12-Volt Batteries in Electric Cars

It surprises many new EV owners: an electric car with a huge high-voltage battery still has a small 12-volt battery too. And when that little battery dies, the car may not turn on at all. Here's why it's there, how to spot trouble and what to do about it.

Why an EV needs a 12-volt battery

The large high-voltage battery powers the motors. But most of the car's everyday electronics run on a lower voltage system, just like a gas car. That includes:

• Door locks and keyless entry.
• Interior and exterior lights.
• Infotainment and displays.
• Control modules and computers.
• Safety systems like airbags.

The 12-volt battery also does a critical job at startup. It powers the systems that close the contactors, the large switches that connect the high-voltage battery. If the 12-volt battery is too weak, the car can't connect its main battery, so it won't go into drive.

While the car is on, a DC-to-DC converter tops off the 12-volt battery from the main pack. Some newer EVs use a lithium-ion low-voltage battery instead of a traditional lead-acid one.

Signs your 12-volt battery is weakening

Watch for these clues:

1. Warning messages about the low-voltage battery or electrical system.
2. Slow or glitchy screens when you first get in.
3. Trouble unlocking with the key fob or phone app.
4. Random electrical faults that come and go.
5. The car won't power on or shift into drive.

These symptoms can be confusing, because the main battery may show plenty of charge. If your high-voltage battery is full but the car acts dead, suspect the 12-volt.

Why they fail

Low-voltage batteries in EVs can fail for several reasons:

• Age. Like any battery, they wear out over time.
• Constant small loads. EVs often keep modules awake for connectivity, app access and security features.
• Long periods of sitting without being driven or kept awake on a charger.
• Extreme heat or cold.

How to jump-start an EV

Many EVs can be jump-started at the 12-volt terminals, which may be under the hood, in the front trunk or behind a panel. Procedures vary a lot by model.

• Check your owner's manual first. It will show the jump points and the correct steps.
• A portable jump pack is often the easiest option.
• Never try to use an EV to jump-start another car unless the manual says it's allowed.
• Don't touch any orange high-voltage cables.

Replacing the battery

Replacement is usually straightforward on some models and more involved on others. Some EVs need the new battery registered with the car's computer, and some require a specific battery type. If you're not sure, have a shop or dealer do it.

Ask for the correct battery spec for your model. Using the wrong type can cause charging problems or warning lights.

To help it last longer:

• Drive the car regularly, or leave it plugged in if your maker recommends that for storage.
• Keep software updated, since updates sometimes improve low-voltage battery management.
• Carry a portable jump pack, especially in winter.
• Replace the battery at the first sign of trouble before a trip.

Some owners also keep a simple battery maintainer on hand for long storage, if the maker allows it. A shop can test the 12-volt battery during routine service, which is an easy way to catch a weak one before it strands you.

How warranties usually treat it

Factory EV warranties generally include a basic warranty plus longer coverage on the high-voltage battery and drive unit. The 12-volt battery is not part of that longer battery coverage. It's typically covered only for a shorter period, if at all, and often treated like a maintenance item afterward.

Many vehicle service contracts also exclude batteries as wear items, though they may cover related electrical components like the DC-to-DC converter. See our hybrid and EV components page for what plans commonly include.

How Warranty Wizards can help

We help EV owners compare service contracts from third-party providers and explain how each one treats batteries and electrical parts. We'll walk through the coverage before you buy. Get a free quote.