Cable length and voltage drop in offgrid solar installation
AWG 8 cable has 2.06Ω/km, so using 5m of it (or 0.005km) would be equivalent to 0.01Ω.

With a 12A current this equates to 0.12V of drop per cable, and 0.24V with both sides of the cable. 

However, connectors can also add a significant amount of resistance, make sure that you are using the right connectors and they are terminated properly. 

0.6V at 12A would be around 0.05Ω of resistance from the voltage drop, which could be reasonable if the connectors are contributing to the resistance. 

If the drop is too much loss consider better cables\connectors and or a higher voltage system (24V) to cut the resistive loss.