I ask because I've got a CVT tent that requires you to run a cable to a 12v outlet to power the LED lights in the tent. I can plug it in fine but there's only power when the key is in the ACC position. If I leave it there all night, would it kill the battery? No other accessories would be running and I'd turn the light off eventually ...

More than likely it would kill the battery however that depends on many factors...
How's your battery charge and age of the battery?
What's the reserve capacity and amp hour rating of the battery?
What's the draw of the lights you want to power?
So many unknowns but it doesn't sound like a great idea. Even with nothing ON the vehicle will draw some power in addition to the lights you want to run.
There's got to be a better option. At the minimum if you plan to go this route carry a jump box in the truck.

Isn't there an automatic shutoff after like 20 minutes? I could be wrong but I thought so... the few times I ran the radio it shut off so it might be the radio vs ACC.

What about a small solar battery setup?

Clip the ends off the cable, install clips on the ends and connect straight to the battery...done!
Or run a dedicated hot (all the time) socket to wherever and plug it in...done!

Maybe lug around a 12v?

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I think nearly all vehicles are supposed to have some form of battery saver to allow car to start - but I've never been brave enough to try. Everyone should experiment and let me know!

In the Modifications, Write-ups, Quick Links and FAQ's section at the top of the forum is this: 12V Outlet Always On by @Antman

Just sayin...

I doubt that the 4Runner has one. Case in point: last year on a road trip I would leave the rear hatch open to listen to music or keeps the hatch lights (led) on while setting up camp and a couple of times it ran low to the point that it was hard to start, but finally did. Fast forward to January and I was meeting my wife out in town and after parking the truck, left it in ACC to listen to music while I waited. The truck died on me and didn't start without a jump. Now keep in mind it was roughly 30-40 degrees out and still the original battery (6 years, 90k miles). Changed it out with an odyssey and haven't tried running it down again.

To the OP, I'd look at the voltage requirements for your lights but I'd bet you could get away with cutting the ends off the cable and running it to a couple of 9V run in series. Take a look at YouTube, there are all sorts of videos about building battery banks.

When planning to use 'Any' accessories overnight (vehicle off), it's advisable to use a 'Dual battery set-up' to avoid any 'unnecessary... surprises' when you get behind the wheel again the next day! A lot of guys here on forum are using this system very successfully! There is also a large resource of threads here on the subject: How to install dual battery systems. Best to you,

rig

Dual batteries or a Goal Zero (or similar) battery pack, at a bare minimum you should have a jump pack.

What worse than camping in the middle of no where with a dead battery? Well there are a few thing but you get the point

Just found this cause I'm looking for a solution to the same problem. I have been trying to find a different cable to run off of an external batteryno luck yet thought

Ok. The price of a new battery after you accidentally drain it will set you back 80-150 dollars. A portable battery pack or multi purpose jump starter will probably cost you half that.

If you don't want to run a dual battery the best option is likely what @jrandom suggested above. Get a Goal Zero Yeti 400 or Anker Powerhouse portable battery pack. They both have a 12v socket that the tent lights can plug directly into. Plenty of juice to run the tent lights and charge other accessories at night and you can plug it in to the back outlet when driving around during the day to recharge. They are also both solar compatible.

you don't need a dual battery setup to run some LED tent lights, that is way overkill, even carrying a yeti 400 or anker powerhouse is overkill for this (a small power pack would be plenty)

just run an always on 12V outlet, LED strip lights like this are usually less than a few watts per foot. If you are someone who falls asleep with lights on and is worried about it you can get a cheap low voltage automatic disconnect (probably <$20) for the outlet. if you're solo in remote areas you should carry a lithium jump pack regardless of what you do.

If it is just for LED lights (don't know why I didn't catch that originally, guess I was going by the thread title?), buy a Luminoodle or a knock-off on amazon (I have a couple of color strands), kits come with their own power cell or will run off USB, some with remotes, other change colors, 5 feet to 25 feet. you can get them from $15-70+.

There was just a small Luminoodle group buy, I bet it enough people showed interest he would start up another, he started a list.

Can't reiterate enough, should still carry a dedicated jump pack regardless of what you choose. Too much bourbon and music... forgetting to turn off your key before passing out... nothing better than a car camp hang over and no way to get to the small town diner/cafe for a big cholesterol bomb breakfast