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.

Progress update on the battery build

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.  

 

Line up and spot welding

The fifteen 21700 cells arrived and after some measurements, adjustments and testing I settled the manner of arrangement of the cells.  I then started to plan out how to go about the actual spot welding.  I noticed that a critical necessity was the tabs for the BMS wires.  Instead of waiting until after installing the connecting strips to then gingerly spot weld the tiny tabs, I made a jig from some cardboard scraps and hot melt glue.  I now had a consistent way to hold the tabs for spot welding a batch all at once and prevents the risk of heat effecting the jackets of the cells when attaching the tabs while on the cells.

There are nine side to side connections of the cells 8 to 15 in the part of the pack I call the upper group and three end to end connections of cells 1 to 6 in the part of the pack I call the lower group.   I made a decision make the batteries run in as much a continuous direction as possible to reduce problems that could result in shorting out from using wire jumpers.  

Commonly, the typical cylindrical cell is much like an elongated cup with a lid and the rim of the cup belongs to the negative side.  Because the nickel strips traverses from positive to the next cell's negative while passing over its own negative rim, it is a vulnerable point.  Only thing that blocks the contact is the thin PVC jacket and an insulating grommet to keep things from touching. touching.  It is easy enough to have a sharp edge of the nickel strip or a minor amount of flex from vibrations to damage this barrier.  To give a little more protection, I used the fish paper stick-on rings around the positive contacts.   The center spots also came in handy to shield the side of the cell from the sharp edge of the BMS tab.

In the above illustration is based on a typical "flat-tip", the weak point is the top part of the negative jacket or body of the cell at the top part where it is crimped and right next to the positive terminal of the cell.  Only the PVC jacket shields the possibility of short-circuit and can easily be damaged due to movement near the positive side or a connector overheating or should a connection break loose and breech the edge of the negative jacket.  The above image is based on a general diagram and most cells used to make packs have a "flat-top" positive cap that brings the nickel strips even more closer.  

It was then time to begin the arranging of the cells into the required a staggered configuration.  The offset was about 3.5mm and I pushed it to a full 4mm, but that still allowed a thin 1.5 to 2 millimeters of space to clear the lid of the module.  Strips of cardboard were glued together to form the 4mm thick shims needed for assembly.  

Illustration of the general way to clamp the cells and maintain the relative position of the offset.  I used 4 or more cardboard shims to allow for the offset while clamping the cells in the wood blocks.  When fully spot welded, the group maintains the position.

  

The actual spot welds were done with the cells standing vertical and shimmed and clamped to the offset needed, however, that image didn't work out... so included is the modeled image.  The challenge was to keep the nickel strips steady while positioning the spot welding tips.  Each connection received a minimum of six spot welds, but I went for eight each.

When both ends were welded... to the needed "flow" configuration, the group could stay in the correct offset positions without support.

Finishing the upper group of eight cells, I moved to planning the lower group that required both side to side and end to end nickel strips.  

image is upside-down compared to my counting descriptions, but I'm too lazy to flip it

 

The lower group required the end to end nickel strips to be installed first.  A simple set of wood blocks were arranged on the table and angled to hold the cells apart just enough to sneak in with the spot welding probes.  

I first spot welded the pre-bent strip to the positive end of the cell (note that all cells will have the fish paper rings installed), then the two cells were moved to the wood jig to make the negative side connection.  This "end to end" procedure was done to cells 1 and 2, 3 and 4, 5 and 6.  This leaves the final connections of 2 to 3 and 4 to 5 being the "side to side"  connections later.  I had discovered that the bend applied to the strip is best round.  I used a 1.5mm wire to put a radius on the bend.  This make it a little easier to line up the cells when it comes time to book fold the connection, otherwise a sharp crease will be harder to fudge the alignment back if you were a little off during the spot weld.

The very last cell to be spot welded is cell number 7.  It requires an off set end to end strip as well an overly built 90 degree connection to cell 6 to handle high amperage as I had concerns of the distance. To make it more complicated, cell 7 has to be built on a slant by lining up to cell 8 at module floor height to cell 6 at 3.5mm height.  All this is need to clear the limited space of the cable outlet area that cell 7 must protrude into.

 

In other examples I've observed, this should be a heavy 14 to 12 gauge wire, but I decided to make essentially a bus bar like connection which will require two strips and a reinforced extra strip.  This will allow it to conform to the "flat as possible" build and to avoid the need to make an odd solder lug to from cell 6 to a just as awkward offset solder lug to cell 7.   To note again, my arrangement avoided the need for three different wire jumpers and lug to lug points.  Is that a good idea?  I don't know.  If the load test proves that this too much heat, then its back to jumpers.

So that's that for now.  I'll have to go back through and add extra spot welds that I rushed to set the positions, but not the required number of spots and finish.  

Next Time

Once I solve the cell 7 and 8 problem, I'll soon install the BMS wires, installing the pin to the socket, the installing of the thermistors, routing all that wire, attaching the main power leads to the lugs, insulating,  

The final steps are the load test to maybe 12 to 20 amps to check for problems before actually installing the pack into the Pint module. 

Few more steps to take in wiring up the Pint battery - the XT60

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.  

 XT60 plug... that took some practice to get a real good solder going, I had to use over 475 degrees Fahrenheit / 246 Celsius and 60/40 solder as the lead free stuff didn't take at all.  The 12 gauge wire was tricky to squeeze into the lug end to solder.  When I finished, I had a neat looking wire and it failed while testing it for routing and position.  The pack to BMS requires a 90 degree bend and the distance was less than 50mm from pack to socket, but now, the total run of wire from the source contact to plug for the positive increases from about 55mm to 130mm, and the negative goes from a long 205mm to about 140mm.  Does is matter will be the question later during the load test.  

Seems the lug was not adhering to the solder at all and chances it needed a good cleaning... acetone to something...I do know the flux works good.  The third try was a quick strip of the silicone sheath and a twist of the strands and then stuff into the lug with no buttering of either.  The 500 degree solder was on top the wire strand bundles and lug at one corner and I kept on feeding the solder into the whole thing until I could see it leak out the edges.  The third plug was complete and the test seems to show it will not separate as the second one did or not so quickly.

The high power output wires takes a paths up the side of cell 15 or 1 depending on how I should count.  Along the side of the cell, but running along the bottom of the module is the positive wire and the negative towards the lid side of the module.  Either way I should start and end the cell taps for the both wire in the optimal direction to reduce strain.

I designed a fitted fish paper protector for the high power wires and it will be the base for the separator for the upper and lower group.  At 285mm by 21mm for the main section and maybe an extra layer in the middle at 150mm by 21mm.

  

Nearing the final plans, the included BMS taps are located and worth noting that the tap should lean towards the negative end of the cell in case of trauma to the outer cover, the direction of the cells are noted and follow the pinout chart from "The Board Garage" and the indication of all the connector strips are marked.  Should the next thing to plan is build some jigs?

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.