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Tesla battery dies while plugged in and owner gets locked out — here’s why

tesla model s
(Image credit: Tesla)

Did you know electric cars actually have two batteries? There’s the big one that drives the car, and a regular 12v car battery that handles the smaller stuff like lights and the door.

The 12v battery can also die on you if you don’t drive very often, even if you keep your car plugged in all the time. It’s all thanks to a quirk of the electronics you might not have known about.

This was a problem faced by television car man James May recently, after discovering that he couldn’t unlock his Tesla Model S. Yes, even Tesla cars have this problem, much to May’s disappointment.

The thing about 12v car batteries is they tend to lose their charge if they’re not recharged often enough. Gas cars do this by driving around, while electric cars utilize power from the larger plug-in battery.

But as May discovered with his Tesla, which is kept plugged in all the time, the big battery stops providing power when it reaches 100%. So while the 12v battery was powering smaller parts of the Model S, like the computer, the larger battery wasn’t topping it up again. 

So it ran out of power, and because the 12v battery also controls the locks he couldn’t get inside to pop open the hood. Cue what can best be described as dismantling a Tesla-shaped jigsaw so May could jump start the battery.

It’s easy to assume that the big battery means electric cars have no need for a traditional 12v car battery. I know I did, until I actually did some reading ahead of purchasing one.

So if you are an EV owner make sure you don’t let that 12v battery die on you. And be prepared in case it does. Make sure you know how to get to it, and you have the appropriate tools to get it back to life. Whether that’s a simple pair of jump cables, a trickle charger or an emergency battery pack.

Nissan Leaf

Nissan Leaf (Interior) (Image credit: Tom's Guide)

Thankfully, not all cars are as tricky to handle as a Tesla. I went out to check my own Nissan Leaf, and the biggest problem I had was actually finding the hood latch. Access to the battery isn’t blocked by plastic sheets, and like many modern cars there’s still a backup lock and key mechanism that doesn’t need power.