I have done the calculations for voltage drop and including the effect where the input terminal voltage to the charger is below its output voltage. Renogy for example recommends 50% higher input current than the stated output current rating. So if the output is current at say 60A@ 14V then the input will be 90A @ 12V (you would need to go through the numbers).According to the wire gauge calculator I use, 60 amps over 50' (round trip distance) @ 12v would have a 6.39% voltage drop. That exceeds the 3% threshold that most of us shoot for.

Keep in mind that 60 amps is the max OUTPUT of that charger. It could very well be drawing more than 60 amps to perform the charge.

In my case, I was using a 40A renogy DC-DC, and based on a 3% loss for 16ft of cable I would never want to drop below 12V at the input to the DC-DC. The current draw from the alternate would just get too high.