Planning Audi A2 Conversion - Page 4 - DIY Electric Car Forums
Go Back  

DIY Electric Car Forums > EV Conversions and Builds > All EV Conversions and Builds

Register Blogs FAQ Members List Social Groups Calendar Search Today's Posts Mark Forums Read

Reply
 
Thread Tools Display Modes
  #31  
Old 08-14-2012, 08:19 AM
Ziggythewiz's Avatar
Ziggythewiz Ziggythewiz is offline
Senior Member
 
Join Date: May 2010
Location: Houston
Posts: 3,973
Ziggythewiz will become famous soon enough
Default Re: Planning Audi A2 Conversion

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.
__________________
1 EV - 11,350 e-miles in 2.5 years (7/25/13) - http://www.evalbum.com/4000
Reply With Quote
Sponsored Links
Advertisement
 
  #32  
Old 08-15-2012, 05:36 AM
drgrieve drgrieve is offline
Senior Member
 
Join Date: Apr 2011
Location: Sydney, Australia
Posts: 286
drgrieve is on a distinguished road
Default Re: Planning Audi A2 Conversion

Quote:
Originally Posted by cpct View Post
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.

Quote:
Originally Posted by cpct View Post
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.

Quote:
Originally Posted by cpct View Post
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.

Quote:
Originally Posted by cpct View Post
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.

Last edited by drgrieve; 08-15-2012 at 05:50 AM.
Reply With Quote
  #33  
Old 08-15-2012, 06:29 AM
cpct cpct is offline
Member
 
Join Date: May 2012
Posts: 55
cpct is an unknown quantity at this point
Default Re: Planning Audi A2 Conversion

Quote:
Originally Posted by drgrieve View Post
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 ...

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.



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



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.

I think it's time you show some proof backing up your crap or you should STFU!



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.
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...
Reply With Quote
  #34  
Old 08-15-2012, 06:41 AM
cpct cpct is offline
Member
 
Join Date: May 2012
Posts: 55
cpct is an unknown quantity at this point
Default Re: Planning Audi A2 Conversion

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
Quote:
. 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
Quote:
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
Quote:
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
Quote:
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
Quote:
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.

Last edited by cpct; 08-15-2012 at 06:51 AM. Reason: Added quotes
Reply With Quote
  #35  
Old 08-15-2012, 10:36 AM
spoland spoland is offline
Member
 
Join Date: Aug 2012
Location: Sweden
Posts: 47
spoland is on a distinguished road
Default Re: Planning Audi A2 Conversion

Quote:
Originally Posted by cpct View Post
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

Battery and Energy Technologies
Achieving cell balancing for lithium-ion batteries
Use Cell Balancing To Enable Large-Scale Li-ion Batteries
Cell Balancing Maximizes The Capacity Of Multi-Cell Li-Ion Battery Packs
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!

Last edited by spoland; 08-15-2012 at 10:39 AM. Reason: Inserted reference to the mentioned article
Reply With Quote
  #36  
Old 08-15-2012, 12:12 PM
cpct cpct is offline
Member
 
Join Date: May 2012
Posts: 55
cpct is an unknown quantity at this point
Default Re: Planning Audi A2 Conversion

Quote:
Originally Posted by spoland View Post
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

Last edited by cpct; 08-15-2012 at 12:36 PM.
Reply With Quote
  #37  
Old 08-15-2012, 12:42 PM
Ziggythewiz's Avatar
Ziggythewiz Ziggythewiz is offline
Senior Member
 
Join Date: May 2010
Location: Houston
Posts: 3,973
Ziggythewiz will become famous soon enough
Default Re: Planning Audi A2 Conversion

Quote:
Originally Posted by spoland View Post
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...ad.php?t=63154
__________________
1 EV - 11,350 e-miles in 2.5 years (7/25/13) - http://www.evalbum.com/4000
Reply With Quote
  #38  
Old 08-18-2012, 07:23 AM
drgrieve drgrieve is offline
Senior Member
 
Join Date: Apr 2011
Location: Sydney, Australia
Posts: 286
drgrieve is on a distinguished road
Default Re: Planning Audi A2 Conversion

Quote:
Originally Posted by cpct View Post
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.
Reply With Quote
  #39  
Old 09-10-2012, 02:22 PM
spoland spoland is offline
Member
 
Join Date: Aug 2012
Location: Sweden
Posts: 47
spoland is on a distinguished road
Default Re: Planning Audi A2 Conversion

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
Reply With Quote
  #40  
Old 10-31-2012, 10:52 AM
spoland spoland is offline
Member
 
Join Date: Aug 2012
Location: Sweden
Posts: 47
spoland is on a distinguished road
Default Re: Planning Audi A2 Conversion

Conversion started!
I started a new thread for it.
http://www.diyelectriccar.com/forums...ion-80415.html
Reply With Quote
Reply

Share or Bookmark this

Tags
"audi a2", audi, hpevs, winston

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

 
Support DIY Electric Car
Sponsors

All times are GMT -6. The time now is 06:20 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.

Ad Management by RedTyger