus bars is very similar to electrolyte and is sometimes confused for it.


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## aeroscott (Jan 5, 2008)

Vary faint just a hint of electrolight smell. 

The 5 amps was balancing power to get back to 3 volts.


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## wb9k (Apr 9, 2015)

aeroscott said:


> I like your passion , but how long have you been into ev's .


I've been working at A123 as the warranty lead for four years now. Before that, I was an "alt energy" hobbyist, working with SLA batteries and solar power, as well as WVO in a VW Diesel Jetta. You can see what I'm driving/riding/building today in my signature line. 

The big advantage I have here is that I have seen and/or analyzed literally thousands of failure cases. Pretty much everything that fails out there comes back through my lab, whether it's cells or electronics. I have a pretty good idea of what works, what doesn't, and what ideas are simply non-starters. 

I understand that what I'm advocating may be out of the reach of many here. Much of what I do would be out of my own reach if I didn't work in the industry. Still, we have to be careful about what we promulgate as "technical gospel". What may be true for the DIY'er may be a total non-starter in the real world. I mean, does anybody here REALLY believe that no EV should ever have any kind of BMS? Does anybody here really believe that's even an option for any real car company? It isn't, period. In fact, it's illegal and it ought to be. The guys using LFP are at lowest risk of terribleness in the end, but they are not without risk of fire and death. Trying to run a car like this with metal oxide chemistries would be a death wish--irresponsible in the extreme.


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## wb9k (Apr 9, 2015)

aeroscott said:


> Vary faint just a hint of electrolight smell.
> 
> The 5 amps was balancing power to get back to 3 volts.


I assume you were charging individual cell groups (rather than the whole module at once). Sounds like some real investigation is in order. If you have to get right up on the bus bars to smell that, it could just be aluminum. If you can smell it a couple feet away, it's almost surely electrolyte, assuming you've correctly identified the smell. Compressing the module with bar clamps is one way to see if you've got a leak. Leaks often seal themselves back up after the VOC's evaporate off and the leftover salts plug the hole. Additional compression (either by applying compression force or creating it by drawing big current through the module) will usually open the leak back up again, temporarily. 

Where did you get the modules?


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## aeroscott (Jan 5, 2008)

Very impressive backround . You sound like the person to build a great BMS system.

When moving the battery stack around some of the metal cooling plates snagged as I was sliding the battery over edges , which bent in and contacted the cell pouches . could I have damaged / shorted the cells.

Why did they make the individual plates this way?


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## wb9k (Apr 9, 2015)

aeroscott said:


> Very impressive backround . You sound like the person to build a great BMS system.
> 
> When moving the battery stack around some of the metal cooling plates snagged as I was sliding the battery over edges , which bent in and contacted the cell pouches . could I have damaged / shorted the cells.
> 
> Why did they make the individual plates this way?


Sounds like you punctured a cell on the bottom when you bent a heat sink. Those aluminum plates are spectacularly fragile. They are there to keep heating even across the module. In the Spark, they sit on a cooling plate that draws off high heat and warms cells in the winter so they can be charged safely. The plates are thin to conserve both weight and space. The downside is their fragility, and dragging the module over any kind of edge is a major handling no-no. You could probably find that the cells can continue to work for quite some time even after being punctured, but don't do it. Leaking electrolyte leads to isolation failures in the module, a situation that gets increasingly problematic (and possibly dangerous) with time. If you're sure the wound is on the bottom side, lay the module on its side and carefully bend back the heat sinks where you think the injury occurred. You should be able to locate the puncture that way. Then, you should unband the module and cut out the leaking cells. Using what you have left may not be simple, but this is the right thing to do at this point.


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## aeroscott (Jan 5, 2008)

Came from a junked Spark . I just unbolted the 4 96 volt units at 105 lbs each. I want to break them in half but lost my banding tool,so been moving them from the house to the motor home and back for charging. I'm jumping 1/2 pack for 48 volt inverter operation. Then switch to other half pack on depleation.


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## wb9k (Apr 9, 2015)

aeroscott said:


> Came from a junked Spark . I just unbolted the 4 96 volt units at 105 lbs each. I want to break them in half but lost my banding tool,so been moving them from the house to the motor home and back for charging. I'm jumping 1/2 pack for 48 volt inverter operation. Then switch to other half pack on depleation.


Sounds like you have identified your first candidate for breakdown and rebanding!


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## aeroscott (Jan 5, 2008)

I saw this problem coming . Could have just contact cemented a piece of 
wood or plastic . Time constraints .

I didn't see any damage to the cells like leakage, but well check now!
Without leakage could I have shorted a cell.


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## aeroscott (Jan 5, 2008)

Any problem on breaking the bands and expansion of the unbanned cells before rebanding.


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## wb9k (Apr 9, 2015)

It's a good idea to compress the module with bar clamps before cutting the bands--they can fly out and snap you in the face if you don't. Don't source or sink any real current until the cells are recompressed. 

If you're really smelling electrolyte, you have a leaking cell somewhere. A shorted cell won't hold a charge, which should be obvious enough by other means, and unless it's also leaking, it won't smell.


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## Sunking (Aug 10, 2009)

wb9k said:


> OK, that all sounds fine on paper, but how do you cope with drift over time? How do you rebalance the pack when necessary?


What drift? Installed about 6 months ago and no drift. All the cells are in series.


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## wb9k (Apr 9, 2015)

Sunking said:


> What drift? Installed about 6 months ago and no drift. All the cells are in series.


Give it time. No two cells are exactly alike. There are small differences in cell impedances that will impose different levels of Peukert loss if nothing else. Add to that temperature gradients within the pack that amplify this effect, small differences in interconnects, it all adds up. 6 months is nothing--you haven't even gone through four seasons yet. Sooner or later ALL packs need balancing. It is a certainty. So...when the time comes, how will you (or your customers, as the case may be) deal with this?


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## aeroscott (Jan 5, 2008)

Just looked over some of the cells on the second pack everything looks ok. Had a bottle of pine cleaner setting near the battery that may have been the smell . I will check the other pack asap.

When I unband and unclamp the pack ,will it expand much as i need to make and insert plywood ends. Would you recamend aluminum or wood and how thick. Or something else.


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## wb9k (Apr 9, 2015)

aeroscott said:


> Just looked over some of the cells on the second pack everything looks ok. Had a bottle of pine cleaner setting near the battery that may have been the smell . I will check the other pack asap.
> 
> When I unband and unclamp the pack ,will it expand much as i need to make and insert plywood ends. Would you recamend aluminum or wood and how thick. Or something else.


You won't see a lot of expansion, but it will be a visible amount. Compress as tight at you practicably can before rebanding, making sure the end plates are as close to parallel as possilble. Our end plates are heavy fiber reinforced ABS. Several materials could be used for stand-ins, maximum rigidity being the most critical aspect of all. Bowing end plates apply pressure unevenly, contributing to concentration gradients in the cells that lead to uneven wear of the cell interior. This may be OK if you're not drawing big loads for extended periods of time, like you would in a typical EV, but is best avoided. I also like non-conductive materials in this spot.


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## aeroscott (Jan 5, 2008)

I have some 1" birch ply. or could go 3/4 dug fir.


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## aeroscott (Jan 5, 2008)

Are you familiar with the Spark BMS. If so how does it work


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## wb9k (Apr 9, 2015)

aeroscott said:


> I have some 1" birch ply. or could go 3/4 dug fir.


Not optimal, but may work. Cut out a groove for the band to lay in like is present on the OEM end plates. You should also be aware that the cells are glued inside there. Cutting the weld straps and bands is just part of the battle. Then, you need to use a solvent to soften the glue on the cell faces so you can CAREFULLY slide a plastic tool (like a wide putty knife) between the cells you wish to remove/keep. This is the hardest part of the operation to complete without damaging a cell. Give the solvent plenty of time to soften things up. We use "SEM Solve", available at any paint supply house.


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## wb9k (Apr 9, 2015)

aeroscott said:


> Are you familiar with the Spark BMS. If so how does it work


The Spark BMS is an A123 BMS with many mods by GM, and GM software. The operation is way too complex for me to describe it here, and IMO, ought to be even more so--I don't think GM's balancing strategy is truly adequate for the real world, especially as the packs begin to age. I firmly believe the BMS of the future will be an order of magnitude smarter than anything that exists today. 

You can get a lot more details if you download the A123 Battery Handling Guide that is published over at http://endless-sphere.com/forums/vi...35&p=761185&hilit=a123+handling+guide#p761185 . That's a great document all the way around which, among other things, spells out the basics of A123's control hardware system.

EDIT: Made the link actually useful....sorry for the garbage link I had in there originally. Go to the top of the thread when you get there.


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## skooler (Mar 26, 2011)

No reason whatsoever to compress the cells other than neat packaging. They only swell when abused.

As mentioned earlier in the thread, I have installed thousands of prismatic LiFePO4 cells in dozens of packs and no issues whatsoever.


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## wb9k (Apr 9, 2015)

skooler said:


> No reason whatsoever to compress the cells other than neat packaging. They only swell when abused.
> 
> As mentioned earlier in the thread, I have installed thousands of prismatic LiFePO4 cells in dozens of packs and no issues whatsoever.


Wrong, wrong wrong. Sorry, that's just flat out wrong. They will swell not only when abused, they will swell under normal use at charge rates over about 1C. Maybe cheap LFP that can't handle big discharge never gets an opportunity to swell, but this is definitely NOT the case for A123 cells.


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## skooler (Mar 26, 2011)

wb9k said:


> Wrong, wrong wrong. Sorry, that's just flat out wrong. They will swell not only when abused, they will swell under normal use at charge rates over about 1C. Maybe cheap LFP that can't handle big discharge never gets an opportunity to swell, but this is definitely NOT the case for A123 cells.


 Sorry but I have never seen a LiFePO4 cell swell when it has not been abused. 

the swelling is due to an irreversible chemical reaction within the cell - that is not normal!


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## wb9k (Apr 9, 2015)

skooler said:


> Sorry but I have never seen a LiFePO4 cell swell when it has not been abused.
> 
> the swelling is due to an irreversible chemical reaction within the cell - that is not normal!


Again, not sure what cells you're using (actually, I think you said Sinopoly, which I have no exposure to whatsoever), but what you're saying is not true for A123's LFP. To my knowledge, it's not true for any LFP, but I can't say for sure. There is gas formation during ion exchange in these cells that is normal and reversible. The gas goes back into solution with a little time, but at high C rates, the amount of gas formed can be significant enough to cause problems to develop if inadequate compression is provided. The gas forms between electrodes in the stack, pushing them apart, causing high impedance zones in the cell that lead to a self-accelerating failure mode that causes premature cell death. With proper compression, the gas cannot form large voids until it gets pushed out beyond the perimeter of the electrode stack where it can do no harm. There were once those inside A123 who said what you're saying here, but experiments have proven them wrong. A123's cells will suffer damage from lack of compression if they are asked to deal with anything more than about 1C. Since steady draw of 10C or more and peaks of 40C or more are regularly thrown at our cells (this ability is what attracts most to them in the first place), no compression on these cells is usually a non-starter. 

Now, you can make prismatic cells that are in hard cans that provide compression to the stack--maybe this is the case with Sinopoly? That can work, but the tradeoff is additional mass and weight--two things you really don't want with LFP if you can help it. So in that scenario, you would be making a tradeoff between performance and ease of assembly / affordability. It seems unlikely that in an EV you would always be drawing less than 1C. How long have these vehicles been in the field and what kind of drive cycles are they seeing?


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