Originally Posted by poodlepeople
4. Will I have any problems charging two batteries in parallel from my tow vehicle (2010 Ram 2550HD diesel) while underway?
Most folks find that their trailer batteries receive little
charge from the tow vehicle while underway.
The reason seems to be that the truck alternator/voltage
regulator sees the engine battery as being charged and
drops the voltage pretty quickly after start up.
The trailer batteries are several feet "down stream" and
the wire between is not big enough to let the regulator
"see" the low charge state in the trailer batteries.
Some have added much larger gauge wire between the
engine and the trailer batteries to help over come this.
(The difference between a charged battery and one
that needs a charge is measured in tenths of a volt.
The wire needs to be really heavy or it will have a few
tenths of a volt drop along it's length.)
The wire that typically comes in a pre-wired truck/trailer
plug is not
large enough to prevent this tiny voltage loss.
The rest of us just use some other means to re-charge
the trailer such as generator, shore power and the trailer built in power center and or separate charger just for the trailer.