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.
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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.
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