Which circuits will turn off when I turn the engine off?

Which circuits will turn off when I turn the engine off?

Postby carlositoelperro [OP] » April 19th 2020, 5:13pm

Hi everyone, this is my first post because I just bought my 2005 Astro base model cargo. I am turning it into a stealth camper, and I am currently working on the electrical setup. I have a small portable power system that can be charged using a 12v cig lighter, and I plan to use that to run my fan, lights, etc. (there will be no fridge or anything else that draws a lot of power)

The problem I have is that the cig lighter is always on, even when the engine is off, so I would risk draining my battery if I use the cig lighter to charge the portable power unit and forget to unplug it. So, I have a few ideas and questions and I am hoping you guys can help me out. warning: I am a newb and have very little idea what I am talking about - hence why I'm here to ask.

Option 1 (preferred) - I've noticed that if I manually turn on the interior dome lights and then turn off the engine and close the doors, that the dome lights turn off automatically after about 15 minutes. Well, I don't need dome lights at all, so I am wondering if I could simply remove the dome light, extend the wiring, and install a cig lighter at the end. That way I could keep the portable power unit charging while the engine is running, but not have to worry about remembering to unplug it when I turn off the engine since the dome lights automatically turn off. I prefer this because I like the idea of having a switch up front to turn the charging on/off, and because it requires the least wiring/work.

Option 2 - If I were to run new wiring to a new cig lighter, I assume I could connect it to an unused spot on the interior fuse box. Is it that simple? And if so, will this ensure that the power draw will stop when the engine turns off?

Option 3 - If I can't do option 2, could I install a battery isolator and run a wire with an inline fuse from that to a new cig lighter? If so, is it literally that simple or is there more to it?

Option 4 - Any other ideas you guys have, please let me know. Just know I'm trying to keep the cost down as much as possible.

Thanks so much and please be gentle, I have no clue what I am doing.
Original Poster [OP]
Fueling (Empty tank)
Fueling (Empty tank)
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Joined: April 2020
Alias (AKA): Jake
Van Model Year: 2005
Van Make/Model: Chevrolet Astro
Extra Info: Base model cargo

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Re: Which circuits will turn off when I turn the engine off?

Postby Rod's Trucks » April 19th 2020, 9:33pm

I am happy to jump in here.
Many ham radio operators have equipment and added batteries that they wish to do similar things with, like run equipment after the engine is turned off, without the danger of running down the vehicle battery.
Your Idea #1: That would work, but it probably more trouble than it is worth, due to giving up the dome lights ( probably used more often than you might expect, particularly when out with nature etc.). I would consider installing a small 12v relay to supply power to an auxiliary outlet (s). ( this combines your Option #1 and #2)
You can use the dome light circuit to trigger the relay, so the relay is energized when the dome light circuit is active. The relay acts like a switch, connecting the battery to the new outlet. The relay will handle more power than the dome light circuit is designed to handle.
There are also some very small 12v timer relays on E-bay They will turn off a circuit after a programmable delay.
There are lots of options to do just about anything you might want.
Option #3 ( battery isolator ): This is probably the best solution, if you plan on using two separate batteries on a regular basis. Again, rather than a dedicated 'battery isolator', you can use an inexpensive relay that is activated whenever the motor is running, and shuts off when the key is turned off. Whether the relay is deactivated when the motor shuts off, or when the key is removed, will be determined by from where you decide to take the activation signal. Fuses that provide a 'piggy-back ' connection can be purchased, ( I think they might be called 'tap' fuses). They provide a very clean way to tap into an existing circuit, for purpose like this.
Many of my ham radio friends, and myself, use a similar scheme. We are usually running high capacity auxiliary batteries ( larger than he stack vehicle battery)
A fuse box circuit that is only 'hot' when the motor is running, is taped. That wire goes to a manual switch, such that when the switch is ON, the tapped power will control a 12V high current relay ($15).
The relay connects a large cable from the positive vehicle battery terminal to the positive auxiliary battery terminal. The negative auxiliary battery terminal just goes to the vehicle frame as would any normal load device. We usually install an indicator lamp in a visible place, to show when that relay coil is energized.
It sounds somewhat complicated if you are electrically challenged, but it is really quite straight forward, in practice.
If you need more specific info, PM me.
Rod J
Issaquah, WA
Rod's Trucks
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