12V battery flat in an electric or hybrid car: causes and solutions

Your electric car or hybrid does not respond, even though the big battery is full. In most cases the cause is the small 12V battery. Below you can read why that battery goes flat, what you should and should not do, and how to prevent it.

By the team at Autobedrijf Willemsbrug, RDW-approved APK (MOT) test station since 1979 · Updated 8 November 2026

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The short answer

An electric car, a hybrid and a plug-in hybrid all have a 12V battery next to the big battery. The 12V battery starts up the car’s systems. If it is flat, the car will not start, even when the big battery is full. According to the ANWB, the Dutch motoring organisation, the 12V battery of a hybrid fairly often goes flat after two to three weeks of standing still, and a charging cable left plugged in for weeks can also drain it. A jump start is only possible in the way the owner’s manual of your car describes. Have the 12V battery tested afterwards.

Battery service at Autobedrijf Willemsbrug: we test the battery, the charging system and the starter motor first, and only replace what is needed. A new battery suits your car (AGM or EFB with start-stop) and we register it with the on-board computer if your car requires that. We tell you the price in advance based on your registration number. Call +31 10 479 00 00. You bring the car and we call you when it is ready. A loan bike is free while one is available. Saturdays too. Our team speaks English. More about our battery service

Why does an electric or hybrid car have a 12V battery?

The 12V battery switches the car on. The big battery, the high-voltage battery, supplies the power for the electric motor. But the computers, the locks, the screens, the lights and the switches that connect the high-voltage battery all run on 12 volts. The ANWB writes that the car needs the 12V battery to switch on the ignition and start up its systems, even before the car can charge.

While the car is switched on, the big battery keeps the 12V battery charged. When the car is off, that does not happen, or only now and then. If the 12V battery is then flat or worn out, a full high-voltage battery does not help. The car stays off.

The 12V battery can be a traditional lead-acid battery or a lithium-ion battery. Tesla, for example, calls the 12V battery of the Model 3 the low voltage (lithium-ion) battery. Which battery your car has and where it sits is in the owner’s manual. A hybrid has a high-voltage battery as well as the 12V battery. You can read more about that on our page about hybrid car service and repair.

How can you tell the 12V battery is flat?

You usually notice it when you get in: the car does not respond, or only partly. According to the ANWB, when an electric car breaks down, the problem is often not the big battery pack but the 12V battery. In 2025 the ANWB Wegenwacht, the ANWB’s roadside assistance service, helped electric drivers 82,000 times. The most common causes were flat tyres and problems with the 12V battery.

What you notice What may be wrong
The car does not respond to the key or key card The 12V battery has too little voltage to power the locks and the computers
The screens stay dark or start up slowly The 12V battery is (almost) flat
The car does not go into READY According to the ANWB, READY is not released if the voltage of the 12V battery is too low
A warning about the 12V battery or the electrical system The car detects that the 12V battery is weak or is not being charged properly
The car starts, but the warning keeps coming back The 12V battery is worn out and no longer holds its charge

Source: ANWB, Accuproblemen and Pech met de elektrische auto. Which warning your car shows is in the owner’s manual.

Why does the 12V battery of an electric or hybrid car go flat?

The main cause is the car standing unused. A car that is switched off still uses power for the alarm, connectivity and the computers. These are possible causes:

  • Standing still for a long time: according to the ANWB, a flat 12V battery in a hybrid fairly often happens after two to three weeks of standing still, especially if the car is not used much.
  • Charging cable plugged in for weeks: the ANWB writes that communication between the car and the charging point stays active. That draws power from the 12V battery, for example if you go on holiday and leave the cable connected.
  • A worn 12V battery: the owner’s manual of the Hyundai IONIQ 5 states that the 12V battery can still go flat, despite the monitoring function, if it is damaged or worn.
  • Extra equipment: according to the same manual, the battery also goes flat if you use it as a power source or if equipment that the manufacturer has not approved is connected to it.
  • Cold: according to the ANWB, cold weather puts more strain on a battery. In winter, a weak 12V battery is the first to struggle.

Some cars monitor the 12V battery themselves. Hyundai calls this Aux. Battery Saver+: the car checks the charge and tops up the 12V battery from the high-voltage battery when needed. Such a function cannot save a worn 12V battery.

Jump starting an electric or hybrid car: what can and cannot be done?

Many electric cars can receive a jump start, but only in the way the owner’s manual describes. According to the ANWB, an electric car can get a jump start from a petrol or diesel car. The other way round, using your electric car to jump start another car, is often not possible. In the Model 3 owner’s manual, Tesla writes that the car cannot be used to jump start another vehicle, because that can cause damage.

The connection points differ from car to car. On the Tesla Model 3 there are separate jump start terminals under the bonnet. On other cars the 12V battery sits in the boot or behind a trim panel. So follow the steps in the owner’s manual, not general instructions written for petrol cars.

Take care with jump starting. According to the ANWB, incorrect handling can damage the electronics, especially on modern cars. If in doubt, call the Wegenwacht or your breakdown service. Do not touch the orange high-voltage cables.

Can I still drive with a weak 12V battery?

That depends on what the car does. This is the rule of thumb we use:

  • After a jump start the car goes into READY and shows no red warning: you can drive. Drive home or to the garage and have the 12V battery tested. Next time the car may not start again.
  • The car starts, but a warning about the 12V battery or the electrical system stays on: according to the ANWB, you must stop if a warning light is red. With a yellow or orange light you can carry on for a while. Look up in the owner’s manual what the warning means.
  • The car does not go into READY, not even after a jump start: do not keep trying. Call the Wegenwacht or your breakdown service. There may be more wrong than a flat battery.

If the car will not start, your breakdown service, the Wegenwacht or a recovery company can bring it to us. Outside our opening hours that is possible at your own risk: park the car in front of the garage and put the key through the letterbox. A car that will not start goes into our schedule as soon as possible.

How we test and replace the 12V battery

We first test whether it really is the 12V battery. A flat battery can also be the result of something else. This is how it works on an electric car or hybrid:

  1. Battery test: with a battery tester we measure the charge and the condition of the 12V battery.
  2. Charging check: we check whether the car charges the 12V battery properly from the high-voltage battery.
  3. Diagnostics: with diagnostic equipment we read out the fault codes and warnings.
  4. Replacement: if the battery is worn out, we fit a battery that suits your car and register it with the on-board computer if your car requires that. We dispose of the old battery.

Our mechanics hold the GMTO certificate, the Dutch qualification for working on high-voltage systems in hybrid and electric cars. They know how to work safely on a car with a high-voltage system. We tell you the price in advance, as one amount based on your registration number (kenteken), and we only replace parts with your approval. You can read more about testing and replacement on our page about car battery testing in Rotterdam. What other maintenance an electric car needs is covered in our guide to servicing an electric car.

How do you prevent a flat 12V battery?

Drive regularly and do not leave the car standing for weeks without preparing it. These points help:

  • Drive a reasonable distance regularly, especially if the car would otherwise stand still for more than two weeks. While you drive, the car tops up the 12V battery.
  • Do not leave the charging cable plugged in for weeks when you go on holiday. That is the ANWB’s advice. Check the owner’s manual for what your manufacturer recommends when the car stands still for a long time.
  • Switch off or unplug devices you have connected yourself, such as a charger in the 12V socket.
  • If the 12V battery warning keeps coming back, have the battery tested before the car stops working.
  • If the car will stand still for longer, also read our article car left standing for a long time: what should you check?

Frequently asked questions

Does a plug-in hybrid also have a 12V battery?

Yes. According to the ANWB, a hybrid, a plug-in hybrid and a fully electric car also have a 12V battery. It switches the car on and powers the computers, the locks and the lights.

Is the big battery broken if my electric car will not start?

No, in most cases it is not. According to the ANWB, the problem is often not the big battery pack but the 12V battery. If that is flat, the car will not start, even when the big battery pack is full.

Doesn’t the big battery charge the 12V battery automatically?

Yes, but only when the car is switched on or charging. According to the ANWB, the big battery pack charges the 12V battery when the ignition is on. If the 12V battery is flat, the car can no longer switch on the big pack.

Can I use my electric car to jump start another car?

No. With many electric cars that is not possible. According to the ANWB, giving a jump start with an electric car is often not possible. Tesla writes that the Model 3 cannot be used to jump start another vehicle, because that can cause damage.

What should I do with the charging cable when I go on holiday?

According to the ANWB, a charging cable left plugged in for weeks can drain the 12V battery, because the car and the charging point keep communicating. Check the owner’s manual for what your manufacturer recommends when the car stands still for a long time.

Is the 12V battery tested during the APK?

No. This is the compulsory Dutch vehicle inspection (APK, comparable to the MOT). According to the APK manual of the RDW, the Dutch vehicle authority (article 5.2.12), the inspector only checks that the battery is properly secured. He does not measure the charge or the condition of the battery. You can read more about the test in our article on the APK for an electric or hybrid car.

Will you come out if my electric car will not start?

No. We are not a breakdown service and we do not give jump starts at your home. The Wegenwacht, your breakdown service or a recovery company can bring the car to us. A car that will not start goes into our schedule as soon as possible.

Do you replace the 12V battery of a hybrid or electric car?

Yes. Our mechanics hold the GMTO certificate for hybrid and electric cars. We first test whether it really is the 12V battery. We tell you the price in advance, as one amount based on your registration number.

My electric car will not charge: what can I check myself?

According to the ANWB (consulted November 2026), first check whether you used the right charge card, whether the cable is firmly connected to the car and the charging point and is not damaged, and whether the charging settings are correct, for example delayed charging. Do not use a damaged charging cable any more. Also try another charging point. If the car still does not charge, the cause may be in the car. Call us with your registration number first, and we will tell you straight away whether we can help. If the car does not respond at all, the cause is often the 12V battery. We test and replace it.

Electric car or hybrid won’t start?

Call +31 10 479 00 00 with your registration number. We first test whether it is the 12V battery and we tell you the cost in advance.

You will find us at Boterdiep 11 in Rotterdam-IJsselmonde, close to Stadion Feijenoord (De Kuip). We work by appointment. For a quick question or check you can walk in without an appointment.

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