Barncat said:
I'm coming at this from a mechanical engineer's perspective. I should have more to report on my project in a week or two.
Being VRUZEND V1.6 or V2.1 (for the price) may be your primary competition here's some more mechanical/electrical consideration as electrical IR is relative to your mechanical design. You are most likely already familiar with the following, but decided to post anyway for other interested VRUZEND DIYer's ...
The threaded ss stud (IMO) should be a larger M4 0.70 metric stud for a couple reasons as it's easier to start the nut with your fingers. Also the VRUZEND precision bus bar holes (on center) in both the V1.6 and V2.1 bus bars will readily accept a M4 0,70 threaded stud which IMO is a better tight fit mechanically and electrically than the current smaller threaded studs. The bus bar holes are precision drilled center hole to center hole for DIY assembly using M4 0.70 threaded studs.
The reason Micah used a hammer and/or woodworking clamps was to make sure the stainless steel cap inserts were completely compressed against the head of the threaded stud. With the later addition of barrel bolts in the kits insured a secure fit with the ss insert remainig fully compressed. Here's the difference in mΩ readings with the ss compression inserts being partially compression and fully compressed with the mΩ reading taken at the adjoining hole connection of the bus bar ...
11.5mΩ ~ not fully compressed with mΩ reading taken at outer hole of ss bus bar
10.3mΩ ~ fully compressed with mΩ reading taken at outer hole of ss bus bar
8.2mΩ ~ not fully compressed with mΩ reading taken at outer hole of ni/cu bus bar
7.0mΩ ~ fully compressed with mΩ reading taken at outer hole of ni/cu bus bar
4.5mΩ between ss bus bar holes
2.2mΩ between ni/cu bus bar holes
2.3mΩ + 0.7mΩ + 0.3mΩ = 3.3mΩ (e.g. 8.2mΩ to 11.5mΩ in above comparison)
For each 18650 cell double the above mΩ's being two VRUZEND caps on each cell (cathode end and can anode end) of an 18650 cell.
With the above mΩ readings one could figure the total mΩ IR of a VRUZEND battery pack kit
IF so inclined figuring all the series and parallel bus bar connections whether ss or ni/cu bus bars.
The attached photo shows how i took the mΩ readings of the ss insert partially compressed and fully compressed (difference of 0.7mΩ). There was an additional 0.3mΩ's when taking the readings of the ss insert with bus bar attached to the cap which is represented in the above comparisons. IMO if a larger threaded screw (M4 0.70) were used it would be a better fit all around and possibly negating the additional 0.3mΩs in the above comparisons.
Bottomline: When the VRUZEND ss insert (V1.6 or V2.1) is fully compressed the mΩ reading between the cell cathode fully compressed contact and threaded stud bus bar contact is only 1.3mΩ (at most) ... so we shouldn't be overly concerned about the compression cap insert being stainless steel when fully compressed.