[VAC] Re: Charge line isolation question

["Dr. Gerald N. Johnson, electrical engineer" <geraldj@xxxxxxxxxx>]

You want isolation for many things. First off you don't want trailer
electricity use to run the tow vehicle battery down. Not being able to
start when truly boondocking but out of supplies or vacation time can be
a real bummer. Its less fun with woods fire or flood approaching. Worse,
the standard starting battery generally doesn't take being so discharged
without doing permanent damage, which tends to lead it to let you down
even when you think you aren't abusing it.

Second, the wire between the batteries in the tow vehicle harness and
the trailer pigtail isn't large enough to allow the trailer battery to
contribute well to cranking the tow vehicle but may overheat the
insulation to the point of destruction while trying. And the deep cycle
battery in the trailer isn't benefited by being used as a starting
battery.

The classic isolator was a pair of diodes (and is still available) that
went between the alternator and each battery. It lets the tow vehicle
alternator charge either or both batteries as needed by regulating the
alternator according to the system voltage of the tow vehicle
compensating for the inherent voltage drop of the diodes. Its important
that the alternator's regulator sense system voltage not alternator
terminal voltage when using the diode isolator. The diode alternator
fails to work properly with one wire alternators with a built in
regulator. The diode drops in the isolator keeps either battery from
being charged properly.

Today, many vehicle towing package options include a relay that is only
connected while the engine is running. The wire to the trailer battery
is fused and its resistance prevents there being excessive charge
current. This option is easy with a computer to decide the engine is
running. Such a relay might also be operated by the stator connection on
some alternators that only has voltage when the alternator is turning at
a charging speed. In some alternator regulators that stator output was
used to run a relay to operate the dash panel alternator light.

The best absorber of a charging surge from an alternator is a good
battery. There are more voltage surges in a charging system from
inductive kicks from the starter while and just after cranking. Either
isolation scheme, diodes or relay while keep those cranking system
surges away from the trailer. Some can come from large changes in tow
vehicle load such as the air conditioner clutch. Sometimes the
alternator regulator can be slow to shutdown the alternator when a very
large load shuts off, but those load changes happen all the time and the
batteries absorb those kicks.

The output of the trailer charging converter tends to include another
diode (some times a pair of diodes) so that if the tow vehicle charging
is at a higher voltage than the converter's output, the converter will
automatically and instantly be disconnected.

Gerald J.