Did you know that electric cars have two batteries? One is the big battery that powers the car, and the other is the regular 12V car battery that handles the lights, doors, and other small things.

The 12V battery can also run out if you don't drive very often, even if you keep your car plugged in all the time. This is all thanks to quirks in your electronics that you may not have known about.

This was the problem recently faced by James May, a TV car guy, who found he couldn't unlock his Tesla Model S.

Batteries in 12V cars tend to lose charge if not charged often enough. Electric cars use power from larger plug-in batteries.

However, as May discovered with the Tesla, the larger battery stops supplying power when it reaches 100% because the Tesla is always connected. So while the 12V battery was powering the smaller components of the Model S, such as the computer, the big battery never replenished it again.

Therefore, he could not get inside to open the hood because he lacked power and the 12V battery also controlled the locks. He dismantled the Tesla-type jigsaw puzzle so that May could jump-start the battery.

It is easy to assume that the larger battery means that an electric car does not need a conventional 12V car battery. I did, but I did some reading before I actually bought one.

So if you are an EV owner, make sure you don't run out of 12V batteries. And be prepared for any eventuality. Make sure you know how to get the battery and the proper tools to revive it. It could be a simple jump cable, a trickle charger, or an emergency battery pack.

Thankfully, not all cars are as unwieldy as Teslas. I went out to check out my Nissan Leaf and the biggest problem was finding the hood latch. Access to the battery is not blocked by a plastic sheet, and like many modern cars, there is still a backup lock and key mechanism that does not require power.