# Planning Audi A2 Conversion



## spoland (Aug 9, 2012)

Hi everybody,

I'm planning to do an EV conversion and right now I am considering converting an Audi A2. I like Audis and drive an A6 right now. The A2 is about enough size for my family (three persons and two dogs) to be able to commute 60km per day. I am hoping that its low weight will give good performance and range and that the aluminium body will make it last long. 
I haven't been doing much auto mechanics for a while (I changed the transmission in a Saab about 20 years ago , but I know the basics about cars and how they work. I don't want to fabricate anything advanced (like adapter plates), but smaller basic stuff won't be a problem.
I'm hoping to get a range of 100-160 km (60-100 miles) and be able to cruise at highway speed 110 km/h (70 mph). My budget is about EUR 20,000 (USD 25,000) including donor car which I haven't got yet. I live in northern Sweden and in winter it gets quite cold here. -20C (-4F) is not uncommon and -30C (-22F) happens a couple of times each year. 
The A2 has a Cd=0.28, A=2.2m2, m=1065kg (I'm guessing 1260kg after conversion=2780lbs)

The parts I have considered so far are:
Motor: HPEVS AC50 (I like the idea of not having to maintain the motor at all)
Controller: Curtis 1238-7601
Adapter: Canadian EV New Beetle adapter
Batteries: 36 Winston (formerly Thundersky) 200 Ah LiFeYPO4 cells giving a nominal voltage of 115V and energy content of 23 kWh. (Not sure if this will work since the nominal voltage of the Curtis is 96V, but some claim it will work just fine) The reason for using 36 instead of 30 cells is mainly to increase range. I don't really know the difference between LiFePO and LiFeYPO4 batteries. Some claim the LiFeYPO4s have better cold performance.
BMS: No BMS. I think I will follow Jack Rickard's advice and undercharge the cells to 3.5V instead of 3.6V to improve their life and not use a BMS.
Charger: A programmable charger that lets me create a constant current/constant voltage charge process with custom current and voltage. If it is possible I would like the charger to take as input 400V,16A which is standard in Sweden if you use the voltage between two phases in a three phase outlet. That would give me a charge power of a little more than 6kW and charge time of around three hours. I haven't found one yet. Any suggestions? Plan B is 230V,16A.
Vaccum pump for brakes: Yes
Power steering: Don't know if I need that. I will probably try without and get an electric 12V pump if needed.
AC: Will probably get a donor car without AC
Heater: Oh yes! I think I will need a 2-3 kW ceramic heater or a water heater

In Sweden we have some peculiar regulations regarding home built autos. To simplify it a little (a lot actually), if you convert a car older than 1993 you can do pretty much what you want, but if you convert a car newer that 1993 things get more compliated. If you only change the engine, the build is called an "altered car"' and then you need the car manufacturers certificate that the new engine will meet all emission an other demands on new cars. In practice this is impossible to get if you don't change to another ICE-engine that is part of the program for that car model. So this is not doable. If, however, you change the car more than this, for exampel change the engine, transmission, and brakes, it is called a "rebuilt car", and then you do not need any cerificates from the manufacturer and everyting gets much easier! That is apart from all the extra work that is needed to change the tranny and brakes that does not contribute neither functionally nor improves security of the final car. So unLEAN! 
Since I want to use an Audi A2 from 2001-2003 I have to do this as well.
Transmision: VW Golf Mk IV, 5 speed (I hope the half shafts will fit)
Brakes: VW Golf Mk IV, discs front and rear plus main cylinder. The A2 has small discs in front and drums in rear so bigger discs will be OK.

Any thoughts on this conversion?
Thanks!
/ Peter


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## EVEngineeer (Apr 11, 2012)

I found that motor on a website for 4.5k, but I also saw that motor and controller in a pack on http://evolveelectrics.com for 4.6k. Also, AC motors probably won't be a good way to go, because of your budget. You can achieve what you are looking for by going with a DC motor, like the WarP 9. That motor will only cost you 2k.


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## cpct (May 31, 2012)

Can you give a reason why you don't want to use a BMS?

I mean, a large part of the budget is batteries, they will be used in harsh environment (very cold) and need to fullfill high range requirements. Leaving out a BMS will shorten the lifespan of your batteries significantly and it does not cost *that* much...


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## spoland (Aug 9, 2012)

EVEngineeer said:


> I found that motor on a website for 4.5k, but I also saw that motor and controller in a pack on http://evolveelectrics.com for 4.6k. Also, AC motors probably won't be a good way to go, because of your budget. You can achieve what you are looking for by going with a DC motor, like the WarP 9. That motor will only cost you 2k.


Thanks EVEngineer!
I think will also have a look at DC motors. I guess the Warp 9 would give me a little less power (32HP compared to 50HP for the AC50), no regen and also I would have to change the brushes from time to time. But, as you mention, with a Curtis 1231 controller for 1,700 I would save almost $1000.


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## spoland (Aug 9, 2012)

cpct said:


> Can you give a reason why you don't want to use a BMS?
> 
> I mean, a large part of the budget is batteries, they will be used in harsh environment (very cold) and need to fullfill high range requirements. Leaving out a BMS will shorten the lifespan of your batteries significantly and it does not cost *that* much...


I'm really not sure about this, thats why I'm asking everyone here. To me there seems to be a debate about BMSes where some argues that this is a must have and some, Jack Rickards of EVTV for example, claims BMS'es are nonsence.
I'm not sure of the precise arguments against BMS but i believe it has to do with the self balancing that occurs in packs of LiFEPOs anyway, the fact that a BMS turns off charging of a single cell based on the voltage over that cell whereas the charging state of the cell is much more complicated than that, and that it is better to undercharge your whole pack of batteries to prolong its life instead of trying to maximise energy content.
I sure am no battery specialist, but from what I can read in different foras there is no consensus regarding BMS in the EV community and they do cost a bit.


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## drgrieve (Apr 14, 2011)

You don't need a BMS as such, but you do need to know how to charge the batteries and have a reliable charger (with a possible fail-safe - timer or secondary voltage or ah counter) and obviously a good ah counter for discharge.

In regards to parts since you are in Europe look at the kostov 9" 220v (or a kostov 10"). Match these up with a Soliton Jnr. Peak output should be around 130kW for the 220v in this combination. It's also a very light motor at 45 kgs.

BTW the warp 9 (with the new brushes) peak output is ~200kw when combined with a Soliton 1. Which is a little more than 25hp!

In regards to cells seriously look at the new calb greys. They are meant to have improved cold weather performance. Get the 100ah cells so you have at least 240v (~75 cells) to counteract voltage sag thus maintaining reasonable current output at a higher rpm.

In regards to brush maintenance. If you keep the motor accessible this is 20 minute job. Not something to worry about. Not sure on brush life but for normal street driving it would seem to be in the "last forever" category of things.


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## steven4601 (Nov 11, 2010)

For all the DC conversions in the E.U.... Warning: It is hard to get EMC (R10) & electrical safety (R100) compliance with (large) DC traction motors. 
If you dont use C.E. certified components with according EMC tests for automotive usage you'll require to do an EMC vehicle test. ~ 2000 to 5000 euro for 1 test. which may fail and you have to do another one. 
Testing can be more expensive than a battery pack. 

Electrical compliance problems comes from the isolation requirement (described in the R100) between the traction pack & vehicle chassis. Carbon dust creates an electrical path between the contacts of the motor and the chassis causing it to fail the insulation test. Also this creates common mode currents causing higher chances it to fail EMC requirements.



Read first what is pubished regarding requirements, call officials regarding regulations towards vehicles. choose parts, budgetize, then buy parts and start converting. 


//Steven


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## Tesseract (Sep 27, 2008)

steven4601 said:


> For all the DC conversions in the E.U.... Warning: It is hard to get EMC (R10) & electrical safety (R100) compliance with (large) DC traction motors. ...


You've been around here long enough to know better. Evnetics controllers from Rebbl have certificates of approval for on-road use, and so do Kostov motors.


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## EVEngineeer (Apr 11, 2012)

I do not know how much if at all of the wiki you have looked at, but this one is important to you at this moment. http://www.diyelectriccar.com/forums/showthread.php?t=73869


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## steven4601 (Nov 11, 2010)

The rebbl emc filter package for the soliton motorcontroller are a solution for reducing the EMI emission, what about the isolation / chassis leak with a DC motor? The easiest way to tackle this would be to use a brand new motor for the test. (as the motor is clean, free from carbon brush dust )


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## cpct (May 31, 2012)

spoland said:


> I'm really not sure about this, thats why I'm asking everyone here. To me there seems to be a debate about BMSes where some argues that this is a must have and some, Jack Rickards of EVTV for example, claims BMS'es are nonsence.
> I'm not sure of the precise arguments against BMS but i believe it has to do with the self balancing that occurs in packs of LiFEPOs anyway, the fact that a BMS turns off charging of a single cell based on the voltage over that cell whereas the charging state of the cell is much more complicated than that, and that it is better to undercharge your whole pack of batteries to prolong its life instead of trying to maximise energy content.
> I sure am no battery specialist, but from what I can read in different foras there is no consensus regarding BMS in the EV community and they do cost a bit.


The problem is that without a BMS, cell differences (not one cell is exatcly equal tot the other) lead to one cell discharging or charging faster than the others. After some 10s of cycles, the differences get bigger and bigger until that one cell is consistently overcharged/overvolted or undervolted.

It will work for a while though.


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## drgrieve (Apr 14, 2011)

cpct said:


> The problem is that without a BMS, cell differences (not one cell is exatcly equal tot the other) lead to one cell discharging or charging faster than the others. After some 10s of cycles, the differences get bigger and bigger until that one cell is consistently overcharged/overvolted or undervolted.
> 
> It will work for a while though.


Just to be clear we are talking about LiFeP04 batteries.

<Begin Rant>

Huh? What planet are your from? Endless Sphere?

There is no such problem. FYI many members on this forum don't use a BMS. They have no issues. There is no cell drift.

Cell drift is a lie.

Perhaps something for you to read up on.

PS Individual cell monitoring BMS's can themselves cause of cell drift. Remove them and the issue is no more.

<End Rant>


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## cpct (May 31, 2012)

drgrieve said:


> Just to be clear we are talking about LiFeP04 batteries.
> 
> <Begin Rant>
> 
> ...


Touchy subject it seems.

I cant say anything about the extent, but drifting principle is not unique to the chemistry. The internal resistance of the cells will never be exactly equal, neither will the capacity. So voltage will drift inherently. Maybe if you equalise the cells initially when assembling the pack the problem can be reduced, but it will not be gone.


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## spoland (Aug 9, 2012)

drgrieve said:


> ...
> In regards to parts since you are in Europe look at the kostov 9" 220v (or a kostov 10"). Match these up with a Soliton Jnr. Peak output should be around 130kW for the 220v in this combination. It's also a very light motor at 45 kgs.
> 
> BTW the warp 9 (with the new brushes) peak output is ~200kw when combined with a Soliton 1. Which is a little more than 25hp!
> ...


Thanks, I'll have a look at those.


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## spoland (Aug 9, 2012)

steven4601 said:


> For all the DC conversions in the E.U.... Warning: It is hard to get EMC (R10) & electrical safety (R100) compliance with (large) DC traction motors.
> If you dont use C.E. certified components with according EMC tests for automotive usage you'll require to do an EMC vehicle test. ~ 2000 to 5000 euro for 1 test. which may fail and you have to do another one.
> Testing can be more expensive than a battery pack.
> 
> ...


Thanks Steven!
I'll keep that in mind.
/ Peter


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## spoland (Aug 9, 2012)

cpct said:


> Touchy subject it seems.
> 
> I cant say anything about the extent, but drifting principle is not unique to the chemistry. The internal resistance of the cells will never be exactly equal, neither will the capacity. So voltage will drift inherently. Maybe if you equalise the cells initially when assembling the pack the problem can be reduced, but it will not be gone.


See what I meen about non consensus?


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## Tesseract (Sep 27, 2008)

steven4601 said:


> The rebbl emc filter package for the soliton motorcontroller are a solution for reducing the EMI emission, what about the isolation / chassis leak with a DC motor? The easiest way to tackle this would be to use a brand new motor for the test. (as the motor is clean, free from carbon brush dust )


The isolation requirement in ECR-R 100 is so easy to meet it hardly qualifies as an obstacle: 100 ohms per volt for DC circuits and 500 ohms per volt for AC circuits. Even the two WarP-9s in our dyno can still pass this test (and by at least an order of magnitude) despite 4 years of abuse.


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## drgrieve (Apr 14, 2011)

cpct said:


> Touchy subject it seems.
> 
> I cant say anything about the extent, but drifting principle is not unique to the chemistry. The internal resistance of the cells will never be exactly equal, neither will the capacity. So voltage will drift inherently. Maybe if you equalise the cells initially when assembling the pack the problem can be reduced, but it will not be gone.


Sorry for the rant just annoying when people post uninformed information as fact.

Please provide proof of cell drift where there is no BMS involved. 

Will wait while the crickets chirp.

Some batteries types do drift. Hence the confusion. LiFeP04 don't drift as there is no internal mechanism for it. The ions can only move when the terminals are connected.

So no cell drift and no cell discharge.


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## cpct (May 31, 2012)

drgrieve said:


> Sorry for the rant just annoying when people post uninformed information as fact.
> 
> Please provide proof of cell drift where there is no BMS involved.
> 
> ...


So I have to prove what you don't have to, haha. But ok, here is what I know.

First, maybe you mean something else then I do when talking about cell drifting. Of course, if the cells are disconnected, there is only some limited self discharge (in the order of a few percent of capacity per month from what I've gathered). 

However, when current is passed through them, the cells can be represented as an equivalent model of a resistor and voltage source. So for a complete pack we have chain of sources and resistors. If even one of the equivalent resistors is not the same as the others, the voltage drop over that cell will be different than for the other cells. This means that after a while, this cell will be critical (too low under load or too high voltage when charging).
As it happens, this equivalent resistance of a cell depends on a lot of possible factors, such as temperature, age, soc, manuf. process etc... Very hard to model. Anyway, no cell will thus behave exactly the same. Sooner or later, not properly equalising the cells in a Li pack will finish it.


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## Ziggythewiz (May 16, 2010)

He's right. No two cells are identical, and over time they will drift...by a couple milliAH. They may drift closer, they may drift further depending on the volts, amps, gravitational distortions etc, so after years of use the imbalance may reach .1 V or so.

Who cares??? Don't try to max your voltage and no worries.


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## cpct (May 31, 2012)

Ziggythewiz said:


> He's right. No two cells are identical, and over time they will drift...by a couple milliAH. They may drift closer, they may drift further depending on the volts, amps, gravitational distortions etc, so after years of use the imbalance may reach .1 V or so.
> 
> Who cares??? Don't try to max your voltage and no worries.


The problem is not that futile. Unless you manually take care of the individual cells regularly, weaker cells will get even more weak as you undervolt and overcharge them without knowing. Instead of 1000's of cycles we're talking about 100's of full cycles before you get a destroyed cell. In many use cases, this could be years for sure.

A BMS just automates what could essentially be done with a multimeter and lots of time.


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## Ziggythewiz (May 16, 2010)

cpct said:


> weaker cells will get even more weak as you undervolt and overcharge them without knowing.


Not gonna happen if you stay away from the ends. CALB recommends operating within 10-90% SOC. Do that and no one is getting under or overvolted.



cpct said:


> A BMS just automates what could essentially be done with a multimeter and lots of time.


Or two voltmeters and no time. All you're looking for is one cell heading up or down the curve ahead of everyone else. Not hard to detect.


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## steven4601 (Nov 11, 2010)

Ziggythewiz said:


> Not gonna happen if you stay away from the ends. CALB recommends operating within 10-90% SOC. Do that and no one is getting under or overvolted.
> 
> 
> 
> Or two voltmeters and no time. All you're looking for is one cell heading up or down the curve ahead of everyone else. Not hard to detect.


False assumption with the 10-90% concept. There's no way to tell all cells are equally in state of charge over time... EVen a BMS cannot be 100% certain if it had not been charged or discharge for prolonged period of time.


When the vehicle is charging, most owners would like to have a great time. Like sleeping or other 'activities' ... Watching the meters you mentioned is , possibly even sleep inducing invalidating your point you'd be able to protect the pack/vehicle/house/family during charging. 

Using a BMS is not inherently safer. It is a dilemma at best. Who trusts to connect a bit of electronics 24/7 to a battery pack capeable of delivering +100kw power? Its better had been designed by skilled designer who is aware of safety requirements, individually tested boards in series production, humisealed, hi-potted all modules to verify its design/safety complies to its specifications. Its not easy...

I can understand why people steer away from BMS'es and spell evil about them. But going without a (safe) BMS is also an accident waiting to happen. And worst is that it chances to go bad is when your away from the vehicle when its charging.

Hope my view on this helps, as I had the same struggle when ordering the BMS for my conversion.


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## spoland (Aug 9, 2012)

steven4601 said:


> False assumption with the 10-90% concept. There's no way to tell all cells are equally in state of charge over time... EVen a BMS cannot be 100% certain if it had not been charged or discharge for prolonged period of time.
> 
> 
> When the vehicle is charging, most owners would like to have a great time. Like sleeping or other 'activities' ... Watching the meters you mentioned is , possibly even sleep inducing invalidating your point you'd be able to protect the pack/vehicle/house/family during charging.
> ...


Thanks all of you for trying to help, but I'm not sure I'm getting any wiser by this. What I think would help me is a report from a scientific test showing how cell drift occurs without a BMS and how a BMS prevents cells from overcharge and overdischarge.
Or, a test showing that self balancing actually occurs when charging LiFePO4 batteries with a proper CI/CV charger and thereby argue that BMSes are not needed if keeping a safe DOD and not charging all cells to the max.
Maybe also scientific proof that a badly designed BMS can actually increase the risk of damaging the batteries as some claim. Or, proof that thay can not.


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## cpct (May 31, 2012)

spoland said:


> Thanks all of you for trying to help, but I'm not sure I'm getting any wiser by this. What I think would help me is a report from a scientific test showing how cell drift occurs without a BMS and how a BMS prevents cells from overcharge and overdischarge.
> Or, a test showing that self balancing actually occurs when charging LiFePO4 batteries with a proper CI/CV charger and thereby argue that BMSes are not needed if keeping a safe DOD and not charging all cells to the max.
> Maybe also scientific proof that a badly designed BMS can actually increase the risk of damaging the batteries as some claim. Or, proof that thay can not.


I don't think there exists detailed measurements concerning Li battery life with BMS etc, because they take a lot of time and Li battery technology still evolves too fast for that.

But this document could be helpful and has a section on safety of Lithium batteries.
http://www.reapsystems.co.uk/_publications/EMA2006final.pdf
True, it is +6 years old but the principles still apply.


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## drgrieve (Apr 14, 2011)

So no proof. No test reports? Just some mumbo jumbo.

And the cells do not discharge at all. There are detailed reports on cells sitting at the same static voltage for YEARS. Why is that? 

And there is a recent report on 500 cycles on headway cells on a test bench. Full discharge and charge. No cell drift in 500 cycles.

Many non BMS members on here. They have no issues with cells drift over the years. They have basically given up measuring for it. Nothing to report but everything still the same as it was last time.

Surely you can come up with one example that shows cell drift over time. Just one .... just make sure it's one without a BMS - since they can cause cell drift.


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## cpct (May 31, 2012)

drgrieve said:


> So no proof. No test reports? Just some mumbo jumbo.
> 
> And the cells do not discharge at all. There are detailed reports on cells sitting at the same static voltage for YEARS. Why is that?
> 
> ...


You seem to be very upset about this. The fact is that there is no serious company that uses large Lithium cells without some kind of monitoring or protection system.

Now regarding your point about drift. It is not clear what you mean by that, but I suppose you refer to cell voltage. The voltage of a battery cell is determined by its chemistry. Pretty logical, as it is determined by a redox potential. Only if they are charged/discharged to the max., will the voltage change considerably because of secondary effects. So no "drift" there.
But because of the equivalent internal resistance, it is the voltage during charging or discharging that tells something about the state of the battery. And that is why a BMS is useful because it will notify you and act before something bad happens. Considering the amounts of energy involved, I find that common sense.

You keep talking about proof but constantly refer to unknown many non BMS members to say that a BMS is never needed, while it is widely acknowledged in the industry that Lithium cells must be protected. There enough examples on the internet of cells gone "bad", e.g. the recall of Dell and Apple some time ago, so why risk it?


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## drgrieve (Apr 14, 2011)

spoland said:


> Thanks all of you for trying to help, but I'm not sure I'm getting any wiser by this. What I think would help me is a report from a scientific test showing how cell drift occurs without a BMS and how a BMS prevents cells from overcharge and overdischarge.
> Or, a test showing that self balancing actually occurs when charging LiFePO4 batteries with a proper CI/CV charger and thereby argue that BMSes are not needed if keeping a safe DOD and not charging all cells to the max.
> Maybe also scientific proof that a badly designed BMS can actually increase the risk of damaging the batteries as some claim. Or, proof that thay can not.


I wouldn't say that LiFeP04 cells self balance - I don't think they do that - nothing I've seen would say yes or no on that. So unless you can match your cells by AH then you need to choose between top balance or bottom balance (which is another flamebait topic).

To keep your pack safe, keep the pack from extreme states of DOD. 20% to 95% seems a fair target.

As I mentioned earlier you need a way to measure AH going in and out of the pack. A reliable charger that does CC CV and can be programmed for different target voltages and accurately turns off when C/20. 

Optionally the device counting AH in could turn off the charger, if the charger doesn't do so. Or you could use a timer as backup.

If you do some basic testing on the battery you choose, or find some reliable data (good luck!) you will work out how to charge to 95%. No charger or BMS can do that without the most important part of the equation - you.


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## drgrieve (Apr 14, 2011)

cpct said:


> You seem to be very upset about this. The fact is that there is no serious company that uses large Lithium cells without some kind of monitoring or protection system.
> 
> That is obvious. You have to monitor the pack. Not sure why you bring this up?
> 
> ...


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## cpct (May 31, 2012)

drgrieve said:


> cpct said:
> 
> 
> > You seem to be very upset about this. The fact is that there is no serious company that uses large Lithium cells without some kind of monitoring or protection system.
> ...


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## Ziggythewiz (May 16, 2010)

All lithium is not the same. Some cells vent when overcharged/discharged. Some explode. There are lithium design charts that show the tradeoffs between cycle life, energy density, and safety. OEMs go for energy density, because range sells cars. LiFePO4 delivers in cycle life and safety.


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## drgrieve (Apr 14, 2011)

cpct said:


> A BMS is a monitoring and protection system


I prefer to call them burn your house down and murder your battery system.

Forget to turn the charger off fire? Shunt and burn? Low voltage warning that means nothing ...

Shunt something that shouldn't be shunted because you don't understand that cell voltage when charging near the knee doesn't correlate to SOC. 



cpct said:


> A BMS can typically also monitor the current so it can give a good idea about SOC. Not sure what a rats nest means here?


Use a shunt. Much safer. Rats nest is what you look at coming from all the cells.



cpct said:


> That is not very hard, every battery will show a lower voltage when under load. Push it hard enough and it will heat up. For weak cells, this is easily achieved. If you monitor them, you'll know it the moment it happens.


Nice sounding theory. Excellent. Then I eagerly await your test results that clearly show that you get cell drift from just using your pack. Please give me a link to these amazing results from Santa. 

The crickets are still chirping waiting for 1 single piece of proof of any of your made up statements. 



cpct said:


> Well, the Tesla Roadster comes to mind. Anyway, they are all Lithium cells. Quality may be different but the failure mode is the same.


I thought we are talking DIY here? And FYI Tesla needs just a bit more than a cell level BMS (which they have none of). What they do have is a very complicated thermal management system for the pack.

It's a credit to them that none have gone up in flames. The newer batteries from Panasonic come with some impressive countermeasures built into the cell itself. Why? Because lithium cobalt is a bomb waiting to go off. LiFeP04 is the chemistry you use if you value your life. 

Why do you think I keep repeating LiFeP04 over and over - I'm trying to get you understand it matters greatly what you are using.


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## cpct (May 31, 2012)

drgrieve said:


> A prefer to call them burn your house down and murder your battery system.
> 
> Forget to turn the charger off fire? Shunt and burn? Low voltage warning that means nothing ...
> 
> ...


Could you just keep it a bit respectful?

Even if LiFePO4 is relatively safe, we're still talking batteries with massive energy contents. You may trust them, but as mentioned, there is a reason BMS are used so often for commercial applications.

You are asking for proof that not using a BMS is harmful for the batteries. Well I don't have numbers, only examples. The same applies to your viewpoint, some DIY'ers run their setups without and have not had problems. How does that prove anything?

You mentioned the obvious need to monitor the pack. If BMS would stand for Battery Monitoring System, then it would be fine? Come on...
If it's a bit professionally made, a BMS can only be benificial for the pack. Just like measuring your cells manually after every drive would be...


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## cpct (May 31, 2012)

Seems like I waited too long to refresh the topic.

In addition, some quick google results show the interest in balancing equipment for series battery packs in general and Lithium cells. They contain enough examples why it is a good thing.
Lithium-Ion continuous cell balancing overview


> . Balancing a battery pack will result in maximizing its capacity.
> . A balanced battery pack is less likely to expose cells to damage at the end of discharge and at the end of charge; thus, maximizing its life.
> . A balanced battery pack means that, with one single charge, the pack is always available to deliver its expected amount of energy - every time - all the time.
> . For Li-Ion, a balanced pack means that unbalanced cells are not continuously overcharged; thus, the safety hazards associated with continuous over charge - even slight continuous overcharge - are reduced.


Battery and Energy Technologies


> The potential failure rate is even worse than this however due to the possibility of interactions between the cells. Because of production tolerances, uneven temperature distribution and differences in the ageing characteristics of particular cells, it is possible that individual cells in a series chain could become overstressed leading to premature failure of the cell. During the charging cycle, if there is a degraded cell in the chain with a diminished capacity, there is a danger that once it has reached its full charge it will be subject to overcharging until the rest of the cells in the chain reach their full charge


Achieving cell balancing for lithium-ion batteries


> Over the life of the battery, the battery may be charged and discharged for hundreds or even thousands of cycles. As this occurs, the individual cells may age differently. Some cells may be
> come slightly (or more than slightly) mismatched with respect to the others. If this phenomenon is not corrected, one or more cells may become undercharged or overcharged...


Use Cell Balancing To Enable Large-Scale Li-ion Batteries


> To deliver a given wattage, high-series cell counts or high voltage are more efficient than a high parallel cell count, so high-series challenges are common. Also, out-of-balance cells can compromise pack reliability and cycle life. The pack will perform to the lowest-common-denominator cell, and cell imbalance will grow over multiple charge and discharge cycles. Self discharge will add to the problem.


Cell Balancing Maximizes The Capacity Of Multi-Cell Li-Ion Battery Packs


> While common, these configurations are not as efficient as they could be, because any capacity mismatch between series-connected cells reduces the overall pack capacity. Cell balancing techniques increase the capacity, and system operating time.


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## spoland (Aug 9, 2012)

cpct said:


> Seems like I waited too long to refresh the topic.
> 
> In addition, some quick google results show the interest in balancing equipment for series battery packs in general and Lithium cells. They contain enough examples why it is a good thing.
> Lithium-Ion continuous cell balancing overview
> ...


Thanks for all the references cpct!
As far as I can read and understand, none of them present any scientific evidence that cell drift occurs in LiFePo4 cells. Cell drift is a fact that they are all based upon, and they do not reference any articles that proves so.
One of the articles (Lithium-Ion continuous cell balancing overview) could be interpreted as though self balancing occurs during discharge since the voltage differences between cells is smaller after discharge than before the discharge.
I'm not sure if I am a sloppy reader, but none of them convinced me. Just to be clear, I'm not convinced of the opposite either!


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## cpct (May 31, 2012)

spoland said:


> Thanks for all the references cpct!
> As far as I can read and understand, none of them present any scientific evidence that cell drift occurs in LiFePo4 cells. Cell drift is a fact that they are all based upon, and they do not reference any articles that proves so.
> One of the articles (Lithium-Ion continuous cell balancing overview) could be interpreted as though self balancing occurs during discharge since the voltage differences between cells is smaller after discharge than before the discharge.
> I'm not sure if I am a sloppy reader, but none of them convinced me. Just to be clear, I'm not convinced of the opposite either!


Yes you are right, the links all state it as a fact. But the point I wanted to make was that balancing of Lithium strings is accepted practice for some time.
Edit: I read the article you mention again. As I understand it, they balance during charging and discharging with their own system, which, if they are to be believed, would perform better than only balancing during charging.
Also, I'm unsure how the process for self-balancing of series cells would work. The current passes through all the cells so an individual cell with a lower voltage than the rest of the pack has no way to "participate" less so to speak. Hmmm still thinking about it...

I had a look around and there are some more "scientific" articles available, but as expected most of them are behind a paywell. I'm not sure I should just copy the contents here because they seem to be fingerprinted when I download them. Maybe some of you also have an IEEE account?

These show life cycle behaviour of Lithium cells, including the degradation of the internal resistance. 
Cycling performance of low-cost lithium ion batteries with natural graphite and LiFePO4
Internal Resistance Identification in Vehicle Power Lithium-Ion Battery and Application in Lifetime Evaluation

I found also one reference specifically testing a string with and without equalisation:
Fundamental characteristics of stationary lithium-ion secondary cells and a cell-voltage-management system
It is however focused on stationary applications (with trickle chargers).

The one below has data on inter cell variance. They had one type of iron phosphate cell with a 2.2% deviation with the remark that a BMS would be "more than required".
Evaluation of performance characteristics of various lithium-ion batteries for use in BEV application
The paper left a bit of a messy impression though.

This one was also pretty interesting
The safety characteristics of lithium-ion batteries for mobile phones and the nail penetration test


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## Ziggythewiz (May 16, 2010)

spoland said:


> I'm not sure if I am a sloppy reader, but none of them convinced me. Just to be clear, I'm not convinced of the opposite either!


I'm just glad to see we haven't driven you off by hijacking your thread for a BMS Holy War. As you can see there are firmly held beliefs on each side. There are those that consider a BMS to be essential while many of us think they are a large expense and add unnecessary complexity with the potential to create every hazard they are meant to prevent.

You'll have to decide for yourself where you sit and what is best for you.

Here's a thread with some BMS free experience: http://www.diyelectriccar.com/forums/showthread.php?t=63154


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## drgrieve (Apr 14, 2011)

cpct said:


> Yes you are right, the links all state it as a fact. But the point I wanted to make was that balancing of Lithium strings is accepted practice for some time.
> 
> I had a look around and there are some more "scientific" articles available, but as expected most of them are behind a paywell. I'm not sure I should just copy the contents here because they seem to be fingerprinted when I download them. Maybe some of you also have an IEEE account?


Thank you for the links.

Yes this is the source of the problem - many folks stating as fact but nothing backing it up. The repetition of a lie or perhaps confusion becomes fact it would seem.

I've searched and haven't found a single life cycle test on a pack that shows that charging a pack of cells and discharging them causes drift.

All tests I've seen indicate no cell drift and no cell self-discharge.

I conclude that a cell level BMS gives your zero benefit (that could be otherwise done in a different cheaper and safer manner) and can only introduce safety issues, battery damage (for example fire or causing cells to drift - this is very common design fault.), and unneeded expense and worst of all cause the owner to believe in something that really shouldn't be trusted and instead they should learn some cell management basics.

Apologies for the attitude - you have been quite reasonable.


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## spoland (Aug 9, 2012)

New ideas,

It seems like the DC track maybe isn't that bad. At least a lot of people go that way and it looks like you get more performance and easier setup for less money. I also found resellers in Europe for Kostov motors and Soliton controllers.
New config:
Motor: Kostov 9" 220V. Really small and light (45kg), yet can give quite enough performace for the small A2 (33kW sustained, 78 kW peak)
Controller: Soliton Jr
Adapter plate: CEV New Beetle
Battery: 72 100 Ah LiFePO4s giving pack voltage 230V and energy content 23 kWh. I hope to put 52 of them where the gas tank and muffler were and 20 in the very front. But, I'll see what space gets available whe I start stripping the car (I have to buy one first also ;-)
Power steering: Stock Audi A2 electro hydraulic
A/C: Will problaby disconnect and not use. It's just needed a couple of days per year anyway 
Heater: Much more important! Twin thermostat controlled, 2 kW, 230V engine coolant heaters from Calix + 12V circulation pump (or MES DEA or similar). The plan is to let the ACC temp flap motor pot control on/off of the heaters which sets at 60-80C water temperature.
BMS: Still not sure about the BMS, I'll definitely need someway of monitoring the cells, maybe measuring half pack voltage difference or something and do manual balancing
Instrumentation: Metric Mind EVision looks nice, it can also control the stock RPM gauge and has half pack voltage monitoring
Charger: Maybe Elcon, still haven't found a way to control the charge curve with these. (other than sending it to the manufacturer)
Vacuum pump: TBD. Maybe a used Audi/VW which I read some people use. Don't know which one though
DC-DC: Meanwell, MES, EV Source, EVolve? I will probaby measure the power reuqirements before deciding

Thoughts?

/ Peter


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## spoland (Aug 9, 2012)

Conversion started!
I started a new thread for it.
http://www.diyelectriccar.com/forums/showthread.php/audi-a2-2004-conversion-80415.html


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