11 replies to this topic

[morscode]

Just finished reading, after searching some very interesting opinions on Battery switches. So here is a question. When I pull into a rest stop or restaurant, it seems the consensus is to disconnect the plug from the tow vehicle, I would like to put a switch in the line that would disconnect the car battery from the trailer. I sounds as though you need two switches, one to disconnect the car and another to disconect the trailer battery. Presently I am disconnecting the trailer plug when I stop for over 30 mins and I disconnect the trailer battery when storing the trailer by undoing the terminals on the battery. Rather weighty when you consider pulling the battery out of the compartment to accomplish this. Any ideas? MM

[Brian]

On my older (non computerized) tow vehicle I installed a relay in the line from the battery to the trailer plug. The relay is hooked to a circuit that is on only when the ignition switch is on. This way, the power wire to the plug is automatically disconnected whenever the car engine is shut off, thereby isolating the trailer battery from the tow vehicle battery. I'm sure this could be done on a newer vehicle but I would be careful when working around late model electrical systems as one slip could cause serious damage to the cars' computer.
As far as the battery in the trailer, I have never used any kind of disconnect switch in my campers, and more than once have had a dead battery because of someone leaving a light or something on. Just last month I apparently bumped the power switch to the pump (in my non-Casita camper), and since the trailer was winterized & the water lines were empty and the system drain valves were open the pump apparently ran dry until the battery was dead. A disconnect of some kind would have prevented it.

[Don in OKC]

MM:

Here's one way to do it.

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[Don in OKC]

MM:

Here's where I put the (trailer) battery disconnect switch.

This was a 'spur of the moment' installation of two years ago, and I mean to do it right (one of these days).

Perhaps it'll give you an idea?!

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[Fred Rosin]

I'm lost here. I bought my trailer used, and the highly informative (just kidding of course) owner's manual didn't mention disconnecting the battery that I recall. I've been using my trailer for about 7 months and have never given it a thought. Why do you need to disconnect it?

Fred Rosin
San Antonio

[morscode]

Fred, I am not a electronic or electrical genius but as been pointed out if you inadvertantly leave something on such as a light, or any DC powered item for a long period of time with out power to the charging unit you can run your deep discharge battery below recomeneded state of charge for a deep discharge battery. I store my trailer in a storage lot without electrical power and find my self wondering if I left anything on. With the Battery disconected there is no worry. MM

[Don in OKC]

Why do you need to disconnect it?

Fred:

A 'silent culprit' is the water pump switch. As long as you have water pressure, the pump doesn't run. But if ya forget............

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Ya probably won't care until you've had a 'dead battery experience'. A disconnect switch is one of those things that doesn't cost much to install and the 'peace of mind' is worth it.

Casita, Scamp and other small trailers don't come with a battery disconnect switch, but the big RVs have em! Admittedly it's not such a big deal to recharge one small battery, but if you believe in Murphys Law, it'll come at the worst possible time.

[Ron and Bernice - Florida]

I find that the antenna amplifier is the easiest thing to leave on by mistake. I left mine on for two days after the Rock Crusher Rally. I am also afraid someone will walk by and turn on one of the outside lights and leave it on, while it is in the storage lot.

I am going to add one. Bob Hunt posted an easy wiring digram to follow, if you need help.

[Steve LaBroad]

Fred,
Other hidden drains are inverters left plugged in. They draw a little current in stand by. Some people install car radios in their trailers. The clock and station preset memories draw current. Battery monitors (volt meters) can also draw current.

Don's solution is elegant if you can find where the 12v plus line from the Bargeman connector is located. Don can give us an idea where he tapped into his.

The marine industry already has battery isolators that disconnect the starting battery from the "house" battery when the starter battery falls below a certain voltage. It cuts out the starter (or car battery) while there's still enough current to start the engine. Here's my make a million dollar invention for Bargeman: The Deluxe Bargeman Connector with built in battery isolator. Youse guys are my witnesses. Just mail my check to the house!

Edited by Steve LaBroad, 17 January 2004 - 06:28 AM.

[Don in OKC]

Here's my make a million dollar invention for Bargeman: The Deluxe Bargeman Connector with built in battery isolator.

Steve:

All they would need to do is put a diode behind the terminal for the 'charge' wire. Allows current to pass one way buy not back 'upstream'. I don't know enough about 'lectric stuff' to know what size and such.

Already in the RV world is the set-up for towing your car behind a motorhome and wiring the 'toad's stop/tail/turn lights to work along with the motorhome lights. a diode kit is added to keep the toads battery and electronics isolated from the rest of the system.

[Don in OKC]

Don's solution is elegant if you can find where the 12v plus line from the Bargeman connector is located. Don can give us an idea where he tapped into his.

Steve:

Look for the trailer charge wire at the car battery. There you should find an added wire on the + terminal. The charge wire should run directly from the battery to the brake controller.

Then check (with a continuity or volt meter) to see if it's the same as the 'charge wire terminal' at the bargman end.

[Steve LaBroad]

Huh! Learned something today. I just hate it when that happens?

Based on a discussion about relays in another forum I ran a little experiment. I read the owner's manual for my 2003 Explorer with trailer tow package. It says it has a relay protecting the trailer battery charge line. That would be the plus source to the trailer. A relay is just sort of an electric switch. In theory, if the relay is tied into the switched circuit, there should be no current at the Bargman plug unless the ignition key's on.

I had an extra male plug to which I connected two wires (plus and ground) and I connected them to a voltmeter. I plugged in the plug and got zero volts. I turned on the ignition and immediately got +12v DC. Turning off the ignition sent the voltage to zero again.

That means, if I turn off the ignition on my tow, the trailer switches to internal battery. The reefer, for example, can't drain the tow's battery. No current is going through the Bargman. I guess you DO get a few extras with a tow package.

It also means that if I plug the trailer into a generator (or solar panel) while plugged into the tow vehicle, the generator's current is stopped at the relay and won't send current "backwards" into the tow past the relay if the tow's ignition is off.

Cool!

By the way, referencing a discussion we've had about battery switches, the relay is protected by a 40amp fuse.