Rev A and on to Rev B

Here are some pictures of the Mark II pack.

I have been having problems with one of my Mark I pack having leaking cells and I think it is due to uneven pressure on the cells.

I pulled the pack and you can see the electrolyte on the cooling fin

I am glad I made the pack easy to remove and replace cells.  This is the third time I have had to R+R bad pouches.

The MkII has very stiff end plates which I think mitigates this problem.  A couple of changes from the Rev - and Rev A design:  I glued plastic plates over the inside of the end plate to prevent possible shorts to the end plates from the tabs.  I also put a sheet of elastic foam between the end plates and the outer most cells.  This foam sheet acts as as spring to control the pressure on the cells due temperature, cycling, etc. and insultation on the threaded rod to prevent a electrical pat from the fins that may be touching the rods.

I am considering adding insulation in between the cells.  I haven't had a problem with it but others have:

http://www.diyelectriccar.com/forums/showthread.php/a123s-gone-bad-78971.html

The factory modules don't have any intracell insulation but obvious some people are having problems.  I have to do some more head sratching as to what the root cause of the problem is first.

Keeping cool

Still not home yet but a couple hour layover is as good as time as any to update the blog.  I have had a few people ask me about the cooling fins on the pack so I am posting so more info.  

I know there is much debate whether thermal management is required for these batteries.  Some say it is not required while others say it is essential for battery life.   In reality they are both correct.   If you are taking a leisurely cruise on a spring day then thermal management will not be a problem for you.  The starting temperature of the pack and the low discharge rates don’t require it.  However if you are driving in more extreme conditions or operating your batteries at a high discharge level then thermal management becomes much more important.  For instance if you are doing laps +20 on a track on a hot summer day then your batteries will NEED cooling, not to mention every other system on your car:

The coolings fins placed between the cells are essential for getting the heat out of the cells.  The fins are made from 8” wide aluminum flashing from Lowes.  It comes in 25’ long rolls that are cut into squares.  Since I don’t like making the same part more than once, I set-up a quick line to crank out a hindered in a matter of minutes using a $30 paper cutter from the office supply store.

The cutter comes with a simple fence and has no problems cutting the soft thin aluminum.

As you can see in the picture I setup the cutter and roll on a bench so I simply unroll it cut and repeat. 

For this pack I used one fin every other cell.  This assures that each cell is in direct contact with a fin and should be minimum if your application requires cooling.  Fewer fins will only cool some cells and result in potentially significant variations in cell temperatures.  For very high power and ambient temperatures you can step up to a fin in between every cell for double the cooling.  I used bare aluminum flashing for maximum heat transfer and it was the only finish I could find in 8” width.  The flashing is also available in prepainted which could provides a thin barrier which could help with voltage leakage from the pouch casing but will decrease the effectiveness of the fin some amount.  Whichever flashing you use make sure that the surface of the flashing and the cells are clean before assembly.  Any dirt or debris that becomes trapped can potentially damage the cell during assembly and/or result in wear over time.

Quick update: The car runs.

Sorry for the long time between post but between work and summer time in Colorado I haven't had much free time to update my build log.  The short story is the car is running now but still tying up loose ends.   Battery packs are all built and installed, 2/0 cable is in with big manual disconnect, DC/DC converter is in and the vacuum for the brakes.  

Really all that is left is some small wiring tasks. I haven't done any real road testing yet as I am finishing the BMS install and more importantly I need to wire a 30amp 220V plug in my garage so I can actually charge up my cells.

I was hoping to have the car finished up this week as I am off to work on the other side of the globe for a few weeks.  So I will probably get the car done in October just in time for the weather to get cold. :)