Why do electric cars have a 12v battery?

  • 12-volt batteries power the low voltage systems in your electric car that always require power, like the ECU, locking system and 4G modem
  • Electric cars use 12v batteries for low voltage accessories because it is simpler, cheaper and safer than using the 400v or 800v main battery
  • While lead-acid 12v batteries are the most common type, Tesla now uses a lithium-ion 12v battery and other car makers will probably follow suit

Logic dictates that an electric car with a huge lithium-ion battery shouldn’t need a 12v battery, yet most electric cars have one.

You might also notice that while your EV has a state-of-the-art lithium-ion battery, the 12v battery is an old lead-acid type.

Why? Because like internal combustion engine cars, electric cars have electrical systems that need to run all the time. A 12v battery is the most efficient way to power them because they require a very low voltage.

You could run all 12v systems off the main battery pack, but this would require a voltage conversion and add unnecessary complexity to the battery management system. It is safer and cheaper to just use a 12v battery!

Want to know more? This guide answers all your questions.

Why do electric cars have a 12v battery?

There are three reasons why electric cars have a 12v battery:

  • Cost. 12v lead-acid batteries are dirt cheap, and there are hundreds of electrical systems in electric cars that run on 12v power. It is cheaper to use a 12v battery than it is to engineer a battery management system that creates a DC-DC conversion from 400v (the voltage that most electric cars run on) to 12v.
  • Simplicity. Lead-acid 12v batteries have been used for over fifty years, offering proven durability and a relatively long lifespan. Designing an electric car to run low-voltage systems with a 12v battery simplifies engineering.
  • Safety. Would you want your keyless door handle to be powered by a 400v or 800v system? 12v is significantly safer and more efficient. By disconnecting the main battery back from critical systems, the higher voltage is isolated.

How is the 12v battery in an electric car charged?

A 12v battery can be charged by the electric motor in drive, although in most electric cars, it is topped up by the main battery directly via a converter that steps down the voltage from 400v/800v to around 14v.

It is more efficient and economical to run the ECU, central locking, alarm, 4G modem and other critical and accessory systems off 12v and have the main battery or electric motor top up the 12v battery in drive or when plugged in.

You can’t charge a 12v battery with an 11kW charger, or any other charger, because it is charged by the electric vehicle itself.

What does the 12v battery do in an electric car?

12v batteries power electrical systems and accessories that run on a low voltage, like alarms, lights and central locking. Systems that run on a low voltage have their own battery to simplify the battery management system.

The 12v battery also powers the car’s ECU (engine control unit), the brain of the car, and the electric motor on start-up. So, without a 12v battery, you can’t drive!

What happens when a 12v battery in an electric car dies?

If your electric car is completely dead, the 12v battery will need a jump start, even if you have power in the main battery pack. This is because the 12v battery on start-up powers your car’s electric motor (which starts the car).

To jumpstart a 12v battery in an electric car, you follow the exact same process as an internal combustion engine car:

  • Attach the positive (red) clamps
  • Attach the negative (black) clamps
  • Start the donor vehicle
  • Start the recipient vehicle
  • Disconnect the cables, and drive!

If you are low on charge in your main battery pack, just plug your car into your home charger or a public charger. If you don’t have the luxury of a driveway, here are a few ways to charge an electric car with no driveway.

How do electric cars start?

An electric car is driven by an electric motor powered by the main battery pack 99% of the time, but not on start-up. On start-up, the 12v battery powers up an electromagnetic field in the electric motor that makes it spin. The spin in the electric motor starts your car, much like the alternator in a petrol car.

Why lead acid for 12v batteries?

Lead-acid is significantly cheaper than lithium-ion, so the simple reason 12v batteries in electric cars are lead-acid is that they are cheap.

Tesla, however, will start using a lithium-ion 12v battery very soon, replacing the lead-acid battery in new Tesla vehicles. It is smaller and lighter than the old lead-acid battery and has greater energy retention, especially in cold weather.

Lead-acid 12 batteries will probably be phased out by lithium-ion 12v batteries over time, just like lithium-ion main batteries will be phased out by graphene EV batteries (next-gen lithium-graphene batteries are a few years away).

Can I just replace my lead-acid battery with lithium-ion?

Yes, you can replace your electric car’s lead-acid battery with a lithium-ion one with no vehicle modifications. The lithium-ion battery should be compatible and will weigh around a third of the weight.

Be warned, though, that the 12v battery isn’t easily accessible in some electric cars and replacing it yourself can easily take more than an hour. 

Also, swapping out the 12v battery will void your vehicle’s 12v battery warranty, which may also void the warranty on 12v electrical systems. If you are unsure about this, you should ask your vehicle manufacturer before proceeding. 

Will 12v batteries in electric cars be phased out?

It is highly unlikely. 12v batteries are a convenient, cheap and safe way to deliver power to electrical systems that always require power. Keeping them powered by the main battery pack would require expensive engineering with a DC-DC conversion that steps down the voltage from 400v/800v to 12v. The extra engineering required for this is enormous, and it is unnecessary when 12v batteries work perfectly fine.

Alfred drives a Tesla Model 3 Standard Range Plus but has his eye on a fully-electric pick-up truck. He'd love an electric Ford Ranger, which should be a real thing in a few years!