Update on the pint battery project

 

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, properly layout the pack plans are 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.  

Size Up of the Battery Module

Dimensions inside the battery module is a left to right of 168mm by 137mm and up to down.  There is a slight bulge in the middle giving another 2mm or so, but it is better to work from the smallest measurements.  This is only the space that the batteries inhabit and ignores the divider that is the wire channel which is made of two plastic tabs.  

 

Real world fit

Although the max width of the module is 168mm and the 8 cells across is 168mm on paper (21x8=168), the slight thickness caused by the vinyl jackets and the cell crimps, make the total width a little more than the 21mm of the specified 21 by 700 millimeter size.

To get the top group of 8 cells to fit without the need to snip out plastic, I looked at staggering the cells. I tried adding shims under cells 9, 11, 13, 15.  The height is around 3.5mm and the remaining headroom from battery to lid will be about 1.5mm.  That means the depth of the compartment is about 25mm.

This is not the final arrangement as the polarity will still have to decided when the time comes, but it doesn't really matter negative or positive first as long as the start and end are established and comply with the BMS order of connections.

My first concept of the 21700 batteries and how they fall into position as mentioned prior.

Numbers Minor Conundrum Encountered and it's all about nothing. 

When I first started my research on the BMS layout, it was from an article by the Board Garage.  Later I found the similar published layout from someone who goes by the name "That-Canadian".  At first glance the two BMS pinout charts could not look any different.  When I gave it a closer look, I notice that they sort of run in a similar pattern.

Two Pinout Charts

Numbers Key

When I wrote out the numbers in a line with the primary ends in the same places, it is sort of easy to see that the charts are the same and only the counting directions were different.  Both the positive and negative sides start or end with the highest and lowest numbers, as well, the middle cell is the same number.  

What still remains it the question of securing the layout

To start off, the cells will have a dab of hot melt glue to position them into their groups and to make it easy to spot weld later.  Then it is the touchy strategy of joining the groups together as an example of cell 9 to cell 8 is a massive connector and possibly on both sides to handle the amperage.  Then the question of how much insulation will fit?  I won't be much or about 1 to 1.5 mm of space remains depending on where.  The lower groups of six cells could be furnished with up to 4mm of foam padding as they don't require a staggered arrangement, but the above group of seven to eight cells may not have much room. Between the groups a reasonable amount of insulation and padding can be added to divide cells 1 to 7, however a special provision to allow cell 1 with a positive wire to be well shielded from any problems or interaction with cell 15.                                                                                                                                                                                                                                                                                                                                                                                  

                                                                                            

Another thought is space needed to run all the wires

Routing for the BMS wires will have to come from three bundles.  It may be necessary to have a groups that are the eight temperature sensor wires and however best to route the 'B' numbered wires into a upper and lower group.  The goal is to not have to alter the battery module in any way.