In a setup of 6 panels (445w) , 2 series of 3 in parallel, do the solar cables need to match in length up to the junction, as they do when connecting batteries inverter, charger etc?
For sure the final NEG/POS do not need to match. But I have read some that say that the parallel connections should match between them and also the serial connection should match between them..
No, the cables do not have to be the same length. The reason it is important for batteries is so that the charge/discharge characterisitc for each battery is the same. That way each battery has the same cycle history and they age equally. That ensures the longest life for the battery bank. The solar panels do not need to have the same consideration.
I would think the resistance of any interconnect cables would not be a significant factor. Remember the solar panels themselves will have an internal resistance of approximately Vmp/Imp. That resistance is essentially in series with the interconnecting cables.
When connecting batteries, the battery resistance is very low so cable resistance does play a role in current balance between parallel-connected batteries.
"solar panels themselves will have an internal resistance of approximately Vmp/Imp."
Where do you get that idea ? Who's definition of resistance is that ? The dynamic resistance is the slope of the V-I curve: dV/dI . The MPPT unit will ensure the panel is operating at the turning point of the power power curve. The power curve has a peak because as more current is drawn, there is a point where the voltage starts to drop. At that point the dynamic resistance is zero: dV/dI = 0.
Current in the panel circuits are a lot lower than the battery circuit because voltages are higher but you would not want a significant difference in cable resistance because that would mean one ( or neither ) set of panels was not working at its max power point.
As long as you have good connections and large multi-braided cables this may be negligible but it depends on cable size and the differences in cable length.
You could try measuring the voltage drop across both sets of cables, both on +ve and -ve runs, or calculate it if you have the spec for the cables. ( Measurment is to be prefered ).
As trevor-bird says this is more important with battery connections because it can mean one set of cells is providing more of the charge cycle than another and they will age differently.
However, when you have 6 panels and one MPPT controller any asymmetry with degrade the ability to find the true optimum operating point.
In place measurement will help you detect how big any differences are. If you have a choice make cabling as near the same as you can.
