R_Lefebvre
Expedition Leader
The problem here is that it will not lower the amps. It will only lower the volts and the difference is simply made into heat. This is a wast of power. Amps is a mater of how much the devise pulls. You need the volts to charge the bat. If you try to charge a bat at to low a voltage it is hard on it. Aside from cost and with the proper fuse/breaker, bigger wire is always better.
That said there can be a problem charging a bat too fast. If it gets too hot it can be damaged. I wonder if there is a heat sensing charging regulator on the market. I know the later XJs (and other new cars) have a temp sensor under the bat for this reason.
Yes, it does lower the amps. V=IR. A long run of 10g is reasonably high resistance, so it lowers the amps in the circuit. As the battery comes up to full charge, the amps drop, and the voltage drop across the wire decreases as well. When the trailer battery is fully charged, the current goes to zero, as does the voltage drop.