Why do EV 12-volt batteries die, and how do you avoid it?

It surprises many new EV owners: the car shows plenty of range in the app, yet it won't unlock or switch on. The usual culprit isn't the big traction battery at all. It is the small 12-volt battery that many EVs still carry, and it can go flat for some of the same reasons as the battery in a gasoline car, plus a few of its own.

What does the 12-volt battery do in an EV?

It powers the car's everyday electrical systems, while a separate high-voltage battery drives the motor. Nissan's Leaf manual describes the two. The 12-volt one "is the same as the battery in vehicles powered by gasoline engines" and powers systems such as the audio, the airbag system, the headlights and the wipers. The high-voltage lithium-ion battery drives the motor and "also charges the 12-volt battery" (Nissan Leaf owner's manual). In the Leaf, a DC/DC converter in the power delivery module does that job. In the Hyundai, Kia and Genesis EVs named in recent recalls, it is the Integrated Charging Control Unit, or ICCU.

Not every "12-volt" battery is lead-acid. Tesla's Model 3 manual calls it the "low voltage (Lithium-Ion) battery" (Tesla jump-starting guide). Check your manual before you buy a charger or a replacement.

When does an EV's 12-volt battery get charged?

Only at certain times: the high-voltage battery doesn't top up the 12-volt battery continuously. This is the key to most EV 12-volt failures.

So a 12-volt battery in an EV can spend long stretches supporting parked loads with only occasional top-ups. Our inference is that this makes the battery's condition and the car's parked draw matter as much as they do in a gasoline car.

Why do EV 12-volt batteries go flat?

These are the causes the makers themselves name:

• Things left on while the car is off. Nissan warns against "installation or extended use of electronic accessories" such as phone chargers and GPS units when the EV system isn't running. It also says long use of the power seat when the car isn't in READY will discharge the battery. And it warns that operating the charging timer repeatedly after charging has finished "may discharge the 12-volt battery."
• Rare or short trips. Nissan lists a vehicle that "is not driven regularly and/or only driven short distances" as a cause of discharge, and says the battery may then need charging to stay healthy. That is the same advice given for gasoline cars.
• Parked features that keep the car awake. Tesla's list of standby energy users includes Sentry Mode, USB power kept on, climate features such as Camp Mode, Pet Mode and Cabin Overheat Protection, and streaming or gaming (Tesla battery information). Tesla lists them to protect the main battery, but they draw it down, and a main battery run down to empty takes the low-voltage battery with it (next point).
• A flat main battery. Tesla says that if the main battery reaches about 0%, the low-voltage battery is no longer supported. A car left that way "may not be possible to charge or use ... without jump starting or replacing the low voltage battery". Tesla estimates an unplugged Model 3 can lose about 1% of its main battery a day parked, and says the rate varies with factors such as cold weather.
• A fault in the charging unit. In the US, Hyundai recalled certain 2022–2024 Ioniq 5, 2023–2025 Ioniq 6 and Genesis GV60, GV70 Electrified and G80 Electrified vehicles because the ICCU "may become damaged and stop charging the 12-volt battery, which can result in a loss of drive power" (NHTSA recall 24V-868). Kia recalled certain 2022–2024 EV6 models for the same problem (NHTSA recall 24V-867). In both recalls, dealers inspect and, if needed, replace the ICCU and its fuse, and update its software. Both replace earlier recalls (24V-204 and 24V-200), so cars fixed under those need the new remedy too.
• An old battery. Like any 12-volt battery, it wears out. Hyundai covers the standard 12-volt battery for 3 years or 58,000 km (36,000 mi), separately from the EV battery warranty (Hyundai warranty handbook).

What are the warning signs?

• The car won't respond. Tesla says that if the car "is unresponsive and does not unlock, open, or charge", the low-voltage battery may be discharged.
• The power switch won't leave OFF. In a Leaf with a discharged 12-volt battery, "the power switch cannot be moved from the OFF position."
• A 12-volt battery charge warning while driving. In a Leaf, a chime sounds and a message appears. Nissan says a light that stays on in READY may mean the DC/DC converter "may not be functioning properly". It says to "immediately stop the vehicle in a safe location", put it in Park, apply the parking brake and have the system checked, and not to charge the 12-volt battery while the light is on. In the Hyundai and Kia recalls, a 12-volt battery that stops being charged could lead to a loss of drive power, so treat this warning as urgent.

How do you jump-start an EV with a flat 12-volt battery?

Use the procedure in your owner's manual and nothing else, because jump-starting procedures differ a lot between EVs.

• Nissan Leaf: a gasoline car can jump the Leaf's 12-volt battery, but the Leaf can't be used to jump another car. Nissan says to connect positive to positive and the final negative clamp to body ground, "not to the 12-volt battery". Don't jump-start while the main battery is charging. Once the car is in READY, disconnect the cables and keep it in READY for over 20 minutes to charge the 12-volt battery. Then have the battery tested at a dealer.
• Tesla Model 3: the manual's procedure uses a portable jump starter: positive to a red jump post under a maintenance panel at the front, negative to a ground bolt next to the brake fluid reservoir, and the negative removed first afterward. If you're using another vehicle, refer to that vehicle maker's instructions. A Model 3 "cannot be used to jump start another vehicle."
• Everyone: wear eye protection, keep sparks and flames away from lead-acid batteries, and never jump-start a frozen battery. Nissan warns that one "could explode". Jump-starting only wakes the car's electronics; it doesn't charge the main battery.

Stay away from the high-voltage system. EV high-voltage systems typically run at 400 to 1,000 volts (AFDC). Nissan and Tesla both say high-voltage cables are orange, and that high-voltage parts and cables must not be disassembled, removed or replaced; our advice is not to touch them at all. Only connect to the 12-volt terminals or jump posts your manual shows. If the car still won't wake, call roadside assistance or book service.

How do you avoid a dead 12-volt battery?

1. Check for open recalls on your VIN, especially on 2022–2025 Hyundai, Kia and Genesis EVs.
2. Keep the main battery charged, and plugged in when parked for long. Tesla calls leaving the car plugged in when you're not using it "the most important way" to preserve the main battery, especially if it will sit for several weeks.
3. Limit parked features such as sentry or camp modes, and unplug accessories when the car is off.
4. Don't sit in accessory mode for long with the car not ready to drive. Leaf models have a 12-volt battery saver that cuts power after a while in ACC or ON, in Park with the doors closed.
5. Drive regularly, or charge the 12-volt battery if the car mostly sits or only makes short trips, as Nissan advises. Use a charger your manual approves for that battery type.
6. Have it tested with age, and after any jump start. For flooded batteries like the Leaf's, Nissan also says to check the fluid level more often in hot climates.

For how a parked car's draw adds up, see parasitic battery drain and how long a car can sit. For reading a 12-volt battery's voltage, see our battery voltage chart. For the high-voltage battery, see how much EV battery degradation is normal and whether DC fast charging hurts an EV battery.