# LiMn vs. LiFePO4 cylindrical battery cells



## Ampster (Oct 6, 2012)

Matej said:


> Hello,
> This battery pack would be for a motorcycle. For a car I would just use Leaf or Volt modules, but since I do not need as much capacity, I am considering building my own pack using cylindrical cells.
> 
> I know LiFePO4 is the preferred safe DIY chemistry, but I am curious if I could get away with using LiMn without BMS. LiMn 18650 and 26650 batteries seem to be popping for very affordable prices and they have higher voltage and capacity than LiFePO4. I think they are primarily marketed toward 'vapers' for their pipes or pens or whatever, and they also seem to be popular among flashlight enthusiasts.
> ...


I have used Headway (cylindrical) LiFEPOs for years on bicycles. I have moved to LMNC Nissan Leaf cells for a stationary pack. I would not run it without a BMS. I am helping a friend convert an AC35 powered VW from LiFEPO prismatic to a leaf pack. Cheaper and better energy density.

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## Karter2 (Nov 17, 2011)

The main advantage of NMC 18650s over any Lifepo4 cell is energy density. (Wh/kg) ...they are simply much lighter for any equivalent pack capacity. That could be useful on a bike.
As with any pack, if you do not use a BMS you have to substiture with good and frequent checks for balance and performance. At your own risk !
There are also cylindrical Lifepo4 cells


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## Tony Bogs (Apr 12, 2014)

Apparently, it is not easy to monitor all cylindrical cells in a large pack.
Tesla recently filed a patent that focuses on safely charging batteries.
http://insideevs.com/teslas-latest-patent-methodology-charging-batteries-safely/

A controller compares an acquired set of parameters from the pack while charging


> _against a set of predetermined profiles indicative of an internal short in one or more cells_


I'd go for prismatic, a temperature sensor for each cell ( ~US$2 per sensor) and a lower DOD.
Saves a lot of space (and wiring) on a motorcycle.


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## Ampster (Oct 6, 2012)

Tony Bogs said:


> Apparently, it is not easy to monitor all cylindrical cells in a large pack.
> ...........


It is only necessary to monitor the group of cells that are in parallel because physics says they will all be at the same voltage. The same thing applies to prismatics. For a while I had a 2P36S pack of Thunderskys. The BMS connection went between the straps that buddy paired the cells.

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## Tony Bogs (Apr 12, 2014)

The quote should be: apparently Tesla indicates that ... in the article on insideevs.com:
http://insideevs.com/teslas-latest-p...teries-safely/

It concerns detecting a short in a cell, i.e. a high current bypass pathway. 



ampster said:


> It is only necessary to monitor the group of cells that are in parallel because physics says they will all be at the same voltage.


My physics and apparently Tesla's differ from yours.

And we all know that Tesla knows its cylindrical cells.


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## Karter2 (Nov 17, 2011)

Tony Bogs said:


> The quote should be: apparently Tesla indicates that ... in the article on insideevs.com:
> http://insideevs.com/teslas-latest-p...teries-safely/
> 
> It concerns detecting a short in a cell, i.e. a high current bypass pathway.
> .......


 "Detecting" ""....would seem to be the relavent word, and only whilst charging !
Tesla have always had individual shorted cell "*protection*" in their packs in the form of fused connection to every individual cell.
This proposal would seem to be a way of checking cell status whilst charging. ( i believe Tesla could always do this with service diagnostics )


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## Ampster (Oct 6, 2012)

Tony Bogs said:


> The quote should be: apparently Tesla indicates that ... in the article on insideevs.com..........
> My physics and apparently Tesla's differ from yours.
> And we all know that Tesla knows its cylindrical cells.


Thanks for the clarification, I took the comment out of context. I think we all agree on the physics of parallel cells and their equal voltage. Yes, Tesla has an ingenuous way to isolate a shorted cell so it doesn't take down the rest of the cells in that parallel group.


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## Tony Bogs (Apr 12, 2014)

Yeah, but it doesn't just concern taking down a parallel bank.
It's a serious safety hazard when a cell shortens out while fast charging
without the proper implementation of detection and protection systems.
That's an issue for Tesla packs with a lot of cells in parallel. 
Apparently Tesla came up with a clever solution.

When a cell shortens out, a BMS will have great difficulties detecting the very uneven distribution of current directly. 
A temperature sensor at each cell will detect the rogue cell and it is much easier to implement in a DIY build than the Tesla solution.
The simple (DIY) solution with temperature sensors is very unpractical if there are a lot of low Ah cylindrical cells in parallel.

So, I'd go for prismatic (max two in parallel).


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## Ampster (Oct 6, 2012)

I agree with what you are saying. If the OP weighs energy density and configurabilty for a motorcycle over safety then a pack of cylindrical cells might serve those two priorities better. I also may not be aware of small prismatics that could be easily configuable.

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## Karter2 (Nov 17, 2011)

Tony Bogs said:


> Yeah, but it doesn't just concern taking down a parallel bank.
> It's a serious safety hazard when a cell shortens out while fast charging
> without the proper implementation of detection and protection systems.
> That's an issue for Tesla packs with a lot of cells in parallel.
> ...


Why would a BMS have problems of detecting a shorted cell during charging ?
The internal short would immediatelly cause the fuse to fail .consequently reducing the capacity of that parallel group and hence increasing the rate of voltage increase.....which a good BMS should be able to detect.
The shorted cell cannot compromise the group once the fuse is blown, so other than thermal runaway, it can do no harm.
If it does go thermal, there is nothing any detection system can do.
But Tesla has also built in thermal supression and containment features to minimise the risk of a major event.
But, unless you are prepared to use similar features, construction , and materials as Tesla, then you are increasing the risk.
Using substandard, untested , reused, or unknown cells is also a big risk.


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## Tony Bogs (Apr 12, 2014)

karter2 said:


> Why would a BMS have problems of detecting a shorted cell during charging ?


I'll have to redirect this question to Tesla. They use the term in the patent paper as quoted in the article on insideevs.com.

But I think what is meant is an extreme uneven current distribution between parallel cells as I mentioned in my earlier post.
The fuses have to be able to withstand very high currents during fast charging. 
They don't blow out that easily.

And in that case a BMS is of little use. The rogue cell will overheat during fast charging. 
Unless the pack is equipped with temperature sensors at each cell or the clever Tesla system.


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## Ampster (Oct 6, 2012)

Very high currents? You are talking about the individual cell fuses, correct? What currents are those fuses withstanding? Those are 3500 mAhr cells and I don't think a Tesla charges at more than 3 C. That is a little over 10 Amps and the fuse is rated for 20 Amps.

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## Tony Bogs (Apr 12, 2014)

Indeed, the discharge current is a lot higher. 
The point of argument still stands: the fuses don't blow out that easily and an extremely uneven current distribution is problematic for the BMS during fast charging.

With a larger number of cells in parallel is it the total current of all the parallel cells that has to be taken into account. 
A rogue cell can suck up quite a big part away from the other parallel cells (Tesla has got a lot in parallel) without the BMS detecting it. 
During fast charging that is, so it can be as high as 5C of even higher for a single rogue cell. Eventually, it will trip the fuse, but by then it probably has already gone thermal. 
Tesla did think of extra protection against fire. But it is of course better to prevent such a scenario with some clever controller solution (i.e. the recently filed patent).

And I bet Tesla is already working on the next gen superchargers with even higher amp rates.

Back to topic: yeah, I'd go for LiMn. And I have seen specs of 26650 cylindrical cells with very high discharge capabilities. Maybe a 2P topology is possible for a motorcycle.


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## Karter2 (Nov 17, 2011)

Teslas cell fuses have been tested at just higher than max discharge rate (~20A)
And presumeably their pack cooling has been designed to deal with cell temperatures at that discharge level.
Since fast charging is much lower levels of current, a "rogue" internally shorted cell will draw current ..potentially over 1500A ...until it blows the fuse
..I dont think that will take long !
Maybe if the cell is only "partially" shorted and draws a (charge) current just less than fuse capacity,..it will heat up much faster than a high rate discharge and cause a thermal hot spot. ?
But ... That senario could happen at any time, charging or not.


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## Ampster (Oct 6, 2012)

20 Amps is a good approximation for those little fuse wires. I think Tony is talking about the high current of the entire module which could be in the hundreds of Amps because current adds when cells are in parallel. The relationship of one cell to the common buss of all those parallel cells probably never exceeds 20 Amps if that. I am not trying to sidetrack this thread but clarify for other readers. I also think it is more informative to use numbers rather than adjectives like "quite a bit" and "very high currents".


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## Karter2 (Nov 17, 2011)

My concern is that a "rogue" cell could short at any time, it wont just occurr during fast charging, it could be during high discharges, regen boosts, or even just due to a physical shock or extreme thermal situation (sub zero parking in Canada).
So any clever system Tesla may build into their superchargers will not protect against the majority of situations.
The best way to deal with this possibility is prevention by the use of effective process control and quality monitoring during manufacturing and assembly.


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## onegreenev (May 18, 2012)

Karter2 said:


> My concern is that a "rogue" cell could short at any time, it wont just occurr during fast charging, it could be during high discharges, regen boosts, or even just due to a physical shock or extreme thermal situation (sub zero parking in Canada).
> So any clever system Tesla may build into their superchargers will not protect against the majority of situations.
> The best way to deal with this possibility is prevention by the use of effective process control and quality monitoring during manufacturing and assembly.


Yes, and if the Rogue Cell internally shorts but never shorts out the fuse, then what?


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## Ampster (Oct 6, 2012)

onegreenev said:


> Yes, and if the Rogue Cell internally shorts but never shorts out the fuse, then what?


How does a fuse short? It is a small wire. Please draw a diagram of what a short a short circuit of a fuse looks like. The readers woukd like to understand what you are talking about. The answer is that it does not get shorted out. It is a1 device that responds to over current by heating up and opening the circuit. Let's not get all excited about a wire not obeying the laws of physics. Karter has the right approach. Quality control.

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## onegreenev (May 18, 2012)

Ampster said:


> How does a fuse short? It is a small wire. Please draw a diagram of what a short a short circuit of a fuse looks like. The readers woukd like to understand what you are talking about. The answer is that it does not get shorted out. It is a1 device that responds to over current by heating up and opening the circuit. Let's not get all excited about a wire not obeying the laws of physics. Karter has the right approach. Quality control.
> 
> Sent from my SM-N910T using Tapatalk


Who's excited. 

Shorts out and blows the fuse. Picky Picky. 

Will a dendrite short circuit internally cause the externally connected fuse wire to blow? Or will it stay connected and just cook. within its self?


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## onegreenev (May 18, 2012)

> Karter has the right approach. Quality control.


Karter knows the right approach. Quality Control is a no brainer.


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## Tony Bogs (Apr 12, 2014)

I used the Tesla patent as an example for safety issues in packs with a lot of cells in parallel (cylindrical like Tesla uses). The TS starter wants to use cylindrical.
Again, apparently, it looks like Tesla is going to add an extra safety feature (source: insideevs.com) on top of quality control, anti fire measures,
10, 20 or 50A, high tech, high or low I2t fuses and whatever else they have done so far. Including a BMS. 
I have tried to clarify the problems with BMS, fast charging and an uneven distribution of current. Not a full short yet in a cell. But it's almost there. 

I repeat, based on the article on insideevs.com it looks like Tesla is going to add an extra safety feature for fast charging.

Makes sense to me as an electrical engineer. I'm definitely not going to use a lot of cylindrical cells in parallel in my DIY builds.


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## Karter2 (Nov 17, 2011)

onegreenev said:


> Yes, and if the Rogue Cell internally shorts but never shorts out the fuse, then what?


Assuming you meant the fuse never fails.(goes open circuit)....?
If so, that cannot happen if the cell has "shorted" internally, as the entire 220+ Ah parallel group is now shorted through that fuse...instant fail ! ..
But, as i said in an earlier post (#14), if the cell only partially fails internally such that it draws a high current just below the fuse capacity, then there is the potential for the cell to overheat.
...but technically that is not a "shorted"cell , its a different failure mode, and not something i have ever heard of , though potentially more problematic.
What i take from this is that Tesla will probably make some improvements/updates to their BMS system to enable better monitoring of the pack during fast charging.


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## Duncan (Dec 8, 2008)

If the fuse does not blow then we have less than 20 amps x 3.4v = 68watts

(was that 20 amps the "live forever" current? or the blow eventually current?)

If I put 68 watts into one of those cells how much will the temperature rise before conduction to it's brothers limits the temperature?

I don't have enough physical details to work it out - but an engineering WAG would be less than 100C

If a cell is at 100C is that enough to start it burning? - I don't think so

So if the fuse does not blow - there is not enough power to cause a major overheat
If there is enough power to cause a major overheat - then the fuse blows


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## Ampster (Oct 6, 2012)

Thank you Duncan and Karter2 for explaining it in simple terms.

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## Karter2 (Nov 17, 2011)

Duncan said:


> If I put 68 watts into one of those cells how much will the temperature rise before conduction to it's brothers limits the temperature?
> 
> I don't have enough physical details to work it out - but an engineering WAG would be less than 100C
> 
> ...


 ... Lets see, ?....20 amp reverse current for 30 mins ?.
.....i have a bunch of unwanted 18650s ,...and a 30amp max charger with a temperature probe and data logging.........


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## onegreenev (May 18, 2012)

I need at least 10 18650's to put in parallel with fuse wires. Im then going to run something at 4 amps continuous and then puncture a cell while discharging. Wager to guess what will happen?


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## Ampster (Oct 6, 2012)

onegreenev said:


> ...... then puncture a cell while discharging. Wager to guess what will happen?


Electrolyte and ions will escape?
If you are trying to replicate a Tesla battery, don't forget the intumescent goo around the cells. 
Seriously, you are going to try to prove to us that puncturing a cell will not cause the fuse to "short" or shorten? Why would anybody expect a 20 Amp fuse to open at 4 Amps? Before figuring out how to puncture a cell why not just hit it with a blowtorch?

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## Karter2 (Nov 17, 2011)

Well, the outcome will depend on..
Type/ brand of cell
Fuse wire rating
Severity and location of the "puncture".
Are you assuming a puncture replicates the effects of a internal short ?


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## onegreenev (May 18, 2012)

Karter2 said:


> Well, the outcome will depend on..
> Type/ brand of cell
> Fuse wire rating
> Severity and location of the "puncture".
> Are you assuming a puncture replicates the effects of a internal short ?


Will try laptop style. Don't want to spend a fortune on the experiment. I will test fuse material to get around 6 amps to blow. 4 amps to run. 

Puncture to replicate road debris that got into the pack. The goal will be to see if the punctured cell will cause the fuse to blow, the cell to burn yet keep the device running or will the device just heat up, die and nothing happens to the rest of the pack. 

Its an experiment to see. 

My take is many are assuming that building a pack with these size cells requires little fuses to prevent fires. Trying to replicate some possible issues we should actually see what happens. See if its required or not. 

Fuse wires are not used within a CALB cell.


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## Ampster (Oct 6, 2012)

Have fun.

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## onegreenev (May 18, 2012)

My guess will be the one cell will take down the rest rather quickly but only by discharging into the shorted out cell. I don't foresee that a fuseable link will be broken. Just discharged cells.


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## onegreenev (May 18, 2012)

Ampster said:


> Have fun.
> 
> Sent from my SM-N910T using Tapatalk


I think it will be.


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## Ampster (Oct 6, 2012)

If you want to emulate a shorted cell, clip a jumper between the poles of this cell. Just make sure the fuse wires are between that cell and the rest of the pack.

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## Tony Bogs (Apr 12, 2014)

I already did a similar test (unintentionally). Apply a negative voltage to a cell with a current limit of approximately 3 amp. 
The cell was too hot to touch after less than 30 seconds. Then the voltage source shorted out.

What 20 Amps (69 Watt) in a thumb sized cell can do?: let you keep a fire extinguisher at hand.


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## Ampster (Oct 6, 2012)

Did you put an Amp clamp on it to see what current was flowing? Are you saying you connected a 3 Amp power supply positive to the negative of the battery and vice versa on the other end?
I am not sure how your example demonstrates what the Tesla cell fuses do and how the cell fuses prevent a shorted cell from quickly discharging the rest of the parallel cells in a module..

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## Tony Bogs (Apr 12, 2014)

The fuse has I2t (square of amps times time) profiles (at different temperatures). In charts. 
How long it takes for the fuse to blow out at a certain amp can be read from the chart.
The specified I2t is usually at 20 or 25 degrees C and at the indicated amps. 
At 20A and at 25 degrees C it will a very long time for a 20A fuse to blow out. 

And yes, polarities reversed. It was a small scale charger test. So it was a short for 3 Sony 18650 cells in series in a diode (inside the power supply) and a short for the power supply. 
Cells were from a notebook. Maybe 2Ah. 

What is demonstrated is that a short is never 0 Ohms (reserved for superconductors only) 
and it takes a while before a component melts down: just like a fuse. 
In my case it took about thirty seconds until the wires on the primary side of the transformer in the power supply melted and fused together. Before the cells went...
Luckily all without a fire..

Tesla filed a patent for an extra safety feature during charging. Seems like they can not fully rely on the fuses during fast charging. I think they're right. Basic electrical circuit theory.


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## Ampster (Oct 6, 2012)

I don't care about the Tesla patent. The physics of superconductors are not relevant to me. What I care about is refuting your and onegreenev's innuendo that the Tesla fuse wires will not blow at currents exceeding 20 Amps. Call me a Tesla Fanboy, but i have a great respect for the engineering and continuous improvement that has gone onto those cars. Neither of you have proven that the fuse will not protect the pack at a current of greater than 20 Amps. It is simple physics that it will take a long time for that fuse to blow at 20 Amps. That doesn't prove a thing. Those cells are capable of 20c or more and that is 60 Amps and that is when the fuse will blow very quickly.
Yes, I understand that other factors could cause a cell to heat up and that patent may help Tesla detect issues during charging. 

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## kennybobby (Aug 10, 2012)

The cells may be able to do 20C, but Tesla doesn't operate them at that level. i don't know what the max limit is now, but for the P90D it was 1500 Amps for about 7 seconds before the power was pulled back by the controller. This is seen in BillD's acceleration run.

1500 Amps / 74 parallel cells = 20.3 Amps per cell, about 7C.

The whiz kid Jason Hughes (aka WK057) tested some cell fuse wires 


> I was able to get them to carry 24A for 60 seconds+ and they would consistently pop within a second or two at 25-25.5A.
> 
> Honestly, that's more power than I had figured they were able to handle. Assuming they're sized for 25A, that's 90W per cell, almost 40kW per module, or close to 8C max before consistent popping.
> 
> Using an NEC-style 25% margin gives me a constant current draw of about 19A possible before popping fuses. That'd be about 6C, or about 485kW nominal for an 85kWh pack.


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## Ampster (Oct 6, 2012)

That was my point. I should have figured Jason explored this issue. He has done some amazing hacks and other investigative musings. My own Outback Radian inverter installation was inspired by his multi Radian Tesla powered off grid system powering his home.

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## Karter2 (Nov 17, 2011)

Tony Bogs said:


> I already did a similar test (unintentionally). Apply a negative voltage to a cell with a current limit of approximately 3 amp.
> The cell was too hot to touch after less than 30 seconds. Then the voltage source shorted out.


??... That doesnt make sense at all.?
There obviously was not a 3 amp current limit in the circuit, because if you connected +ve to negative, and negative to positive, all this does is replicate a pair of cells in series with a 2 ohm resistor !(for a 3 amp current)
And even If you connected +ve to +ve and -ve to -ve with a 3 amp limit, that is just a simple charge circuit, which also shouldnt cause excessive overheating ?
But 20+ amps in either case would be a very different situation.


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