12 Volt Constant Power with Car Off

Where in the car do you want your device to be?

Maybe just look in the wiring diagram (more info) for a suitably-powered circuit that is nearby?

 

If you want your device in the rear hatch of a Gen 3, you've got the great convenience of the battery being right there. Add your fusing (best tapped on the car side of the 140 amp "MAIN" fuse there) and your low-voltage disconnect, and run your devices from right there.

 

What's the amp load your device will use? As always a wire screwed to the metal chassis is ground/negative wire. Then if you're pulling lots of amps run a wire direct to 12v terminal. If you aren't pulling lots of amps use a test light and clip one end to the chassis and use the pointy end into wires and plugs behind the dash until you find a wire that lights up with engine off and splice into that.

 

What’s the device, and how much amperage will it draw?

That’s not a “get out of jail free” card: in a few months you may need new battery due to steady depletion.

 

The connection point I recommend for larger loads back by the battery is this one right here, over the side of the 12v battery terminal:



That puts your tap on the protected side of the 140 amp "MAIN" fuse that is built into that terminal.

It is still up to you to supply a smaller fuse (or some smaller fuses) appropriate to the wiring and accessory (or accessories) you're adding.

Depending on the low-voltage disconnect you are using, some of them may have internal current-limiting features; the one I use will limit its output electronically to 15 amps without blowing a fuse. I still need to put smaller fuses on the downstream side for any of my accessories that couldn't handle a 15 amp fault current, and of course there needs to be one upstream of the LVD sized appropriately in case of a fault in the LVD itself or the wiring to it.

Worth thinking about. I am using an LVD in my setup, but I have it configured to switch on when the voltage goes above 13.x (which is when the car goes READY), and switch off an hour after the voltage drops below 12.8 (meaning the car's been switched off). So I get that extra hour, but I'm not using it to supply a constant drain with the car powered off. (The LVD I'm using also has an immediate cutoff if the battery drops below 11.8.)

If your load will really be constant, it's worth calculating how many amps it draws, multiply by the hours it will be on between uses of the car, and compare that to the 45 amp-hour capacity of the battery. If it ends up not being a substantial drain in those terms, then you just need to make sure you're using the car often enough and long enough to replace the energy (or you have some other charging arrangement in place).

 

You've got a couple different power consumption "averages" in that post.

If they give a figure of ¼ kWh per 24 hours, same as 1 kWh per 96 hours, just a shade over 10 watts average (that'd be 1000 Wh in 100 hr). That's probably what they are saying it will use to hold temperature once it is cooled down and cycling on and off.

The "other" average of 45 watts (roughly 3.8 amps times 12 volts) is probably its average power when actually running. So it will draw at that rate when you first hook it up, if it is starting out at ambient temperature, for as long as it takes to get down to the set temperature where it starts cycling.

Some of these coolers can also be line powered, so you can save that initial power draw by cooling it down on line power first, before taking it out to the car.

If it really averages ten watts when cool, that's less than an amp at 12 volts. If five hours is the longest power-off period you expect, that's less than five amp hours, out of a 45 amp hour battery, so you'll use about 11% of the battery capacity. If you start with the cooler warm and it draws 3.8 amps for the first hour and then an average amp for the next four, that would be about 20% of the battery capacity. That's a slightly taller order but you can do that from time to time with no big worries about the battery, and you can avoid it by precooling the cooler.

You might want to plan about an hour of driving (or just having the car in READY) to be sure you get that battery charged back up.

I would still recommend adding your connector in the usual way, negative to body ground (the same screw where the battery black cable screws to the body is an easy one to reach), and positive to the stud on the protected side of the 140 amp fuse, and with your own fuse added, sized appropriately for your wiring and accessory.

If you find a solar panel that can give you an amp or two for some of those five away hours, you might get your cooling just about for free.

 

In the photo back in #7, the fuse-protected connection stud is the one down over the side of the battery, that has a cable attached to it.

I just had my tap wire terminate in another ring terminal of that size, removed that nut, and added my wire there and put the nut back on over both. (One disconnects the battery ground terminal before doing such things, of course.)

 

If I were you I'd buy a 55amphour like this: https://ebay.us/vN4JsP for $113 and mount/box it next to this fridge so it's a fully portable system you can even float down a river with. Then when you're driving or vehicle is in ready mode you plug/connect it to car's 12v battery for charging. And beyond that add a solar panel and battery management system.