Isolator In Trailer Charge Circuit

[toyota_mdt_tech]

OK, I have always had trucks with an isolator, mechanical type, so when my trailer is hooked up and plugged in, it's connected as long as I am in the "run" position on my truck.

So when I shut it off, the trailer battery becomes isolated from the truck, so if I leave a trailer light in, it doesn't run the truck battery down.

I didn't realize this till I found my truck batteries dead from my trailer being plugged into it.

I am considering installing a continuous duty solenoid run by the key in the run position.

Has anyone else run into this?

Now that I am aware, I unplug my trailer when I am stopped with the trailer hooked up for a while.

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[AK Bob]

I haven’t had any battery discharge problems with my fifth wheel, but I generally unhook while parked.

You bring up a good point; thinking a 40A N.O. relay contact in the hot wire going to the trailer plug at the fuse box, controlled with auxiliary power, would be an easy, inexpensive solution.

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[toyota_mdt_tech]

I haven’t had any battery discharge problems with my fifth wheel, but I generally unhook while parked.

You bring up a good point; thinking a 40A N.O. relay contact in the hot wire going to the trailer plug at the fuse box, controlled with auxiliary power, would be an easy, inexpensive solution.

The solenoids I have used are rated at maybe 150 amps, probably overkill. I think a small relay may get warm.

I usually use a 30A Cole Hersee auto reset circuit breaker; this type will trip and reset if over 30 amps for too long.

It acts as a current limiter this way.

If the trailer battery were really low, my truck alternator would work hard to bring it up, maybe exceeding the circuit's rating.

This allows a little boost on and off till the battery level comes up, then the breaker closes and remains closed as long as the current is 30 amps or less.

Of course, that can be adjusted to 40 AMPS or whatever.

I'm curious what size wire is used with no limiter in place now.

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[AK Bob]

The solenoids I have used are rated at maybe 150 amps, probably overkill. I think a small relay may get warm.

I usually use a 30A Cole Hersee auto reset circuit breaker; this type will trip and reset if over 30 amps for too long.

It acts as a current limiter this way.

If the trailer battery were really low, my truck alternator would work hard to bring it up, maybe exceeding the circuit's rating.

This allows a little boost on and off till the battery level comes up, then the breaker closes and remains closed as long as the current is 30 amps or less.

Of course, that can be adjusted to 40 AMPS or whatever.

I'm curious what size wire is used with no limiter in place now.

Now I understand your concern, so your 12V supply to the trailer plug comes straight off the battery.

Mine uses what looks like #10 AWG going to the trailer plug but is factory fused at 30A.

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[toyota_mdt_tech]

Now I understand your concern, so your 12V supply to the trailer plug comes straight off the battery.

Mine uses what looks like #10 AWG going to the trailer plug but is factory fused at 30A.

But it's always hot, even with the key off.

Ideally, hot when the key is in the "run" position, not acc or off.

OK, 10AWG is good, probably a 30A circuit; what prevents it from charging heavier than 30 amps, let's say a large battery in the trailer is dead, start the truck up, 160A or maybe 220A alternator?

Sure, the fuse would blow, but it won't reset.

So you would have to charge the battery up first, then hook it to the truck, so it would maintain the fully charged battery but maybe not charge a low one?

I have a mind like an engineer is my issue, so I think too deeply about many things.

but all valid concerns.

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[AK Bob]

But it's always hot, even with the key off.

Ideally, hot when the key is in the "run" position, not acc or off.

OK, 10AWG is good, probably a 30A circuit; what prevents it from charging heavier than 30 amps, let's say a large battery in the trailer is dead, start the truck up, 160A or maybe 220A alternator?

Sure, the fuse would blow, but it won't reset.

So you would have to charge the battery up first, then hook it to the truck, so it would maintain the fully charged battery but maybe not charge a low one?

I have a mind like an engineer is my issue, so I think too deeply about many things.

but all valid concerns.

Yes, I agree unless you can control the current going to the low battery, you would need 4 or 2 AWG going to the plug and the trailer battery.

The only practical way to solve the issue, in this case, is to limit the current going to the low battery.

Having the truck battery disconnected from the trailer battery when the truck is not running will prevent a possible no-start condition.

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[toyota_mdt_tech]

Yes, I agree unless you can control the current going to the low battery, you would need 4 or 2 AWG going to the plug and the trailer battery.

The only practical way to solve the issue, in this case, is to limit the current going to the low battery.

Having the truck battery disconnected from the trailer battery when the truck is not running will prevent a possible no-start condition.

I did install a 30A auto reset circuit breaker at my trailers battery, this will pop and reset over and over as it charges in burst, and when the battery is up, it will close and continue to charge.

Just caught me off guard one day, fortunately, I checked it a day before heading to my cabin, found battery dead.

Put on battery charger.

A big alternator can get warm/hot trying to charge a dead large capacity battery.

Anyone in here have the dual alternator option?

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