buy a 12Ga 3 wire extension cord, cut the ends off it and solder the green wire to either the black or white on one end do the same at the other end soldering to the same color you did at the first end. you now have a cable with 1 conductor 12ga and the other 10ga. total resistance will be about 0.13 ohms. voltage drop with 5A will be about 0.6V
and an easy way to do a group of cables is to use anderson connectors on both ends. that way you could make the 50ft two 25ft real easy and plug and play to go to whatever length you want.
BTW the 0.6V drop won't have much of an effect.
Or the big kanuna option is to get 30ft of 30A trailer cords and do the same trick. but now you have one conductor at 10 ga and the other at 8Ga
for each 30ft section you would have only a 1/4V drop not enough to worry about.
I have close to 120ft of cable made this way with anderson connectors to string whatever length I need.
FWIW I have three 160 W portable panels going to an MPPT controller and using the 30A cords wired as described. Usually panels in series to minimize drop. a 100W panel would be no problem.
NOW THE BIG THING YOU MUST PAY ATTENTION TO. The controller MUST be at the battery end, NOT at the panel end!. Otherwise the controller will output a voltage it expects the battery to see and the voltage drop will mess up the charging rate big time. And NOT in the direction you want. Charging current will drop like a rock, even 1/4V will really drop the charging rate. If the controller is on the panel, remove it, make a panel board or some way to place it even temporarily within a few feet of the battery so the cable goes from the panel to the controller, not from controller to battery.
