# A123 20ah cells, testing the connection method



## rwaudio (May 22, 2008)

Some of you may have noticed I decided to go with A123's in my 944 conversion. I don't need a lot of range, but I am hoping for quite a bit of power so these cells meet both criteria. It's also hard not to borrow some of Crodrivers findings and he likes these cells so why not. Especially since it appears that he's using them in his Concept_One electric supercar.



















I'm sure one of his battery modules cost as much or more than my entire conversion including the car, but it is an elegant package and will put my foam insulated boxes to shame.
Visit Rimac Automobili for more info on his amazing creation.


These aren't "new with warranty / cost a small fortune cells" I took a gamble on some grey market cells and these appear to be cells that were rejected and destined for recycling. All of the ones I've tested have good IR (1 - 1.5mohm), but only come in at 18 - 18.25Ah. Because of this the cells were castrated... well had their tabs removed.

There is plenty of tab material still covered by the plastic casing, it's just a pain in the ass to uncover and make useful without going to far and puncturing the cell. Below is a cell that is "half" done, the casing material has been removed but the remaining adhesive needs to be scraped off to reveal the aluminium and copper tabs.










With some of Yabert's help on connection method ideas I settled on a simple sandwich type connection with block - tab - block - repeat. There are 4 aluminium blocks for each end terminal and 7 blocks for all of the main series connections. 










As you can see from Yabert's rendering above that there will be a lot of different blocks to connect these cells. Initially he had a line on a company that would laser cut the green parts for a decent price, but I would still have to make all of the red and blue blocks, then due to limitations on available material thickness to be laser cut it just makes more sense to make ALL of the pieces myself. Below are a few pieces from the first batch of parts used for assembly.










Each block goes through many steps before it's done, first the material is cut to length, then marker holes are drilled using the CNC router, then the marker holes are fully drilled on the drill press, along with some parts that get a countersink, some threaded etc. Each 16S3P module has 65 of the "green" blocks above, 15 "blue" blocks, 16 "red" blocks and 2 large terminals shown below.
That's a total of 98 blocks per module or 588 blocks for the 6 modules in the final design. The pink foam spacer in the center of the pack is just what I had on hand, I have 0.06" plastic sheet that will be cut to size for the actual pack assembly. During assembly each side of each cell tab is cleaned with a stainless steel wire brush, along with the aluminium blocks and treated with No-Alox, this is another time consuming process but the initial results are worth it. The battery shown below is only 2S3P for testing, I will be assembling them into 16S3P modules for the car.










Above charging with my dc/dc converter charger, it's simple and reliable but 3 parallel cells are too much for this method without some current limiting. During the charging process the dc/dc converters overheated and shut down, they do turn back on automatically when they cool off though, but I'm sure doing this too often would shorten their life span. The cells charged up perfectly to 3.5v per cell though. (Please ignore the lack of proper terminals, I have terminals for the high current connections, but I neglected to buy small terminals for this purpose)










The cell tabs first need to be uncovered, this is time consuming surgery and must be done carefully or risk damaging the cell. Then two holes are drilled in each tab to allow the screws to pass through that hold the blocks together. I created a jig on the CNC router out of MDF that allows me to align and clamp each cell and drill the 4 holes with a built in guides.



















The jig is simple and effective allowing me to quickly and precisely drill the 4 holes I need. If I were to do it again I would probably find/make metal sleeves to line the pilot holes to keep the holes tight with repeated drilling.










I wanted to do some high current discharge testing to make sure that the connections are sound and don't heat up at high currents. I was only able to test this pack to about 700A but all of the terminals and the cells themselves were cold and the pack only sagged to about 3.1xV per cell (measured ~6.25V under load after 5 or 6 seconds when stuff started to melt and smoke). I can't say the same for the load, I wasn't exactly setup for 700A discharges and melted the insulation off some 4 awg wire and made my resistor bank hotter than I ever have before. OOOPS! The cells perform exceptionally well though in all tests I've done to date.

My final comment is it's a good thing these cells perform otherwise all of this work to prepare and connect them just wouldn't be worth it. I can't wait for the day that a large prismatic has the same or better performance to these A123 pouches but already has 2 big screw terminals at the top.

Okay, ONE more comment, I'm sure you are wondering "what did these cells cost!?!?!?! Around $1.25 per Ah assuming 18Ah per cell.


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## dougingraham (Jul 26, 2011)

Nice work all around!

So each module will have a nominal 51.2V and 54AH rating. That would be 2.76kwh. How many modules are you planning to series together?


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## Tesseract (Sep 27, 2008)

Ingenious. Completely impractical, but ingenious nonetheless!

You might want to measure the tab-to-tab voltage drop at a constant current load well within the capability of your test setup (e.g. - 100A) to make sure the interconnection resistance is consistent. In particular, I would be especially concerned about using aluminum blocks on the copper tabs as the difference in electronegativity between those two metals is more than enough to result in rapid corrosion of the aluminum:

http://www.corrosionist.com/Aluminum_ Galvanic_Corrosion.html

Copper (or brass) blocks for the copper tabs would be a safer choice, but if it's going to take you a while to put these modules in your car then it might be instructive to test the tab-to-tab voltage drop (after bringing them back up to full charge) every few days to a week to see if it goes up. 

Note, you can't use protective dielectric grease like NoAlOx on the blocks to prevent corrosion as there won't be enough clamping force to fully expel the grease from the joint (it's called "dielectric" grease for a reason, you know  ).


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## Yabert (Feb 7, 2010)

Wow! Rw. Impressive build. Nice work.

I hope you take the time to remove aluminum oxidation on both face of the small block before mounting.





rwaudio said:


> I can't wait for the day that a large prismatic has the same or better performance to these A123 pouches but already has 2 big screw terminals at the top.


The real problem isn't the performance of the current prismatic cells(8-10C is enough). 
The problem is a 50 Kwh pack weight more than 1000 lbs and take a lot of space.
When a 50 Kwh battery pack will weight 350 lbs and take few cubic feet by volume, the life will be easier..


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## rwaudio (May 22, 2008)

dougingraham said:


> Nice work all around!
> 
> So each module will have a nominal 51.2V and 54AH rating. That would be 2.76kwh. How many modules are you planning to series together?


I have enough cells to build 5 modules, but 6 modules is the design. Which is enough range and a little over 300lbs



Tesseract said:


> Ingenious. Completely impractical, but ingenious nonetheless!
> 
> You might want to measure the tab-to-tab voltage drop at a constant current load well within the capability of your test setup (e.g. - 100A) to make sure the interconnection resistance is consistent. In particular, I would be especially concerned about using aluminum blocks on the copper tabs as the difference in electronegativity between those two metals is more than enough to result in rapid corrosion of the aluminum:
> 
> ...


HAHA, that's the type of comment I love, and completely impractical is right! The problem is in series groups I have to connect an aluminium tab to a copper tab, so unless I use some fancy plated parts I'm connecting an aluminium tab to copper bars, or a copper tab to aluminium bars. 

It looks like tin plating would be half way in between, would it be "safer" if the copper tab was tin plated for example? I have tin plated some copper traces on prototype PCB's before and although it would be a slow painful process I could probably plate the copper tab. (there aren't enough slow painful processes in this battery assembly yet)

I will take your advice and test the connection resistance over repeated charge/discharge cycles though, I can do safe continuous testing to about 200A.


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## rwaudio (May 22, 2008)

Yabert said:


> Wow! Rw. Impressive build. Nice work.
> 
> I hope you take the time to remove aluminum oxidation on both face of the small block before mounting.


I have been insanely picky about this step, I remove the oxidation from the cell tabs and all connecting surfaces of the blocks.

Thanks for your help with the initial design.


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## steven4601 (Nov 11, 2010)

About the oxidation and aluminium, you will need something to keep the air and moisture out. Contact-grease or anti corrosion grease would be a good idea. Even some high temperature rated 'lithium based'  bearing-greases are suitable for this.


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## Salty9 (Jul 13, 2009)

Have you investigated using sheet copper (such as copper flashing)? It could be bent around the battery tabs and inert spacers inserted between the copper folds.


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## Tesseract (Sep 27, 2008)

steven4601 said:


> About the oxidation and aluminium, you will need something to keep the air and moisture out. Contact-grease or anti corrosion grease would be a good idea. Even some high temperature rated 'lithium based'  bearing-greases are suitable for this.


No, you cannot use grease here - it isn't possible to generate enough clamping force to expel the grease to allow a good metal-to-metal interface. Greases like NoAlOx are only appropriate for connections which utilize high compression force.

Plating the copper with a metal that is in between copper and aluminum on the galvanic series is really the only thing that can be done here.


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## MalcolmB (Jun 10, 2008)

How about using lead foil between the Cu and Al? A quick Google brings up several sources.


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## rwaudio (May 22, 2008)

Everything I read on galvanic corrosion says it needs water/salt water or some other external substance to speed up the process. It's natural progression is supposed to be very slow. The other thing is that unless I am mistaken every LiFePO4 prismatic cell has one aluminum terminal but most people are using copper bars for connection. If this was a quick process all sorts of lithium conversions would be having problems by now.

Also with the grease idea, I think Stevens idea is to cover the completed connections to keep out moisture out, not to include it before/during assembly.

On a side note, Tess is 100% correct, there isn't enough clamping force to get rid of the grease on such a large flat connection. I had minimal No-alox left on the connections before clamping but I had less voltage drop on clean/dry connections that were simply prepared with a wire brush. 

I have increased the test pack to 4S and powered an 800W heater (resistor bank) for almost an hour. I have to say these cells behave much nicer in a 3P configuration and subtle cell differences in capacity etc are slower to appear and are much easier to catch. (No wonder people like 160-200ah cells, I would imagine they behave very well in a series pack even without a BMS) This goes for both charging and discharging. I'm going to take the pack appart and remove all of the no-alox and reassemble. The packs will be in a clean/dry area of the car, all housed in a box below the floor of the hatch and accessible through the hatch floor.

I had hoped to not test each cell, but in the end it might be a worthwhile process, in my 4S3P groupings I had one group fully discharge (to 2.4v) at around 53.5Ah while another group didn't drop below 2.95v so that group is probably closer to 56 or 57Ah. The murphy's law say I'm going to group the 3 lowest capacity cells together and that will limit the rest of my pack, but if spread out I could probably maintain a 54Ah average. Lots to think about.....


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## LithiumaniacsEVRacing (Oct 9, 2010)

Very nice work, I will be starting a new thread showing my design for the Haiyin cell tab connections.


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## LithiumaniacsEVRacing (Oct 9, 2010)

Your pack build is similar to mine. I have a few questions if you don't mind answering? Are you placing double stick tape between each cell? Are you using fiberglass sheets between each parallel pack? Are you wrapping the cell packs in wrap to prevent chaffing in the battery box?

Thank you.



rwaudio said:


> Some of you may have noticed I decided to go with A123's in my 944 conversion. I don't need a lot of range, but I am hoping for quite a bit of power so these cells meet both criteria. It's also hard not to borrow some of Crodrivers findings and he likes these cells so why not. Especially since it appears that he's using them in his Concept_One electric supercar.
> 
> 
> 
> ...


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## Duncan (Dec 8, 2008)

Hi Rw

Have you thought about applying a copper coat to your aluminium bars 
(bloody spell checker keeps trying to change it to aluminum - even i know that not how you spell aluminium)

I think its a chemical process - you would have to look up the chemicals - but it should help all of the issues


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## rwaudio (May 22, 2008)

LithiumaniacsEVRacing said:


> Your pack build is similar to mine. I have a few questions if you don't mind answering? Are you placing double stick tape between each cell? Are you using fiberglass sheets between each parallel pack? Are you wrapping the cell packs in wrap to prevent chaffing in the battery box?
> 
> Thank you.


Hi Ron,

Yes I will be using double sided tape on the cells and the insulating spacer. This initial test pack is just stacked because I knew I would have to take it apart. I am using a plastic spacer between cell groups, partly because it's easy to cut on my CNC router and cheap and has a fairly high melting point, if the plastic melts I have bigger problems with boiling electrolite. I hadn't actually thought of fibreglass which is probably a much better solution for a drag car. I will be banding on some end plates that will give me mounting points, but the pack will be in a foam insulated aluminium box. Pack heating will be a big thing for me.

What do you plan to wrap the pack in? Some sort of tape or?
What are the tabs made of in your cells? Do you have the copper/aluminium issue?



Duncan said:


> Hi Rw
> 
> Have you thought about applying a copper coat to your aluminium bars
> (bloody spell checker keeps trying to change it to aluminum - even i know that not how you spell aluminium)
> ...


I had not thought about a copper coat, but once I have 5 cell packs in the car (I have to buy more cells for the 6th group) I might try copper connecting bars instead and compare voltage drop to the aluminium packs. It would give me an excuse to take the packs out one at a time and test the capacity and IR of each cell and group them together to get the most out of the pack. Then I wouldn't have time constraints of trying to get the car done, and running on 5/6 of the pack would still give me plenty of range and power. At that time I would visit the idea of having plated/coated copper/aluminium bars made to connect to the aluminium or copper tabs. Given the elapsed time and charge/discharge cycles I would also be able to evaluate changes in voltage drop in a cell group etc.

Keep the idea's coming though, once I get the pack sorted out (and I don't expect that to happen on the first assembly) I expect it to run trouble free after that for many many years.


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## Salty9 (Jul 13, 2009)

Pure aluminum is a very reactive metal (check the thermite reaction) rendered inert by the surface oxidation coating. I found this site : http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JESOAN00015600001200D564000001&idtype=cvips&gifs=yes&ref=no that immersion plates copper on Al using a laser to disrupt the surface coating. It might be worthwhile to see if mechanical abrasion of Al submersed in copper sulfate solution would produce a copper coating.

Edit: I learned of immersion plating in helping a trucker by cleaning his battery terminals and leads in a snowstorm with my pocketknife. When I wiped the blade I saw that it was copper colored.


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## Lopezjm2001 (Nov 22, 2011)

Hi rwaudio,

I have just started putting together a 230V 20AH battery pack for my PHEV Prius Gen2. I have been using the same A123 20AH pouch cells without tabs and I just happened to be lucky enough to find the right tool for the job from my workplace. It is a little pneumatic handheld linisher. I can do one cell in a matter of minutes.


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## Lopezjm2001 (Nov 22, 2011)

You can get more detail on my A123 tabless cell tab exposure method on Priuschat:

http://priuschat.com/forums/prius-p...planing-canview-bms-conversion-lithium-7.html


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## IamIan (Mar 29, 2009)

rwaudio said:


> Everything I read on galvanic corrosion says it needs water/salt water or some other external substance to speed up the process. It's natural progression is supposed to be very slow.


Just a few thoughts ...

Any type of battery including Galvanic cells needs an electrolyte ... although water and or salt water are common fairly good environmental electrolytes , there are others ... the worse the electrolyte and the less of it the less galvanic corrosion will occur ... just keeping it dry will be a major reduction ( dry air usually makes a very poor electrolyte on its own ).

Sense the Galvanic cell has a specific polarity ... and so does the actual battery ... if the voltage and current flow are setup opposite the galvanic potential , it will also reduce the effect ... although it will also reduce battery pack voltage a few tenths of a volt or so ( depending on alloys ) per Galvanic cell... or I guess one could intentionally set it up the other way , so the Galvanic current is contributing to the total Battery Voltage... the the second case would not slow the galvanic corrosion... where the first case would.

Most Aluminum Alloys have about ~2,000 Ah/kg worth of galvanic potential as an Anode ... if you are able to measure the small amounts of galvanic current you are getting you should be able to estimate the rate of corrosion , and the potential service life of the connection point.

If you know the connection points or packs initial tested electrical resistance ... you can use future pack resistance measurements to get an idea of the amount of corrosion you might have ... corrosion at the contacts will increase the resistance of that connection... general measurement won't distinguish between battery changes in resistance and contact changes in resistance ... but as long as it is within a reasonable margin from the initial condition , it is probably a non issue.


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## rwaudio (May 22, 2008)

IamIan said:


> Just a few thoughts ...
> 
> Any type of battery including Galvanic cells needs an electrolyte ... although water and or salt water are common fairly good environmental electrolytes , there are others ... the worse the electrolyte and the less of it the less galvanic corrosion will occur ... just keeping it dry will be a major reduction ( dry air usually makes a very poor electrolyte on its own ).
> 
> ...


That's a lot of good info, and goes along with what I've read that cool/dry air won't accelerate the process to the same extent other things would. I believe I have the galvanic potential going in both directions, since the more positive and more negative side of each dissimilar metal connection would be the same. I will be watching for change in resistance over time, but I'm not sure if that would come out in overall pack sag at a certain current draw, or changes in individual cell voltage at high current draw. I will be using cell log 8's initially and you can program in alarm warnings triggered by cell voltage differences, IE a certain delta V between the highest/lowest cell in the group of 8. I will have to see how my cells do first though since I will be top balancing with the dc/dc converter charger I could have an unavoidable voltage difference towards lower SOC.


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## IamIan (Mar 29, 2009)

rwaudio said:


> I will be watching for change in resistance over time, but I'm not sure if that would come out in overall pack sag at a certain current draw, or changes in individual cell voltage at high current draw.


A few thoughts.

The corrosion should be more detectable over more connections ( the whole battery pack )... sense the connections look reasonably uniform ... but the cell resistance would also show up in the same measurement .. I'm not sure how you would be able to distinguish them ... although you might not care much what the source of the resistance is if it reaches some point you don't like.

Once the whole pack is built up I would recommend plotting an initial base line of several of the pack functional characteristics ... which you could then later compare annually or every now and then to watch how the pack is aging over it service life ... some things are obvious ... Wh , Ah ... but if you also do an initial base line test to determine the dV of the whole pack with all the connections at some known SoC and some known Amp rate you could repeat that dV test in the future to quantify the whole packs change in resistance ... doesn't matter as much what the specific SoC and Amp rate are as it is that you know what they are so you can consistently repeat the test at a later point ... this method would not differentiate between cell resistance and connection resistance... but might be enough of a total resistance feedback to be useful.

In the alternative ... if you measured the initial resistance of a small representative sample of a few of the connection points you could then latter measure them again to more directly track the connection point resistance over time / use... should be able to be tested without taking things apart... you should also be able to measure the galvanic Voltage of the connections , the lower the voltage the slower / less galvanic corrosion you will have... if it is a low enough galvanic voltage it might be slow enough to be considered insignificant ( compared to other vehicle eventual service needs )... by knowing the connection resistance and the Galvanic voltage you also know the galvanic current flow , which could also be used to estimate how if it will be slow enough to be a non-issue.


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## LithiumaniacsEVRacing (Oct 9, 2010)




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## LithiumaniacsEVRacing (Oct 9, 2010)

Just some recommendations, do not use an aluminum box, any crack in the foam will cause a short. Lexan is very very strong and your best option. Separate each parallel set with very thin G10 fiberglass, this can save your whole pack incase of a fire. 

I do have one tab that is aluminum, but I decided on copper connectors to be error free. When in doubt use copper, it will never fail. Each of my parallel packs are wrapped with basic duck tape. The double stick tape I use is sold at Lowes (2" wide Shuretape Brand) and the duck tape is (2" wide Scotch Brand), worked well with the first pack. 



rwaudio said:


> Hi Ron,
> 
> Yes I will be using double sided tape on the cells and the insulating spacer. This initial test pack is just stacked because I knew I would have to take it apart. I am using a plastic spacer between cell groups, partly because it's easy to cut on my CNC router and cheap and has a fairly high melting point, if the plastic melts I have bigger problems with boiling electrolite. I hadn't actually thought of fibreglass which is probably a much better solution for a drag car. I will be banding on some end plates that will give me mounting points, but the pack will be in a foam insulated aluminium box. Pack heating will be a big thing for me.
> 
> ...


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## LithiumaniacsEVRacing (Oct 9, 2010)

Using a block reduces cell tab heat, better that welding tabs. When building high amperage packs cooling the tabs is a must!



LithiumaniacsEVRacing said:


>


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## rwaudio (May 22, 2008)

LithiumaniacsEVRacing said:


>


I'm glad to see your design is coming along Ron, the "holy" block is interesting, just make sure you have enough material remaining to conduct 3000A (or is it 6000A?) without adding to the voltage sag. 

I'll stick with aluminium for now and monitor any change in resistance in my "control block" in theory any reaction should be slow, in practice who the hell knows.

I did finally give in and setup an automatic single cell charger and discharger so that I can measure the capacity of every single cell and group them accordingly.

The discharger is simple... EV200 contactor controlled by the relay outputs of one of these: http://www.ebay.com/itm/AC-DC-1V-500V-Range-Programmable-Voltmeter-Volt-/200682026483?pt=LH_DefaultDomain_0&hash=item2eb994b5f3#ht_3516wt_1396 turning on a resistive load for any terminal voltage above 3v and turning it off at 2.25v it's not a CC load, but it doesn't matter I'm not monitoring the discharge, it draws 100A from a full cell tapering a bit as the cell discharges.

When I hear the contactor click off I grab another cell and hook it up and move the "dead" cell to the charging pile, charge under controlled conditions and write the capacity on the cell.

I haven't decided if I want to top or bottom balance the pack to start (once the dc/dc charger is done it will always be top balanced) but before I get there I have a choice. Either way is easy, charging the single cells takes them to the same place at the top, or I could run them through the discharger one more time for what I believe would be pretty consistent bottom balancing.


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## LithiumaniacsEVRacing (Oct 9, 2010)

Thanks, that is why we are drilling small holes into the sides and not the mating area. The amount of blocks needed to complete my pack creates too much additional weight, by removing some copper I will save about 75lbs in total. Good luck with your pack, keep us updated.



rwaudio said:


> I'm glad to see your design is coming along Ron, the "holy" block is interesting, just make sure you have enough material remaining to conduct 3000A (or is it 6000A?) without adding to the voltage sag.
> 
> I'll stick with aluminium for now and monitor any change in resistance in my "control block" in theory any reaction should be slow, in practice who the hell knows.
> 
> ...


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## drgrieve (Apr 14, 2011)

I've seen A123 grey cells with tabs selling for ~$24 a piece. If you could sell the module pieces to put a EV pack together with tabbed cells how much would that add to each cell price? 

For example 4p80s split over 2 or 3 boxes.

Just thinking that there might be a market for this that you could supply.


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## rwaudio (May 22, 2008)

drgrieve said:


> I've seen A123 grey cells with tabs selling for ~$24 a piece. If you could sell the module pieces to put a EV pack together with tabbed cells how much would that add to each cell price?
> 
> For example 4p80s split over 2 or 3 boxes.
> 
> Just thinking that there might be a market for this that you could supply.


The cost of aluminium is minimal, copper would be higher but not too crazy however most of the cost would be in the machining time required. I'm going to get a few of the normal tabbed cells and do some playing. If it works well perhaps I'd consider selling blocks/screws/spacers.

My setup is tweaked for 3P though, and the screw length has to be exact to work well. Too short and you don't get enough threads in a soft aluminium/copper block, too long and you mess with the spacing and/or cut through the insulator and short some cells out.


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## DIYguy (Sep 18, 2008)

Just wondering why no one has considered silver-soldering (brazing) the tabs. I mean the RC packs are all soldered inside. No worries about corrosion or deterioration or fasteners. Also lighter pack. On the down side is removing a cell. 
I think the solder connection can be improved by adding silver also. Silver solder is much stronger, particularly with gaps and a higher melting temp. 
I believe the A123 packs have tabs with a connector that are laser welded.

RWaudio and I discussed this off line a while ago. . but his tabs are cut off so, bit tough to do with that set up.


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## IamIan (Mar 29, 2009)

DIYguy said:


> Just wondering why no one has considered silver-soldering (brazing) the tabs. I mean the RC packs are all soldered inside. No worries about corrosion or deterioration or fasteners.


Maybe I'm missing something ... ?

Between silver and aluminum there is about a ~0.75V galvanic potential difference ... and about a ~0.2V potential difference between silver and copper ... I don't see that as better than the galvanic corrosion potential of straight copper and aluminum about ~0.55V.


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## DIYguy (Sep 18, 2008)

Ya, u may have a point about the silver content with aluminum. Higher silver content is good with copper but for Aluminum I think it has to be limited to 5% or less. More Cadmium Zinc for Al IIRC. Anyways, would be worth looking up the best alloy but point still being soldering/brazing connections may be worth consideration. That's the point I was wanting to make.


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## Genius Pooh (Dec 23, 2011)

Hi. It's a good idea but too hard work for me.

I will do same work for my sedan..

I qurious about what is your motor? 

I'm very tired of buying and getting good motor.

My current choice is using four Me1002 motors.

What is your motor?

And Why do you not contact directly A123 system and buy their power pack?

They sell 23 kw and custom 20ah pouch modules.


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## LithiumaniacsEVRacing (Oct 9, 2010)

I use lexan for all my battery boxes, trust me lexan is very very strong, it stops bullets. When I flipped the Camaro at the track, the box held together like steel. The many benefits of lexan are:

1. You can tap into lexan for screws.
2. You can cut lexan with any wood cutter.
3. You can design lexan to fit any dimension.
4. Lexan is not conductive, so no chance of shorts.

Just my 2 cents. 

Merry Christmas! 



rwaudio said:


> Hi Ron,
> 
> Yes I will be using double sided tape on the cells and the insulating spacer. This initial test pack is just stacked because I knew I would have to take it apart. I am using a plastic spacer between cell groups, partly because it's easy to cut on my CNC router and cheap and has a fairly high melting point, if the plastic melts I have bigger problems with boiling electrolite. I hadn't actually thought of fibreglass which is probably a much better solution for a drag car. I will be banding on some end plates that will give me mounting points, but the pack will be in a foam insulated aluminium box. Pack heating will be a big thing for me.
> 
> ...


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## rwaudio (May 22, 2008)

Genius Pooh said:


> Hi. It's a good idea but too hard work for me.
> 
> I will do same work for my sedan..
> 
> ...


I assume this is directed at me.
I'm using the Netgain Motors Warp 11HV, one motor that simply replaces the gas engine and still uses the stock transmission.

As for the reason we can't buy from A123 directly is the little * at the bottom of most of the pages...
* Modules are not available for consumer use or aftermarket conversion kits

They want OEM's only, we are getting surplus or reject cells from the Korea factory.


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## Genius Pooh (Dec 23, 2011)

rwaudio said:


> I assume this is directed at me.
> I'm using the Netgain Motors Warp 11HV, one motor that simply replaces the gas engine and still uses the stock transmission.
> 
> As for the reason we can't buy from A123 directly is the little * at the bottom of most of the pages...
> ...


 
Wow.... Is that cell good??? hasn't serious condition?? and what is price? my ebay seller offer me 19 dollor per cell..


And Can you tell me Netgain motor's continous power and peak power? 

There is too many words and I can't find out which one is correct


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## rwaudio (May 22, 2008)

Genius Pooh said:


> Wow.... Is that cell good??? hasn't serious condition?? and what is price? my ebay seller offer me 19 dollor per cell..
> 
> 
> And Can you tell me Netgain motor's continous power and peak power?
> ...


Yes the cells are good, but I don't think it has anything to do with why they won't sell us any. It's just their business plan, and we aren't in it.

I don't have the continuous rating of the motor handy but I'd guess in the 30-50kw range. Peak power in my set-up will be around 260kw but others have taken slightly modified motors much higher.

you can find more info on Netgain motors at www.go-ev.com


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## Genius Pooh (Dec 23, 2011)

rwaudio said:


> Yes the cells are good, but I don't think it has anything to do with why they won't sell us any. It's just their business plan, and we aren't in it.
> 
> I don't have the continuous rating of the motor handy but I'd guess in the 30-50kw range. Peak power in my set-up will be around 260kw but others have taken slightly modified motors much higher.
> 
> you can find more info on Netgain motors at www.go-ev.com


 
It's Amazing.... I will Try me1002 4 motors But your try is much more powerful and lighter than me....

I hope to hear working stage's success story. Good Luck


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## rwaudio (May 22, 2008)

As I reach the completion of my cell testing and I've charged/discharged and handled hundreds of cells (not to mention the cell preparation as mine are the tabless cells) I'm getting to know them very well. While charging I use a simple clamp made from some of the blocks and spacers that I had made for the final pack assembly, I've noticed that it doesn't take much for a single clean connection to be "bad" IE high resistance. It can be a spec of dirt/sand (in my case most likely a grain of sand from the sanding discs that I use to prepare the cells) or a connection that isn't quite as tight as it should be, or a case where two layers of tab material get sandwiched where there should be only one layer (the second layer being a very small stray leftover from hole drilling or similar)

I've also noticed that I'm using too small of hardware to make tight consistent reliable connections. (M4 flat head stainless fasteners) Some times I would tighten a connection and it would be just as tight as the rest, however given a few days of physically moving the pack that a few connections would get loose and this is with minimal current going in/out so it's not heat cycling like it would in the car. I finally got everything tight in my 12v pack after a few tries and it's working beautifully however I wouldn't want to have to take apart my main pack repeatedly to make sure everything is tight.

Although I take anything Jack R of EVTV says with a grain of salt, I do believe he is correct about the use of Nord-Lock washers. I recommended they try them where I work in a high vibration environment and our engineers got some to play with, so far they like them. 

I've already redesigned my basic cell connection to use a single 3/4" spacer between each pair of 3 cells instead of having a block between every tab. I found that having a block between every tab resulted in a higher than expected total resistance of the test pack. Although each individual contact resistance is small they add up when you have individual cells all with spacer blocks between the tabs. This is confirmed by the IR measurements of the chargers I'm using, when they measure(calculate) the IR of a battery under test. I was getting 3-4x the resistance using blocks than I was getting measuring the IR of a single cell. I've gotten much better results by clamping 3 tabs directly together and measuring from the single block attached to those 3 tabs. It's a lot less parts in the pack, less material, much simpler and it leaves enough room for a traditional socket head cap screw instead of the countersunk flat head screw. This gives me the room and the surface to use a Nord-Lock washer on every connection, as well as switching from a 2.5mm Allen key drive M4 to a 4mm Allen drive M5 cap screw. This is a significant difference in the torque that can be applied to the fastener consistently.

I will be using a pair of M8 cap screws and Nord-Lock washers to connect the terminals of each 12v module to the next. I've got the parts on order and will put together a couple of test packs soon to see how well everything works and how smoothly it all goes together. At that point I will do a comparison between the new version and the existing 12v module.

I actually like the idea of the flat pack that a few people are working towards, it would allow tab to tab connections everywhere without any blocks or spacers in between cells, any blocks or spacers would be for clamping only. I think this would be the ideal for a high current pack. I don't have enough room for that given the shape of the hatch in my 944 and the fact the transmission is in the rear.


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## windjunkie (Jan 17, 2012)

Great info. It sure does make sense to keep those connections clean.

Does anyone use conductive grease for their connections? I'm new here but I thought in such cases it might make sense to treat the connectors so they wouldn't need to be serviced as often due to wear / impurities and thus increased resistance.

A quick search reveals "jetlube" and a few others designed to connect dissimilar metals under high current loads:

http://www.heavyequipmentforums.com/showthread.php?4394-Electrically-Conductive-Grease

http://www.cruisersforum.com/forums/f55/proper-battery-connections-62128.html


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## rwaudio (May 22, 2008)

windjunkie said:


> Great info. It sure does make sense to keep those connections clean.
> 
> Does anyone use conductive grease for their connections? I'm new here but I thought in such cases it might make sense to treat the connectors so they wouldn't need to be serviced as often due to wear / impurities and thus increased resistance.
> 
> ...


I used noalox on the first test pack with very poor results, the clamping force is not high enough to squeeze out the grease and provide a good connection. Clean and dry is the way to go here.


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## windjunkie (Jan 17, 2012)

I'm thinking more about putting the bulk of the cells inside the rear passenger compartment (where the rear seat used to go) and enclosing the front engine (motor) compartment to guard against road spray getting to the battery boxes.


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## rwaudio (May 22, 2008)

After months of testing I found that my old connection method resulted in higher than expected connection resistance (most likely due to the large number of connection blocks) 

So to reduce the number of parts required and try to reduce the connection resistance of the pack, groups of 3 tabs are sandwiched together. For a 3P series connection there are now 3 blocks instead of 7, and the end terminals use two blocks instead of 4. Now there is only a single block between each series connection instead of 5. This is a drastic decrease in the parts I need to make, as well as the complexity of trying to hold and align 6 tabs, 7 bocks and 2 screws. I also moved from M4 to M5 screws and from a flat head (countersunk) to a normal cap screw. I now use a 4mm allen key vs a 2.5mm to tighten the screws which allows much higher fastening torque to be applied. I also incorporated Nord-lock washers into the design.

I still have to revise the end terminal design slightly for proper clearance as well as the extreme end terminals that will connect to the 2/0 ring terminals. When testing with the PL6, the IR of the cells is much closer to the expected value of 3 parallel cells. I can also tighten all of the connections much better and the Nord-locks seem to be doing exactly what they are designed for.

I believe I now have the build it and forget it pack that I was hoping for initially. I will have cell level monitoring using my Cell Log 8 Breakout Modules, but I believe any worry of loose or bad connections over time will be virtually eliminated.


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## boekel (Nov 10, 2010)

rwaudio said:


> I also moved from M4 to M5 screws and from a flat head (countersunk) to a normal cap screw. I now use a 4mm allen key vs a 2.5mm to tighten the screws which allows much higher fastening torque to be applied. I also incorporated Nord-lock washers into the design.


What is the reason you don't use 'normal' hex-head screws?
This would give you the possibility to check the torque and if necessary re-fastening or if there's a bad cell you can remove it without having to take the whole pack apart.
You might also get higher torque with it (if necessary)


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## Ziggythewiz (May 16, 2010)

If you were to use the flatinum approach you could get rid of the middle block and have only tab-tab connections. Depends, of course, on the dimensions you have to work with.


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## rwaudio (May 22, 2008)

boekel said:


> What is the reason you don't use 'normal' hex-head screws?
> This would give you the possibility to check the torque and if necessary re-fastening or if there's a bad cell you can remove it without having to take the whole pack apart.
> You might also get higher torque with it (if necessary)


Choice, availability, and personal preference. If I switched to inch sizes it probably would have been easier but I've been able to maintain almost entirely metric fasteners in the car. By switching to this style of fastener it allows me to use the Nord-locks so I don't believe it will be necessary to check torque.



Ziggythewiz said:


> If you were to use the flatinum approach you could get rid of the middle block and have only tab-tab connections. Depends, of course, on the dimensions you have to work with.


I also like that approach, but the 944 has a rear transmission so my battery box is designed to fit around it. I think a well designed pack using that method has potential though.


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## mrbigh (Dec 31, 2008)

Any updates on the build????


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## rwaudio (May 22, 2008)

mrbigh said:


> Any updates on the build????


Related to the connection method no. The new method simply works well, requires a minimum number of parts and is simple to assemble.

For the car I put some of the updates in my build thread, but I usually capture everything on my blog, link is in my signature.


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## somanywelps (Jan 25, 2012)

rwaudio said:


> Related to the connection method no. The new method simply works well, requires a minimum number of parts and is simple to assemble.
> 
> For the car I put some of the updates in my build thread, but I usually capture everything on my blog, link is in my signature.


I know that page is up, but my firewall is throwing sequence number errors when I try to connect.

What kind of server is it and what brand of firewall are you behind?


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## rwaudio (May 22, 2008)

somanywelps said:


> I know that page is up, but my firewall is throwing sequence number errors when I try to connect.
> 
> What kind of server is it and what brand of firewall are you behind?


It's a wordpress blog pointed at my domain hosted on my company site..
Try this:
http://electricporsche.rwaudio.com


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