Updated · Batteries
What the 12-volt battery does
The big high-voltage battery moves the car. The small 12-volt battery boots it: it powers the computers, door locks, screens and the relays that connect the main pack. If it's dead, the car stays dark and won't go into Ready.
Instead of an alternator, a converter tops it up from the high-voltage pack while the car is on. In many hybrids, the high-voltage system starts the gas engine, so the 12-volt battery doesn't have to and is often smaller than a gas car's.
Why it dies
These batteries often fail with less warning than in a gas car, because there's no slow crank to hear. A dash message about the 12-volt battery deserves attention. Common causes:
- Age and heat, the same as any car battery
- Sitting parked for weeks, since the car charges it mainly while it's on
- Cameras, alarms and app features that wake the car while it's parked
- A light or accessory left on with the car off
Where it is and how to jump it
In many hybrids, the 12-volt battery sits in the trunk or under a rear seat, with a jump terminal under the hood in the fuse box. In EVs it may be under the front trunk lid or behind a panel. The owner's manual shows the right connection points.
Connect only to the 12-volt points the manual names, and never touch orange cables; they carry high voltage. Many makers also warn against using a hybrid or EV to jump another car, so check before you offer yours.
Replacement isn't always plug-and-play
Many hybrid 12-volt batteries are AGM because they sit inside the cabin or trunk, and some need a vent tube connected. Some newer EVs use a lithium low-voltage battery that calls for the exact part. A few hybrids with a lithium 12-volt battery have a reset button for it; the manual shows where.
Matching the original part and type matters more here than on most gas cars.
Locked out by a dead 12-volt
When the 12-volt battery dies, keyless entry and electronic door handles may stop working. Most key fobs hide a mechanical key, and the manual shows the backup way in.
If a child or pet is inside and you can't get in, call 911 first. Otherwise, we can jump or replace the 12-volt battery in your hybrid or EV where it's parked, and if you're locked out with the key inside, we can open the car.