Interesting to see how the internal BMS copes with a relatively large series connected string. How do the 4 cell sub-pack BMS units communicate with the charger to let it know that one cell in the group has reached full charge cut-off voltage?
I tried something similar a few years ago, using individual cell BMS units I made up. These were essentially just voltage clamps, like those used on some consumer lithium cell packs. They worked OK on their own for a few cells in series, as long as the charger maximum voltage was carefully set to ensure that the current reduced as the cells came up to full charge and the BMS shunts kicked in. The system failed big time once I tried to build a series pack with more than half a dozen cells, though, as the variation in total pack voltage became too great, leading to occasions when the charge current through some of the activated BMS cell shunts was far too great, causing them to overheat.
The fix I used (and one that's used on a lot of multi-cell BMS units I've seen) was to have each cell monitoring circuit communicate with the charger (or just cut the charge current). This way the charge current is reduced as soon as one BMS cell shunt starts to turn on, so reducing the heat dissipation in the activated cell shunt(s) whilst allowing enough balancing charge current to flow to bring the other cells in the pack to full charge.
Anyway, back on topic. The simple RC-type plug suggestion, fitted in a block so that they can only be plugged in one way, would work for you. If you want a simpler to make high current switch connector, then take a look at gluing some Dean's Ultra connectors together. I found that these can easily be glued together, side by side, using super glue. It makes for a 70A capable multi pin connector that can only be connected one way around. The only downside is that the force needed to plug and unplug them is pretty high.
