I just walked past a Tesla parked (unoccupied), with its headlights and taillights on, in the middle of the afternoon.
So I’m wondering what will happen..
With a traditional car, of course, we all know the results–after a couple hours the owner will return to find a car that won’t start, with a dead battery. . And probably be unable to even unlock the doors
But I know absolutely nothing about Teslas, or electric cars in general. Obviously
electric vehicles have a lot more battery power. So what happens when you leave the lights on ?
Also, a related question:
why in the 21st century is it still possible to accidently leave the lights on, after you turn off the engine? All the other electric stuff in the car turns off automatically. Why not the lights?
All the ‘normal’ car stuff, things like the windows, radio, headlights etc, still run off a 12v system with 12v battery. The car will then have some strategy for keeping the 12v battery charged. If the headlights are left on long enough to drain the 12v battery, the car won’t start and you’ll have to jump it. And by jump it, I mean like you’d do with any other car, you’re just connecting the 12v systems.
In fact, the cars that don’t have a physical key will and/or electronic doors have to keep this in mine. My Mustang Mach-E has power leads accessible from outside the vehicle. Hooking 12v up to them opens the hood so you can access the rest of the 12v system and get the doors opened.
I can only reply for how Tesla does it. I expect other EVs are similar, but I can’t say for certain.
There are two batteries:
The 12v battery, just like any other car, AKA the low voltage (LV) battery.
The big battery that the car uses for motive power, AKA the high voltage battery (HV) or traction battery.
The primary job of the 12v battery is to close the contactor to bring the traction battery online. Once that is done, the car has lots of power available, and can run the lights probably for days.
Just like your phone or laptop, Teslas have several power modes. In “idle” the traction battery is connected, and is being used to power the computer, and cameras. Sentry mode is “idle”, and the traction battery is connected. In “sleep” the traction battery is disconnected, and the 12v battery monitors a few sensors to know when to wake-up the rest of the car.
My guess would be that the Tesla you saw was in sentry mode, and it turned the lights on as a warning that someone was too close.
Normally, the lights on a Tesla are in “auto” mode, like many other cars have. If they are turned to “on”, then when you go to leave the car it will remind you that the lights are on, just like any other car. I think, but I won’t say 100%, that newer versions of the software will put the lights from “on” to “auto” when the car is put in park, which would be the equivalent of turning them off.
When the traction battery is connected, it charges the 12v battery. If the car is sleeping so long that the 12v battery gets low, then it will connect the traction battery to charge the 12v battery. At some point (probably months), the traction battery may get low enough that the car will not use it to charge the 12v battery, which may die. Simply leaving the lights on a few hours should not kill the 12v battery.
If you define “traditional” as pre- about 2010, then yes, a dead battery. Many many ICE cars of more modern vintage are smart enough to not run themselves out of battery to start the engine.
Tesla also has the leads to open the “frunk”. They are hidden behind the front tow hook. If the 12V is dead the leads attached to a 12v (or I’ve been told a 9V transistor radio batter) will pop the frunk so you can get at the 12V battery.
Without 12V the electric door locks, trunk/frunk, etc won’t open. (I assume many modern cars are like this).
Four years ago, James May (one of the presenters of the BBC program Top Gear) had a Tesla Model S with a dead 12-volt battery and he had to spend quite some time to get to it. Here is a Jalopnik article describing his ordeal and here is a YouTube video.
I wish they’d make the frunk latch work on 5v-12v. A random person, I think, is more likely to have access to a 5v power bank than a 12v source. Granted, you’ll still need 12v to jump the battery, but you can keep a jump pack in the frunk.
This is mind-boggling stupidity!
The jalopnik link says that James May had to pull two emergency latches, remove multiple panels, and remove a heating duct, just to reach the dead battery. And this was for his car which was parked inside his own garage, fully charged !
Because “the car’s charging system turns off when the big battery is fully charged, and, then, the big battery stops charging the smaller one, too.”.
How stupid are the engineers who designed this? You have a simple 12V battery which is essential, and without which the entire car is dead…And 6 inches away, you have a huge battery pack with a complete charge—but you don’t think to use it to put a trickle-charge into the critical 12V battery?
As far as being unable to unlock the doors, it depends on how ‘traditional’ the ICE car is. Our 2009 Accord and 2016 Civic both have actual keys as part of the remote key fob that you can use to open the door if either the battery in the key fob or the car battery itself is dead.
Last fall, we bought a 2025 CR-V which just has the remote fob. My wife’s been the sole driver of the CR-V so far; I’ll have to ask her if she knows how to get into the car if either battery fails.
I recall reading about one of the earlyDetroit compact cars, where you had to remove the steering column to change the fourth spark plug.
The Model S - especially the early ones - were not noted for simplicity of construction.
I assume part of the problem - which is why it is an issue with the Model 3 also - is that the feed from the big battery is computer-controlled so this would require the computer to be running 24-7. OTOH, systems like Sentry and the radio listening for the key are already running, so I’m sure there would be a solution. The other issue is that 90% of these 12V problems are a battery that cannot hold a charge, meaning tha “trickle” would have provide enough power to be able to actually be the 12V feed for any real usefulness.
A lot do. When I turn my car (2023 Sonata Hybrid) off, the headlights stay on for about a minute (if they were previously on) then turn off. If I turn them on when the car is off, they automatically turn off after 30 minutes, or at least the dashboard flashes a warning that they will. I suspect the Tesla you saw has the same features.
Our Lyriq has one keyhole for the tailgate. That’s it. A Ford Mach-E recently locked a toddler inside and bricked itself such that a tow truck driver as well as a Ford tech could not open anything. They ended up breaking a window. I’m not sure what are the implications of the 48V architecture in the Cybertruck but it seems obvious/likely that a 12v jump pack won’t work.
My late friend was an accountant who worked for a number of different small companies. He told me the story from back in the late 70’s or early 80’s of the fellow who drove into the dealership with his new fancy Cadillac with electric everything, and tapped on the window saying “get me out”. Something failed under the hood, and all the fancy electronics, including door locks had stopped working.
I should note too that a common problem with my Tesla Model 3 is warning newbie passengers to use the electric door button. The Model 3 doors have a not-obvious lever that you can pull to open the door in an emergency. Like many new cars, there is no top window frame, the window slides up a quarter inch or so into the metal trim channel at the top after closing (My BMW does the same). In an emergency, the risk of bending or pulling off that trim is a secondary consideration. But… don’t if you don’t have to, if the button works and lowers the window that quarter-inch to open cleanly.
A while back, someone posted pictures from an Uber they were in that had the door handles covered in tape, almost like they were trying to ‘trap’ the person in the car. A little bit of googling told me that people cover the door handles because they’re electric (sliding) doors and riders will yank the handle right off the door if they try to use it like a normal sliding door. This forces them to wait for it to open on it’s own.
I’m guessing Uber/Lyft give drivers these stickers since all I’m finding now, but the picture I saw was just tape. It did look a bit unsettling.
Also, I saw this:
Even on my Mach-E I have to yell directions through the window if I forget that my passenger (friend/family, not uber/lyft) has never been in my car. Push the button, the door pops open about a half inch, then use the handle to pull it open.
And since I always forget this car doesn’t unlock the doors when you put it in park, even if they understand what I’m trying to get them to do, they still can’t open it.
