Renogy recommends the DC-DC input side be sized for 50% excess of the output current. According to my calculation ()IIRC) a 3% voltage drop at 16 ft is about a minimum DC-DC input voltage of 12V at 14.5 alternators. I used a 40A but the voltage drop and currents will be proportionate for a 60Amp DC-DC. Any more drop than that will be exceeding 90 Amps in the case of a 60A DC-DC.My guess, because the input volts to the B2B are low due to volt drops, the unit is having to work hard ( the currents on the input side may be well in excess of 80 amps) and is reaching protective thermal limit, thus causing automatic reduction of power conversion and a lowered output current
This is quite common and can be seen in other B2B units from Sterling and Victron.
A further consideration is that you maybe overloading the alternator and/or the cables from alternator to starter battery and dropping volts along that path.
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...