Coming to a finish but not the end

 

WARNING!   The following article is from a tinkerer who can't do anything the easy way.   The information provided is based on many assumptions and should not be followed too closely without your own research and testing. DO YOUR RESEARCH, TAKE PLENTY OF PRECAUTIONS , HAVE A PLAN, ALWAYS ERR ON THE SIDE OF SAFETY.  Any injury, critical malfunctions, explosions and death due to following elements of details or procedure contained in these articles are at the risk of anyone reading these article and attempting to follow this information without a reasonable amount of knowledge in constructing Lithium Ion batteries and a strong knowledge of electricity is doing so at your own risk!  Constructing large Lithium packs require an understanding and respecting of the dangers as well taking plenty amounts of precautions in constructing even a small pack.  Do not attempt constructing any type of battery packs without proper prior instruction or training and again, having basic electrical knowledge, a proper layout the pack is important.  This is a project that I am researching and the total amount of equipment I own would make this worth trying.  The actual expense if one were to start from scratch would out strip the amount in savings that is listed in my blogs.  The information within these blogs are starting points for what is required to construct a battery pack and does not have all the answers, such as, technical details of voltage drop issues, internal resistance considerations, charging methods or structural techniques and so on. Those details depend on the application of the battery pack and exceed the scope of these articles.  

Final Connections to the cells 7 & 8

The lower group of six cells was awaiting the final connections, but I had decided to use a staggered row arrangement much like the top group.  The staggering will net about 2 millimeters more space and might get a little more wiggle room for the odd fit of cell number 7.   

I still have to settle on the details of that last cell's unusual connections and came up with an odd solution that may not look straight, but is stable enough and can carry the maximum amps.  The theory is the cell 7 not only has a odd angle from cell 8 to cell 6, but will end up being canted slightly when the two way angled nickel strip is secured.  This creates a slightly off center fit when pressed into position and is needed since the available gap to join cell 7 to cell 8 is barely enough to slip a single strip of nickel in.  

Then there is 6 & 7

Almost there, but it looks like the 'L' shape connection will have more room on bottom of the module.  The limited space and lifting the position of cell 7 high enough towards the lid will leave little margin for error from possible abrasion damage.  While testing the fit of the XT60 plug, I realized that this final connection will have to wait until I finish the XT60 part.  By not making the pack fully connected, I give myself some wiggle room for mistakes while testing out the fit of the insulator and the placement of the soldering tabs.

XT60 is the critical part

The testing of the length and the direction of the tabs are important.  The goal is to not allow for an easy path to short out the pack.  Logically, the tabs are very close to one another.  I worked out in a few illustrations so far of how it should work and look.  The fish paper separator is illustrated more precisely than is necessary.  The things that are important are the tab directions.  Should a solder or weld to battery fails, the movement towards the other output is not possible.

BMS time

   The delicate work is to populate the 24 wires to the 26 pin connector and run all the wires as neatly as possible and install the thermistors and make a choice of how much wire I can cut off of them.  Then I have to use thermal glue as well decide on the thermistors appropriate locations to best serve the monitoring of temperatures.

I am heading in the direction of how the "Board Garage" arranged the connections.  This in part was because I already describe the cells in the direction of the cells based on the "Board Garage" article.  I've read that the longest and shortest wire should not have too large a variation, not found much discussion on that yet.  I got a crimp tool that can handle 26 gauge wire and have the .3 pins and the plug itself at the ready.  The random advice is to not have too large a variation in wire lengths or just keep them the same.  I started looking at the longest run of wire and as well testing the amount of resistance for that wire.  Just in case, I'll look at a similar wire and compare the resistance.

Thermistors to locate

From the datasheets and the pinout charts, I am voting that it is not critical the polarity.  The decision is to put them were they will do their job and hopefully not get a false reading.  Seemingly the cells 1 and 15 will experience the most heating from use as well the center cells 7, 8 and 9.  So I'll look to cells 14, 13 and 10, 9 and 4, 5 and 3, 6.  The thermal glue will work best if it remains flexible after it cures.  The glue should have the consistency of a thick silicone?  If not, then it will be 2, 6, 13, 9.  The reason being that they are cells I deem as the least influence by certain high draw situations.  Not knowing the monitoring thresholds or limits.

Routing

Once the BMS wires and thermistor wires are crimped to the plug, the laborious task of making things neat will be necessary.  The final clearance for the lid to close is going to depend on a few factors.  The cardboard shims will need to be modified and coved to shield against heat.  Most likely I'll have to use some dots of hot glue to anchor the wires and if it works to do, tape it all down with the Kapton tape.    

Concept revision 6 of the pack and as of this entry, I'm up to revision 9

Insulation and Securing

I was also awaiting some supply items.  I changed my mind on taping down the fish paper as well steering away from using layers of reinforced tape.  I'm thinking of adhesive fish paper and some wide Kapton tape since the goal is to cram all of this into a tight space, layer of tape maybe cutting it too close.  Opting to leave it to the shrink wrapping the outside layer for last until I can complete the load tests.