Using 12V while hooked up to the tow vehicle

sofraj wrote:
When I went to install the battery to winterize, nothing 12V would work. Long story short, 12V in the camper will only work when the tail/marker light cable in attached to the tow vehicle (2012 Toyota Highlander).

Your trailer is wired with two separate 12 vdc lighting circuits and neither has anything at all to do with the other - 1) the exterior lighting (running lights, brake lights) and 2) an interior system which includes interior lighting. Anytime there is power on Pin 4 of the trailer's Bargman connector, be it supplied by the trailer battery OR the trailer's converter when plugged into shore power OR the tow vehicle that interior 12 vdc system will be operational. When you say "12V in the camper will only work when the tail/marker light cable in attached to the tow vehicle" what is actually happening is that by connecting that tow vehicle cable to the trailer you're supplying + 12 vdc to Pin 4 of the Bargman connector and thereby supplying power to the trailer's interior 12 vdc system, including the lights. Since this seems to be the only 1 of 3 potential sources of power for this circuit that works that would suggest you probably hooked up the battery incorrectly, thereby blowing the reverse polarity fuse(s) in the converter ... so check for correct battery connection and check the fuse(s). You may have also tripped the circuit breaker connected inline with the cable connected to the battery's positive terminal ... if it's an auto resetting CB it will reset on it's own once the fault is removed, if it's a manual reset you'd have to reset it yourself. If the manufacturer installed a fuse instead of a circuit breaker then you'd obviously have to replace the fuse, of course once the cause of the fault is removed.

sofraj wrote:


Is this normal? I thought it should work stand-alone.

No of course it's not normal. You probably are feeding the trailer off the 12 V system in the tow vehicle.

YOU NEED A MULTI-METER, even if the current problem turns out to be something simple like a switch or fuse or improper connection.

And the maintenance charger should be a small AUTOMATIC tender type charger. A true trickle charger will not do.

When I am in these situations I always measure across my batteries to make sure they are being charged before I leave.

A fully charged battery will read somewhere around 12.6-7VDC using a multimeter around the battery terminals. When the charger is connected this voltage reading will jump up to 13.6VDC or more depending on the charging mode telling you the charger is connected and working...

Everytime you run down your batteries below 12.0VDC you are driving a nail in the Battery performance coffin... I suspect your batteries are damaged now if you indeed ran them way down in charge state. Hopefully they were disconnected somehow when you left them... A $15 dollar cheap multimeter from WALMART/LOWES is a great thing to have around... Use mine almost daily when around my RV units...

Roy Ken

Mine is located near the battery, I replaced the fuse with a circuit break same amperage as the fuse. I also used the manual re-set just in case of a short to ground.

Many have a three way splice on the tongue of the trailer where the umbilical cord ties to the battery cable and the trailer 12 volt distribution panel.
Some companies even hide the fuse or circuit breaker in the box at this location.

enblethen wrote:
Old-Bisquit: The fuse/circuit breaker for the power to the rig could be between the splice at the umbilical cord/ rig wiring and the battery.
OP: was battery disconnect while on trickle charge? If waas and installed with incorrrect polarity, you could have blown the fuse.

Umbilical cord does NOT provide trailer 12V SYSTEM except to charge battery. (and trailer exterior lights)
OP had battery out of trailer on trickle charger.then reinstalled battery

Trailer 12V SYSTEM only working when connects to tow vehicle via umbilical cord which ONLY provides a charge circuit...does NOT connect to trailer 12V except thru trailer battery

Blown reverse polarity fuses kills 12V system either from battery OR from converter

Old-Bisquit: The fuse/circuit breaker for the power to the rig could be between the splice at the umbilical cord/ rig wiring and the battery.
OP: was battery disconnect while on trickle charge? If waas and installed with incorrrect polarity, you could have blown the fuse.

If tow vehicle/trailer cord allows for trailer 12V to function then trailer battery is not fully charged.

The tow vehicle/trailer cord only supplies 12V Charge line to battery.....does NOT connect to trailer interior 12V system except thru charging battery
(connects/runs trailer exterior lights/brake/turn signals)

If trailer interior 12V system functions when connected to tow vehicle then battery in-line fuse is good, converter reverse polarity fuses are good and battery disconnect switch (if you have one) is on.

Only thing left is a low voltage battery

It should work without being plugged into the tow vehicle. You probably have a battery disconnect switch some where that was turned off. Or, you didn't have something connected properly.