how can I get "alternator only power" from truck?

[jeffsw6]

I would like to change my 7-wire center pin to provide charging power for an accessory battery. I only want it to be powered when the engine of the truck is running, that way the truck battery won't run down if things are left on while the rig is parked.

How do I get power from the truck only when the engine is running? I would rather not have to tear into the dashboards and spend all day poking at things with a VOM if I can just tap into the alternator cabling inside the engine compartment or something.

 

[Texan4Life]

maybe a battery isolator?

Or maybe have your power wire be fed through a relay so that you can turn your power wire on and off from the cab? or just skip the switch and power the relay straight from something that is only on when the ignition is (like a cig lighter)

So then when the ignition is on (engine running) the relay closes giving power. turn the engine off, relay opens, and your power wire is kaput.

so then you would go something like this

Battery --> fuse or circuit breaker ---> relay ---> trailer plug

ground your relay, and then with the signal wire run to a power source that get's power with the ignition.

...also why not use the black wire (12v battery power)...

 

[jeffsw6]

I don't want to use a switch in the cab because I might forget to turn that switch off and then deplete the truck battery. Same reason why I don't want to just charge the trailer battery from the black wire, if the trailer loading lights get left on at a job site, then I would have not only a dead trailer battery, but a truck that can't start, too!

I know about relays but I do not know where the easiest place is to get the close signal from. The cigarette lighters in my Suburban are always on, even when the truck is parked and lights/etc are all turned off. This is nice for charging cell phones but it means I can't use that for the relay. Not sure about the cig lighters in my other truck.

I was hoping I could just tie into the alternator terminal or something else straight-forward.

 

[Texan4Life]

well I got to looking up battery isolators, and wow the prices have come down the last time i shopped them

So with the whole relay setup I was basically making a manual battery isolator. since usually they were around $100

Battery isolator - Wikipedia, the free encyclopedia

for example:
[ame=http://www.amazon.com/NOCO-IGD90HP-High-Performance-Battery-Isolator/dp/B00316KFV4]Amazon.com: NOCO IGD90HP Grey 90 Amp High-Performance Battery Isolator: Automotive[/ame]

 

[jeffsw6]

A "battery isolator" kit is not what I need. You can use that to dedicate a battery to starting purposes but you can't make it do this. I would have to add a second battery to the trucks, and there is no room in the engine compartments for more batteries.

 

[LD1]

First, double check that 12v wire on the plug. I dont know, but I thought they were only hot with key on?

Unfortunatally though, you are going to have to do some prodding with the VOM because wether you wire it straight, or use a relay, you HAVE to find a wire that is switched by the key. Simply tapping the alternator is no good since it is a solid connection straight to B+. So that would be just like wiring your trailer battery straight to the truck battery. You need yo do some prodding. Their may be things in the underhood fuse box that are hot ONLY when key is on. It is simple, check some spare fuse locations for one that DONT have power. Now turn the key on. If it has power now, use that for your tap:thumbsup:

 

[timster2]

A "battery isolator" kit is not what I need. You can use that to dedicate a battery to starting purposes but you can't make it do this. I would have to add a second battery to the trucks, and there is no room in the engine compartments for more batteries.

Your information is incorrect you don't need a second battery in the engine compartment. The battery isolator "isolates" each battery from the other. When the vehicle's alternator is putting out power it directs the alternator output between the two batteries. You do not need both hooked up at the same time. You do need to make sure your battery isolator is capable of handling the full output of your alternator as it can direct all of the alternator output to only one battery. Make sure you use some kind of circuit protection to your trailer so you don't burn up any wires in case of a short or over current.

A common use is to dedicate one battery for starting but not the only use for this system. I have designed and wired several truck systems with this setup for accessory inverter systems. The only thing different here is a trailer plug in the circuit.

 

[tcartwri]

I use this on our service truck. It only connects when it senses a charging voltage level from the alternator

Automatic charging relay

 

[Fixemall]

Before you get all complicated, how about a lighted switch AND an LED on the trailer that let's you know it is on, between the two I doubt you will forget to power it down.

 

[tcartwri]

There is one other important issue in a circuit like this, and that is if you run the battery down in the trailer, and then reconnect to the truck for charging, it will draw a potentially very large current until the battery voltages level out. So your hot wire for this circuit should be at least a #8, and an equal sized ground wire from the back of the plug to the truck chassis.

This is the reason they put a small dc-dc chargers in battery breakaway circuits.

This would be a fire issue on the truck, so make sure you think it through completely.