[A/S] RE: ran down BOTH batteries...

["David & Bret" <dtbw_1999@xxxxxxxxx>]

Dan,
If you succeeded in draining BOTH the tow and trailer battery, then you
either have a malfunctioning battery isolator or one was never
installed.  Battery isolators are NOT installed with factory towing
packages.  The whole idea is to only allow current to flow to the
trailer batteries when the engine is running and the alternator is
producing current.  There are simple relay type isolators (though they
tend to be unreliable over time as the relay contacts fail), diode type
(which have a side effect of significantly reducing the available
charging current to the trailer batteries, and the high efficiency solid
state switchers (which overcome the problems with the diode type, but
retain the reliability factor).  I like the Hellroaring
(www.hellroaring.com) solid state switch as it has the ability to be
switched totally OFF, to automatic mode )ie on when alternator
available, off otherwise), and to bi-directional, full time operation.
The bi-directional mode could come in handy should you have an
alternator failure many miles from a repair location as it would allow
you to draw on the trailer battery to power the engine electronics and
this could amount to 100-200 miles or more of distance before exhausting
both batteries (at which time the engine would quit).  The off setting
allows me to tow while depending on the solar panels on the trailer to
charge the batteries.  BTW, the solar panels on a sunny day, when the
Hellroaring is switched to bi-directional can also help power the engine
electronics (another advantage of solar).  

The cost is not significant with even the more expensive high efficiency
switcher such as the Hellroaring, costing around $150.  Installation is
fairly simple and can be a DIY.

You asked about wiring size.  The charge line should be 10 gauge AND the
ground wire should be 8 gauge as it has to carry not only the trailer
battery current return path, but the trailer exterior lighting return
path.  A 10 gauge charge line will typically allow about 30 amps of
charging current to flow to a discharged trailer battery.  This is
adequate to charge the battery over several hours of towing. But, the
charge line wiring has nothing to do with running down the tow vehicle
battery.  If these are undersized, you will simply not get adequate
charging to the trailer battery.

David