I am trying to find a way to automatically charge a trailer battery from tow vehicles that may be either 12v or 24v. The trailer battery is 12v and I intend to keep it charged via the tail light circuit. I am trying to avoid the necessity of the driver having to switch to either 12v or 24v before using the trailer, hence avoiding damaging circuitry when they forget. I thought I could do it with a small solar panel regulator which is supposed to handle either 12 or 24v automatically, however it seems to use the input voltage to output either 13/15 v or 26/27v approx, so that doesnt work. Any ideas?