Making Flexible Busbars

Since the van is mobile, and as its cells won't be clamped as precisely and predictably as in OEM packs for EVs, flexible busbars / jumpers will be used to avoid mechanical stress on the cell terminals and nuts. Mechanical stress can resul from vibrations, misalignment, temperature variations, cell expansion, etc.

Several types of flex busbars / jumpers exist. The ones made of many stacked thin layers of copper are great. Very low resistance, as long as the stack under each bolt remains delamination and oxidation free. They can be expensive though.

Braided busbars are extremely flexible. They are typically used to ground equipment, not to carry high current. However, it is unclear to me how good and reliable the electrical bond is between the braid and the end terminals.

Some high current versions exist, though, that seem to present a solid bond at their terminals. Dunno how readily available and affordable they are.

Specifications and goals for our flex busbars / jumpers:

2 AWG (33mm²) equivalent wire section
~0.1mΩ max resistance from mid eye to mid eye
Tinned copper lugs and wires (no bare copper)
Distance between cell terminals= 3.75"

In the end I built my own DIY flex interconnects, for various reasons (custom length, availability, cost, performance, etc). They are built traditionally, out of crimped lugs + cable. 2 AWG is too stiff for flexible connections that are only ~4" long, so two 4 AWG cables were used instead, stacked vertically for easy flexing.

It worked very well, the end result is notably flexible, and making them was faster than expected.

The mid eye to mid eye resistance is an excellent 0.08mΩ, and the cell terminal to cell terminal resistance is only 0.12 to 0.15mΩ. And that's before lapping the top of the cell terminals and applying Noalox.

Consequently, at the 250A max sustained current, the power losses due to the cell to cell series interconnects will be limited to 0.00015Ω (R) x 50A² (I²) = 0.375W per interconnect. That's 5.6W total for the 4S5P Winter pack, when drawing 3000W.

Oversized cabling + low resistance between cell terminals = cool busbars and cell posts. Excellent !

But how does it compare to the standard 2x20mm, 72mm spacing, plated copper rigid busbars ?

Copper section (i.e. current carrying capacity): 4AWG + 4AWG = 42.2mm² vs 2mm x 20mm = 40mm². Marginally better.

Resistance: it is higher at 0.08mΩ vs 0.06mΩ, but probably just because of the longer length. So let's look at the resistance per inch. When straightened, the flex busbar is ~5" long from mid eye to mid eye. Resulting in: 0.08 / 5 = 0.016mΩ/inch vs 0.06 / 2.75 = 0.022mΩ/inch. A tad better. Consequently, had the rigid busbar been 5" long, it might have measured ~0.11mΩ (lot of fudge here, let's just take away "same or worse").

Conclusion: not too shabby !

Here is the process used to build them:

Mark and cut the cable

Remove the insulation and cut a wedge on each end

If the two #4 wires hardly fit in the #2 lug, cut a few strands
(only if #2 is oversized for your need)

Stack the wires wedge against wedge, crimp to specs

Apply heat shrink tubing

Finally, while still hot from the heat gun, each busbar / jumper is mounted on cell posts spaced as needed so they take the proper shape and length as they cool down. 1 done, 14 to go !