second 12 volt battery - 2020 edition

If we're talking about a purpose-built refrigerator that uses about one amp-hour every hour to keep the food cool, and the battery is forty-five amp hours, and division still works the way Mrs. Silver taught me, the OP might end up having no problem keeping the food cold even through a gap of several hours or less between having the car READY.

It's certainly not off the scale of feasibility; it's the kind of thing you could set up, keep a meter around, observe for a while, and escalate to something more complicated if experience warrants.

If I did it, I would have a jump pack in the glove box to avoid any frustration if something didn't go according to plan. But I have that anyway. I would also want the refrigerator to have an automatic preserve-the-battery cutoff if the voltage gets low ... and it does.

 

As Chapman said, all of those things will 'work' with the standard battery, it's just down to your individual requirements, how long each device is on, how often you have the car in 'ready mode' etc as to wether you'll need to upgrade the battery or add a second one.

It should be pretty obvious if you start using the devices and get a flat battery before you're finished using the devices then you need a bigger battery.

 

The pragmatic approach in a nutshell, right there.

 

Oh, in READY it won't.

We've all been thinking about how long you might get away with leaving the car unattended and not READY, and still come back to safe cold food.

 

Right. And then the pragmatic conclusion would be that you need more battery if you want to be away that long. But I think you'll probably get comfortable run times.

An important consideration is how to know if the fridge stopped, and what the inside temps got to, and for how long.

Fancy solution is a datalogging thermometer. Seems like these new Dometics have some kind of feature where they can alert you on your phone (I guess provided they have power and cell coverage). I would almost be more interested in knowing that it has a small backup battery to just keep logging the temperature while off, and show me a graph when I got back.

I had a colleague who used a low-tech solution: if he was going out of town for any length of time, he would leave a coin or something on top of an ice cube in the ice cube tray in the freezer. If he came home and the coin was in the ice cube, he would know stuff had thawed, even if power came back on after and looked ok when he got home.

I guess it's not quite as complicated a problem with the Dometic, because if it turns off at some point while you're away, it's not going to turn back on and look all innocent before you return.

 

The battery could technically go flat in 'ready mode', if you were drawing more power than the DC-DC charger can provide (around 80a I think?) for a period of time, but that's not gonna happen with the devices you've listed.

I've done it before while driving in cars with big sound systems and small alternators

 

I think you are playing with fire.
And you are drawing too much power from a battery that wasn't designed for it.
Thus, the 2nd battery......
Keep that jump pack charged and handy...and set money aside for a new 12v battery,
sooner than you think....

 

Shut off from READY? Or from ACC or ON?

 

There is an in-line fuse for the socket, right? And you are using sufficient gauge wire, right?

Unless I've been taught wrong, I don't think that's how it works. When the car is in the READY position, you draw 12v from the high voltage battery through a DC to DC converter. The 12v battery is irrelevant and charging it is just another load for the high voltage battery. If you disconnected the 12v battery while the car was in ready mode, I think you'd still have 12v to everything else. A second battery would be important in ACC mode because then it is solely responsible for delivering 12v as the high voltage battery is not involved.

Yes, what does "shut itself off" mean?

 

The converter only outputs a certain amount of amps. Which is probably enough to run the computers
and accessories, and charge the 12v battery. When you put other things on it, you are overloading it.