Every electric car still carries a small 12V battery, and when it goes flat the car cannot wake up, even with a fully charged main battery. This guide explains why the 12V battery fails before anything else, which UK models fail most often, what the recalls cover, and what an owner should do when it dies.
Table of Contents
- Why the 12V battery matters in an electric car
- Electric car won’t start: how to tell it is the 12V battery
- Why do electric cars have a 12V battery?
- How the 12V battery is charged in an electric car
- Why the 12V battery fails early in an EV
- Can an electric car be jump-started?
- How long does an EV 12V battery last and when to replace it
- Which parts of an electric car are high voltage?
- What to do when the 12V battery dies
- Conclusion
- Frequently Asked Questions
Why the 12V battery matters in an electric car
The 12V battery can leave an electric car completely unresponsive, even when the main battery still has plenty of charge. The reason is that the low-voltage system powers the control electronics and other systems needed to bring the vehicle’s high-voltage system online.
Definition. The 12V battery in an electric car is a small low-voltage battery, usually in the front compartment or under the boot floor, that powers the computers, locks, lights and the relays that connect the high-voltage battery to the rest of the car. It is charged from the main battery through a DC-DC converter, not by an alternator.
In the Which? survey of 63,334 UK car owners published in May 2026, 3.8% of electric cars aged under four years had a 12V battery fault in the previous twelve months. For petrol cars the figure was 1.4% (Which?, 2026a). The sections below explain the mechanism, the recalls and the fix.
Electric car won’t start: how to tell it is the 12V battery
An electric car with a flat 12V battery does not behave like a petrol car with a flat battery. There is no slow crank and no clicking starter. The car simply does nothing. The typical picture is a key or phone that will not unlock the doors and a screen that stays black or flickers. Warning lights may come on together and the car may refuse to shift into drive. A flat or failing 12V battery can therefore prevent the vehicle from starting and can disable electrically operated systems such as the door locks. The RAC lists dimming lights, a battery warning light and malfunctioning electrics among the signs of a weak 12V battery.
One point matters more than the rest. A high state of charge in the main battery does not prevent a car from becoming completely unresponsive if the 12V battery is flat.
A 12V battery can also read close to 12 volts on a multimeter and still fail under load. A voltage reading alone does not clear it. The 12V load test in the EV and hybrid high-voltage safety check measures what the battery delivers when the car demands current. The load test gives a much better indication of whether the battery can deliver the current the vehicle needs than a voltage reading alone.
Why do electric cars have a 12V battery?
Electric cars keep a 12V battery for the same reason every other car has one. The dashboard, lights, wipers, locks, airbags and control units all run on low voltage, and they need power before the high-voltage system is live. The RAC states that most electric cars use a 12-volt battery to power important systems (RAC, 2026). The AA’s EV technical liaison engineer, Lee Houston, explained that EV 12V batteries are typically smaller and lower in capacity than those in petrol cars. They do not have to drive a starter motor. They only switch the system on and run the 12V components (Houston, in Fleet News, 2024).
There is a safety logic as well. The high-voltage battery is disconnected from the car by contactors when the vehicle is off. The 12V system helps initialise and control the high-voltage system, including enabling the contactors to close when the vehicle needs to enter its ready state. A separate low-voltage supply lets the car keep the high-voltage system isolated when the vehicle is at rest and wake it safely when needed.
How the 12V battery is charged in an electric car
A petrol car charges its battery with an alternator driven by the engine. An electric car has no alternator. Instead a DC-DC converter steps the high-voltage battery down to around 14 volts and feeds the 12V battery and the low-voltage circuits.
The detail that catches owners out is when the converter runs. Houston stated that, on the vehicles he was discussing, the DC-DC converter charges the 12V battery when the vehicle is active and in ready mode. In the vast majority of cases, the 12V battery is not continuously supported simply because the vehicle is plugged in and charging. A car sitting on a driveway plugged into a home charger can therefore fill its main battery to 100% while its 12V battery drains.
Draining is the other half of the problem. Houston noted that electric cars are usually connected vehicles and may output data at rest, which means the 12V battery is discharging while the car is parked (Houston, in Fleet News, 2024). Telematics, app connections, alarm systems and sentry-style camera modes all draw on the same small battery.
Why the 12V battery fails early in an EV
Three factors can work against the 12V battery: a smaller battery, a heavier standby load, and a charging strategy that depends on software getting the wake-up cycles right. When the battery is repeatedly discharged more deeply than intended, it ages faster. Lead-acid batteries lose capacity with repeated deep discharge.
UK failure rates by fuel type
Which? asked 63,334 UK owners between April and June 2025 about faults in the previous twelve months. For cars aged up to four years, owners of full hybrids were four times as likely as petrol owners to report a 12V battery fault. Owners of electric and plug-in hybrid cars were more than twice as likely (Which?, 2026a).
| Fuel type (cars 0 to 4 years old) | Owners reporting a 12V battery fault |
|---|---|
| Full hybrid | 5.5% |
| Electric | 3.8% |
| Plug-in hybrid | 3.5% |
| Diesel | 1.8% |
| Petrol | 1.4% |
Breakdown data from the recovery organisations points the same way. The AA reported that the top third of EV breakdowns were the same faults as petrol and diesel cars, namely 12V battery problems and tyres (The AA, 2023). The RAC’s Nick Mullender said that electric vehicles face the same risk as internal combustion engines because they also depend on 12-volt batteries for starting (RAC, 2025).
The models with the most 12V faults
The averages hide a wide spread between manufacturers. In the Which? data, 16.1% of Hyundai electric car owners and 7.5% of Kia electric car owners reported a 12V fault, against 0.6% for Volvo, 1.5% for Tesla and Skoda and 1.6% for BMW (Which?, 2026a).
| Model (0 to 4 years old) | Owners reporting a 12V battery fault |
|---|---|
| Lexus LBX (full hybrid) | 29.2% |
| Hyundai Ioniq 5 (electric) | 27.3% |
| Toyota Yaris Cross (full hybrid) | 19.1% |
| Kia EV6 (electric) | 18.8% |
| Kia Soul EV (electric) | 18.8% |
Michael Passingham, the Which? researcher who wrote the report, gave the cause plainly. In Hyundai’s case it appears to be a software problem that is not keeping the batteries properly charged, or is draining the battery while the car is off. For Toyota and Lexus it appears to be an issue of parts quality (Which?, 2026a).
The ICCU fault and the UK recalls
The Hyundai and Kia problem has a name. The Integrated Charging Control Unit, or ICCU, is the unit used in the Hyundai Ioniq 5, Hyundai Ioniq 6, Kia EV6 and Genesis GV60 models to charge the 12V battery from the main battery.
When the ICCU fails, the 12V battery can stop receiving charge and gradually discharge. The vehicle can then enter a failsafe mode with a progressive reduction in motive power and, once the 12V battery is fully discharged, may not restart.
The UK recalls cover the Ioniq 5 and Ioniq 6 (R/2024/164), Kia EV6 (R/2024/197) and Genesis GV60 (R/2024/165). Hyundai and Genesis specify an ICCU software update followed by a check and, where necessary, replacement of the ICCU and fuse. Kia’s procedure first checks for DTC P1A9096: if it is absent, the ICCU software is updated; if it is present, the ICCU and fuse are replaced and the software version is checked (DVSA 2024a, 2024b, 2024c, 2024d).
The Driver and Vehicle Standards Agency has recorded three UK safety recalls for this fault.
| Vehicle | DVSA Reference | Date | Vehicles Affected | Defect Wording (DVSA) |
|---|---|---|---|---|
| Hyundai Ioniq 5 | R/2024/164 | 6 May 2024 | 16,297 | The ICCU may become defective and stop charging the 12 volt battery, which can result in the vehicle entering a failsafe driving mode |
| Hyundai Ioniq 6 | R/2024/164 | 6 May 2024 | 2,538 | The ICCU may become defective and stop charging the 12 volt battery, which can result in the vehicle entering a failsafe driving mode |
| Kia EV6 | R/2024/197 | 20 May 2024 | 4,499 | The ICCU may become damaged over time and may not be able to charge the 12-volt battery, which can discharge gradually while driving with a progressive reduction of motive power |
| Genesis GV60 | R/2024/165 | 22 July 2024 | 1,724 | The ICCU may become defective and stop charging the 12 volt battery, which can result in the vehicle entering a failsafe driving mode |
No UK recall for a 12V charging fault was found for the Nissan Leaf, MG4, Vauxhall Corsa-e or Tesla Model 3 at the time of writing.
The recall addresses the known failure mode. It does not, by itself, establish that the DC-DC converter in an individual car is delivering the correct voltage, or that a 12V battery already weakened by under-charging is still healthy. The annual EV and hybrid health check measures the converter output and the 12V battery together, which makes it a very useful check after a recall repair.
Can an electric car be jump-started?
Yes, in most cases. Which? states that the 12V battery in an electric car can usually be jump-started. It gives Hyundai’s Ioniq Electric and Kona Electric as examples of cars with conventional 12V auxiliary batteries that accept the standard procedure. Tesla owners are advised to call Tesla roadside assistance first (Which?, 2026c). The car’s handbook gives the location of the 12V battery or the jump terminals.
An electric car should not be used to jump-start another car. Which? states that an EV must never be used as the donor vehicle, because doing so can void the warranty and damage its electrical systems (Which?, 2026c). Although both vehicles may have a nominal 12V system, an EV’s 12V system is designed to power its own electronics and initialise the high-voltage system, not to supply the current required to start another vehicle.
A jump-start gets the car moving. It does not explain why the battery went flat, and a battery that has been fully discharged once is more likely to do it again.
WARNING:
An electric car can usually receive a 12V jump-start, but it should not be used to jump-start another vehicle.
How long does an EV 12V battery last and when to replace it
The RAC puts the life of a car battery at between three and five years. The signs of a failing battery are the same in any car: dimming lights, a battery warning light and electrical functions that misbehave (RAC, 2026).
No UK motoring body publishes a separate lifespan figure for the 12V battery in an electric car. The Which? data above does, however, show that 12V battery faults are significantly more common on some EV models than others.
Cold weather can make a weak 12V battery more likely to fail. The RAC states that lower temperatures and wintry conditions can significantly reduce a battery’s capacity to deliver power (RAC, 2026). On the first Monday of 2025, a quarter of all RAC breakdowns were flat batteries, and battery faults are the leading cause of RAC callouts across the year (RAC, 2025).
Two habits reduce the risk. The RAC advises electric car owners to take the car for occasional drives during long idle periods and to keep the high-voltage battery above 20% in winter (RAC, 2025). On EVs that recharge the 12V battery only when the vehicle is active and in ready mode, driving is important because the DC-DC converter is not continuously maintaining the 12V battery while the car is parked.
For an EV that does not maintain the 12V battery while parked, a long period of inactivity can therefore leave the car without enough 12V power to start, even though the high-voltage battery remains charged.
Other EVs use battery-management systems that can maintain the 12V battery while the vehicle is charging or parked, so the behaviour varies by make and model.
Replacement is not always a simple swap. On some models the 12V battery sits beneath trim close to orange high-voltage cabling. The HSE training rule described below applies to anything on that side of the car (HSE, 2026). A new battery fitted to a car with an unresolved charging fault may also fail again, because the underlying cause has not been addressed.
Which parts of an electric car are high-voltage?
The Health and Safety Executive defines dc voltages between 60 and 1500 volts in electric and hybrid vehicles as high voltage. It states that additional skills and training are necessary to work safely on these vehicles. High-voltage systems should be isolated and proven dead by testing before any work is undertaken (HSE, 2026).
In practice the boundary is easy to see. High-voltage cables are orange. The traction battery, the drive motor, the inverter, the on-board charger, the DC-DC converter, the heater and the air conditioning compressor all sit on the high-voltage side. The 12V battery and its black and red leads sit on the low-voltage side. An owner can check or jump the 12V battery. Everything on the orange side, including the DC-DC converter that charges the 12V battery, is workshop work.
What to do when the 12V battery dies
The sequence below assumes the car shows no life and the main battery is not empty.
- Check the main battery state of charge on the app or the last known reading. If it is above zero, the 12V battery is the first suspect.
- Check the owner’s handbook for the location of the manual door release and the 12V battery or jump terminals.
- Jump-start the 12V battery from a booster pack or another car, following the handbook, or call the breakdown provider. Never use the EV as the donor vehicle (Which?, 2026c).
- Check for an open recall on the GOV.UK recall checker using the registration. Ioniq 5, EV6 and GV60 owners should confirm that R/2024/164, R/2024/197 or R/2024/165 has been carried out (DVSA, 2024a, 2024b, 2024c, 2024d).
- Have the 12V battery load-tested and the DC-DC converter output measured before assuming the problem is solved. A battery that has been flattened once needs testing, not a second chance.
If the car needs to be jump-started repeatedly, the charging system should be checked rather than assuming the battery itself is the only problem. That is the point at which a diagnostic check can help identify the cause before the next breakdown.
Conclusion
The 12V battery in an electric car is small, works hard at rest, but it is essential to the vehicle’s low-voltage systems and to bringing the high-voltage system online. A flat 12V battery can therefore leave an EV completely unresponsive even when the main battery still has plenty of charge.
A single flat battery may need a jump-start to get the car moving, followed by a load test to establish whether the 12V battery is still healthy. If the battery goes flat again, the charging system and the vehicle’s low-voltage electrical system need to be investigated.
On the Ioniq 5, Ioniq 6, EV6 and GV60, that should include confirming that the relevant DVSA recall has been completed.
OGS Hybrid & EV Repair in North London tests the 12V battery and the DC-DC converter together, on every make, as part of the EV and hybrid high-voltage safety check. Owners whose car has gone flat more than once can book a diagnostic to find out whether the battery, its charging system, or another part of the low-voltage system is responsible.
Frequently Asked Questions
Why do electric cars have a 12V battery?
Electric cars use a 12V battery to power the dashboard, lights, locks, control units and the relays that connect the high-voltage battery, all of which must work before the main battery is live. The RAC confirms that most electric cars use a 12-volt battery for these systems, and the AA notes that EV 12V batteries are smaller because they never drive a starter motor.
What happens when the 12V battery dies in an electric car?
The car may become completely unresponsive. The screen can stay dark, the doors may not unlock, and the car will not shift into drive, even when the main battery is fully charged. The 12V battery powers the low-voltage systems needed to wake the vehicle and bring the high-voltage system online.
Can an electric car be jump-started?
Yes, in most cases the 12V battery in an electric car can be jump-started in the same way as a petrol car’s battery, using the terminals or jump points named in the handbook. Which? gives Hyundai’s Ioniq Electric and Kona Electric as examples, and advises Tesla owners to call Tesla roadside assistance first.
Can an electric car be used to jump-start another car?
No. An electric car should not be used to jump-start another vehicle. Although both vehicles may have a nominal 12V system, an EV’s 12V system is designed to power its own electronics and initialise the high-voltage system, not to supply the current required to start another vehicle. Which? warns that using an EV as a donor can void the warranty and damage its electrical systems.
Does the main battery charge the 12V battery while the car is parked or plugged in?
Usually not. The AA’s EV technical liaison engineer stated that DC-DC converters only charge the 12V battery when the vehicle is active in ready mode, and that in the vast majority of cases the 12V battery is not supported even while the car is charging. Connected cars also drain the 12V battery at rest by sending data.
Which electric cars have the most 12V battery problems in the UK?
In the Which? 2026 survey of 63,334 UK owners, the Hyundai Ioniq 5 had the highest 12V fault rate among electric cars at 27.3%, followed by the Kia EV6 and Kia Soul EV at 18.8%. Across brands, 16.1% of Hyundai EV owners and 7.5% of Kia EV owners reported a fault, against 0.6% for Volvo and 1.5% for Tesla.
Were the Hyundai Ioniq 5, Hyundai Ioniq 6 and Kia EV6 recalled in the UK for the 12V fault?
Yes. DVSA recall R/2024/164 covers 16,297 Hyundai Ioniq 5, and 2,538 Hyundai Ioniq 6; R/2024/197 covers 4,499 Kia EV6, both because the Integrated Charging Control Unit may stop charging the 12-volt battery. The Genesis GV60 is covered by R/2024/165. The remedy is a software update and, where a fault code is present, a new ICCU and fuse.
Does cold weather affect the 12V battery in an electric car?
Yes. The RAC states that lower temperatures significantly reduce a battery’s capacity to deliver power, and on the first Monday of 2025 a quarter of all RAC breakdowns were flat batteries. The RAC advises EV owners to drive the car occasionally during idle periods and keep the main battery above 20% in winter.
How long does an EV 12V battery last?
A 12V battery typically lasts around three to five years, although its actual lifespan depends on the vehicle, battery condition and how it is used. Cold weather, repeated discharge and charging problems can shorten its life. If a 12V battery has been fully discharged or has not been charged correctly, it should be tested under load rather than judged by age or voltage alone.