DIY Electric Car Forums banner

1 - 20 of 49 Posts

·
Registered
Joined
·
1,317 Posts
Discussion Starter #1
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.
 

·
Registered
Joined
·
2,044 Posts
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?
 

·
Registered
Joined
·
3,218 Posts
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 ;) ).
 

·
Registered
Joined
·
1,626 Posts
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 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..;)
 

·
Registered
Joined
·
1,317 Posts
Discussion Starter #5
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

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 ;) ).
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.
 

·
Registered
Joined
·
1,317 Posts
Discussion Starter #6
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.
 

·
Registered
Joined
·
514 Posts
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' :rolleyes: bearing-greases are suitable for this.
 

·
Registered
Joined
·
747 Posts
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.
 

·
Registered
Joined
·
3,218 Posts
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' :rolleyes: 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.
 

·
Registered
Joined
·
1,317 Posts
Discussion Starter #11
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.....
 

·
Registered
Joined
·
1,956 Posts
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.

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.
 

·
Administrator
Joined
·
6,203 Posts
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
 

·
Registered
Joined
·
1,317 Posts
Discussion Starter #15
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?

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
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.
 

·
Registered
Joined
·
747 Posts
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.
 

·
Registered
Joined
·
12 Posts
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.
 

Attachments

·
Registered
Joined
·
1,296 Posts
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.
 

·
Registered
Joined
·
1,317 Posts
Discussion Starter #20
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.
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.
 
1 - 20 of 49 Posts
Top