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Discussion Starter #1
Enclosed trailer with a battery to run lights when not hooked to truck tapping into the running light wire @ rear of trailer. When towing, battery gets some "charge" , but the battery will feedback to both my truck and the running lights of trailers when truck/key is off.

Putting a diode in-line should stop the feedback shouldn't it? Does the black banded diode go on the truck side "incoming" of the wiring, or the battery side?

I
 

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Enclosed trailer with a battery to run lights when not hooked to truck tapping into the running light wire @ rear of trailer. When towing, battery gets some "charge" , but the battery will feedback to both my truck and the running lights of trailers when truck/key is off.

Putting a diode in-line should stop the feedback shouldn't it? Does the black banded diode go on the truck side "incoming" of the wiring, or the battery side?

I
I think this is what you are looking for

 

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Without increasing the size of the wiring I doubt you will ever put any charge into your battery especially if you add a diode into the mix.

Even running 8B&S from the front of a vehicle to a trailer it will struggle to put any meaningful charge into a battery.

Relying on the vehicles tail light system will not help things because of voltage drop and it will not carry enough amps to be useful.
 

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If you have a 4 pin trailer harness, it is not designed for battery charging, you'd need a 7 pin harness with a 10 gauge or larger wire from the vehicle's battery to charge the trailer battery.

In the case of an rv and some enclosed trailers , the position lights are not hooked up to the battery, only the interior lights.

Sent from my SM-T700 using Tapatalk
 

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Discussion Starter #5
OK. Just looking for a way to provide trailer lights when not hooked to truck, have a small 12v lawnmower battery and hoped it would get some charge when I hooked up to the truck.

I still need to know which way to put the diode so my trailer light system will not work if I do put my spare battery in line to light my trailer as it sits.

Black line on the battery side or the trailer light side.
 

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OK. Just looking for a way to provide trailer lights when not hooked to truck, have a small 12v lawnmower battery and hoped it would get some charge when I hooked up to the truck.

I still need to know which way to put the diode so my trailer light system will not work if I do put my spare battery in line to light my trailer as it sits.

Black line on the battery side or the trailer light side.
(Answering as an electrical engineer: )
Current (viewed as movement of positive charge) flows from the + terminal of a DC source to its - terminal (or ground on modern vehicles.) You want to place and orient your diodes so that desired current flows from anode to cathode (the barred end, usually marked with a physical stripe.) The cathode will be closer to your trailer battery's + terminal than to (blocked) loads in the truck, so that undesired current would have to flow from cathode to anode (which it cannot.)

I don't think a 12V battery in the trailer, with the same chemistry as the truck battery, is going to charge properly with a diode put in series between it and the charging source in the truck. The voltage will be lower due to the diode's forward drop. You might get it charged halfway, however.

I strongly recommend posting a schematic of what you plan to build rather than hoping for the best and coming back with questions when it does not work. (You would then need a schematic anyway.)
 

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Discussion Starter #7
Thanks

Answering as a non-EE Production Manager, I just want the thing to work :)

I have learned what I need. I will put the diode in to prevent drain back into system and continue to use the garden tractor battery I have been using. It has lasted 3 years, and cutting off the trailer light feedback drain will more than double its life, charging from truck or not.
 

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Speaking as someone with no professional qualifications in electrickery!

You could look at using a VSR (voltage sensitive relay). These are used in boats, RVs etc, when there is a starting battery for the motor(s) and a 'house' battery to run accessories. These need to be connected in parallel when the motor is running so both get charged, but disconnected at lower voltage to save the starting battery from being discharged.

If you put the VSR on the trailer, it would connect trailer to vehicle if the voltage was high enough (motor running) and disconnect if not. You would not have the voltage drop that you'd get with a diode. Rolex's previous comments regarding size of wiring to get a reasonable charge current still apply.
 

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Speaking as someone with no professional qualifications in electrickery!

You could look at using a VSR (voltage sensitive relay). These are used in boats, RVs etc, when there is a starting battery for the motor(s) and a 'house' battery to run accessories. These need to be connected in parallel when the motor is running so both get charged, but disconnected at lower voltage to save the starting battery from being discharged.

If you put the VSR on the trailer, it would connect trailer to vehicle if the voltage was high enough (motor running) and disconnect if not. You would not have the voltage drop that you'd get with a diode. Rolex's previous comments regarding size of wiring to get a reasonable charge current still apply.
Something like this could well be part of a viable, robust solution. However, there is one detail above and associated difficulty which would be enough to make me go beyond just connecting a VSR sensibly. In the OP's usage, the tow vehicle battery and the trailer battery could get to very different charge states. Connecting a well charged battery to another battery which has been discharged very much will cause high currents to flow until the batteries' charge states equalize. Either the wire and VSR contacts will get hot, or if the trailer battery is a lot smaller than the other battery, it will limit current and get hot. I cannot predict a certain failure or disaster, but the prospect for problems is serious enough that I would want to see the issue dealt with responsibly rather than taking a "hope it works" approach.
 

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The isolator uses diodes, and as in the description in the link, there will be a voltage drop that may inhibit efficient charging. The VSR does not have that issue, but as Trepidator pointed out it has its own issues if the batteries are in a very different state of charge.
My DIY instinct suggests that might be solved by putting a suitable resistor in the circuit between the batteries (ie. a headlight globe?), but you would do well to look for an answer from the less electrically challenged amongst us.
:confused:
 

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well I use a battery isolater to charge my camper's battery when I drive and to prevent unintentional discharge of my truck's battery when I am using the camper.
 

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I don't know how new your tow vehicle is but modern vehicles have their alternator output controlled by the computer, after the initial start and after things under the hood heat up the computer cuts back the alternators output often as far back as 13.2v then when you add in voltage drop over the distance, then take away the 0.7v for the diode you don't have much to play with, if you were to use a relay instead of the diode you could gain 0.7v but I still think you are doing more damage to the battery than you are doing good.

I don't know what prices are like where you live but solar is very efficient and cheap to install, (here in OZ 100w panels are great value) just throw a 100w panel on the trailer's roof, it will charge the battery when on the move or parked during daylight hours and your battery will love you for it.
 
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