It is important to use the proper size wire between your MPPT solar controller and battery bank. Here's why

One of our customers was getting less than 20% of the rated power from his solar panels and he called us for help.  He had 400 watts of solar, was using a Victron MPPT controller and two CMPower 120 Ah LiFePO4 batteries.  He should have been getting 25+ Amps of charging power from the controller to the batteries but was only getting 4 Amps.  We spent several hours troubleshooting; charging and discharging the batteries, testing the solar panels reprogramming the MPPT controller all to no avail.  He texted me a picture of his installation and happened to mention he was temporarily using a 25 foot long 12 gauge wire between the controller and the batteries.  I suggested he try a short 10 or 8 gauge wire and see what happens.  Magically, the system began to work perfectly.  I reproduced the same situation in our shop and achieved the same results.  The bottomline:  The wires between the controller and the battery bank MUST be adequate to handle the load otherwise the performance of the solar system will be severely restricted.  Don't skimp on the wire.  The same applies for the wire from the solar panels to the controller.