Most hybrids and EVs keep a conventional 12V battery that runs electronics and wakes the car. When it fails the car can refuse to “start” even if the big battery is full. It is priced like any car battery replacement, and often must be AGM. Some cars hide it in the boot or under a seat, so check the manual and consider a professional. Never touch orange high-voltage cables.
See hybrid battery replacement cost and electric car battery replacement cost for the large pack.
Why it exists
The high-voltage battery powers the motor, but the car’s computers, lights, locks and the contactors that connect the big battery all run on 12V. When you “start” the car, the 12V battery wakes the system and closes the contactors. If the 12V battery is weak, the car can fail to start even if the main battery is full.
Typical symptoms of a failing 12V battery
- The car will not turn on or “ready” light does not appear.
- Warning lights or messages about low voltage or system faults.
- The doors, locks or key fob behave erratically.
- The car works after a jump or a charge, then fails again.
- In some EVs, the car loses connection to its app.
How it differs from a normal car battery
Many hybrids and EVs use an AGM battery, and some are in unusual places such as the boot or under a seat, where venting needs care. Some cars need registration after a replacement. The battery is cycled differently, because it is recharged from the high-voltage battery by a converter rather than an alternator. Always follow the manual: it may have specific procedures, and some cars have special jump points.
Replacing it
If you are comfortable and the battery is easy to reach, you can replace it like any other, with the same safety steps. Never touch orange high-voltage cables or open the high-voltage battery. If the battery needs coding or the location is awkward, use a shop or mobile service. Compare costs with the replacement cost calculator and check AGM vs EFB vs flooded for the right type.
Jump-starting a hybrid or EV
Check the manual first. Many cars have a designated jump terminal under the bonnet and a specific order. Never connect jumper cables to the high-voltage system. Our general jump-start guide explains the basics, but your car’s manual takes priority.
Care tips
- Take regular drives to keep it charged, since sitting for weeks can run it flat.
- Keep a portable jump starter suitable for your car.
- Have the 12V battery tested annually once it is over three years old.