Electromobility is a megatrend of the future, especially on short and medium-haul routes. When people think of electric cars, they first think of the lithium-ion drive battery, but they tend to overlook the fact that (almost) every electric car is also equipped with a 12 V lead-acid back-up battery.

The back-up battery is also colloquially referred to as the backup, on-board power supply, auxiliary or secondary battery.

The lithium-ion traction battery is also known as a high-voltage battery (HV). 


Lead-acid batteries are part of e-mobility.

When you think of electric cars, the first thing you think of is the built-in high-voltage traction battery with lithium-ion technology, often with rated voltages from 288 to well over 400 V. The first electric car with an onboard power system of 800 V is the Porsche Taycan (Concept Car Mission E). Formula E relies on a 900 V system!


It is often overlooked that (almost) every electric car is also equipped with a 12V lead-acid battery.

After all, hardly any electric vehicle can still manage without a lead-acid battery to support and supply the on-board power supply. In addition to start-ups that are looking toward the future, renowned vehicle manufacturers such as the BMW Group also rely on a lead-acid battery for the power supply of the electrical system. The electrical system of the purely electrically powered BMW i3, for example, is stabilised with a Banner lead-acid battery, which supplies the 12 V consumers in the car.


In terms of technology, conventional batteries, but also EFB or AGM batteries are used, depending on the electric car manufacturer.

EFB = Enhanced Flooded Battery, the cycle-resistant starter battery
AGM = Absorbent Glass Mat, the acid is absorbed in a glass mat, meaning that it cannot leak

Main tasks of the back-up battery in an electric car

1. 12 V power supply of the electrical system - when the electric car is switched off and while driving

It goes without saying that a lot of consumers work with 12 V voltage in electric cars as well (e.g. central locking, interior light, tools, etc.). These have been perfected over decades and are used every year in millions of new cars (combustion engines and electric cars). Clearly no e-car manufacturer wants to redevelop them, especially as they don’t consume a lot of power. The 12 V electrical system power supply is therefore also used in electric cars. At the moment, the stability, availability and lower risk mean that a lead-acid battery is used for the 12 V electrical system in all electric cars. This battery for the electrical system does not act as a starter, but supplies the 12 V consumers in the vehicle and thus also supplies the on-board computer, which ultimately controls the high-voltage battery.


2. Safety requires redundancy- when the electric car is switched off

This is because the 12 V back-up battery also serves as a buffer to create redundancy for functional safety aspects. For example, it must be ensured that the steering assistance (power steering) of the electric car continues to function when the engine is switched off so that manoeuvrability is maintained. It is not only in electric cars that the power steering systems of today usually operate electrically.

Furthermore, the driver assist systems are supplied with power from the 12 V back-up battery. The English term “Adaptive Cruise Control (ACC)” has become established in the international automotive industry. Here, the focus is often on safety aspects, but also on increasing driving comfort. Another aspect is the improvement of economic efficiency.



3. High-voltage safety - when the electric car is switched off

Before the high-voltage battery can be accessed, high-voltage safety must be ensured. In order for this to be tested, control units must be in operation. These draw all their energy from the 12 V lead-acid-based electrical system battery.

If the 12 V back-up battery is discharged, the on-board computer cannot start up. If this occurs, the frequency converter is not controlled and starting up is not possible.


4. Carry out staus changes - in the event of failure of the high-voltage system

A second source of energy in the vehicle is necessary to reach the safe state “vehicle stationary” from the safe state "high voltage off". This requires: windscreen wipers, lights, brakes, steering. … this is a basic safety requirement.

All these functions must be guaranteed so that the high-voltage system can be switched off at full speed in the event of an emergency. Many safety-relevant components continue to be supplied with power, for example dipped headlights, audio, autopilot and safety systems, all motors that are not part of the drive - window lifters, windscreen wipers, side mirrors, power steering, seat motors, ...; furthermore brake boosters, interior lights, instrument clusters including main display, power steering and some more.


5. do not jump start with electric car! (BEV Battery Electric Vehicle)

For technical reasons, the jump-start points of electric cars, if any, are not designed to jump-start other vehicles. In this case, it is better to seek the help of a breakdown service. Conversely, however, an internal combustion engine may properly jump-start an electric car.


A brief word about the electric car open day.

The following situation is not uncommon: When charging the Li-ion drive battery, the car doors are left open, the heating or interior lighting is left on, then the 12V on-board battery can discharge before the Li-ion drive battery is fully charged. Result? The charging plug can no longer be disconnected!

Banner tip: This function is controlled via the vehicle electrical system, which, as the name suggests, is also connected to the vehicle electrical system battery. 

Good to know: Depending on the electric car manufacturer, the 12V starter and on-board power supply battery is charged during the journey by recuperation (brake energy recovery) and is always charged during the charging process of the high-voltage battery - i.e. regardless of the state of charge (SOC State Of Charge) - or only when the high-voltage battery is charged to approx. 80%. A look at the operating instructions of the respective electric car can be really helpful. Some electric car manufacturers recommend not charging the high-voltage battery to 100%, which could be a possible cause of partially or deeply discharged 12V starter and on-board batteries.

Replacing a back-up battery


The back-up battery must be replaced every 2-3 years during the annual service, depending on the e-car manufacturer. It is THE safety-relevant component.

Assuming that the high-voltage system were switched off for safety reasons while travelling at night on the motorway at 130 km/h, the back-up battery must continue to reliably supply energy to the vehicle lighting, for example.


At the latest when the error message „Check ELEC System“ (= check electrical/electronic system) appears on the on-board computer display, often in combination with the red battery warning light, it is high time to think about replacing the back-up battery.

Please note:

Never install a conventional wet-cell battery in an electric vehicle that is equipped with an EFB or AGM battery as standard. If necessary, an EFB or AGM battery must be used again. In an identical housing and of a similar performance class.

PS: Minor deviations in capacity or performance during cold start have no effect on the safe and optimum power supply of the electrical system. 

Fahrzeug Motorraum



  • The electric car must be in an electric/electronic „deep sleep“ so that it does not issue error messages.
  • First unlock the bonnet (but do not open it yet), leave at least one side window open*, then close (leave) the car and wait (with the keycard at a safe distance) for about 20 minutes. Only then open the bonnet, of course without the keycard inserted. Please note: Many control units in the vehicle already go into standby simply when the keycard is near the electric car.
  • Search for the electrical system battery or take this information from the vehicle operating manual. The battery is not always installed at the front of the vehicle, but can also be located in the interior/glove compartment or in the boot. However, this does not affect the procedure for changing the battery.
  • The battery in many cars is fitted with a plastic cover. Remove the cover.
  • Another highly exciting hint: Now disconnect the backup battery from the on-board power supply, usually one (orange) plug connection has to be pulled apart, the high-voltage system is thus deactivated! Do not forget: After replacing the battery, plug the connector back together. Otherwise, the general rule is: Keep your hands away from high-voltage components and cables (recognisable by their orange colour)! This is reserved for motor vehicle specialists with the appropriate equipment and additional qualifications (HV high-voltage training). No matter whether it is non-electrotechnical, electrotechnical work or electrotechnical work under voltage (often with nominal voltages of 288 to over 800V)!


*Even better: leave a door propped open so that you can get back into the electric car if necessary. Why? Depending on the repair, the 12V starter and on-board battery could be disconnected. If this happens without external voltage maintenance, the doors could remain locked depending on the car model! In the worst case scenario, the workshop visit may take a little longer than planned. This information also applies to DIY (do it yourself) service and repair work.

This is how you replace the battery in an electric car.

Here’s our recommendation after a few real-life test cases: 

1. Remove the old battery


You can recognise the electrical system battery by its box shape and the two terminals, which both have a cable connected to them. With newer batteries, the terminals are often coloured red and blue or red and black  the positive terminal is always marked red.

Banner Tip: 

We recommend, without exception, that before disconnecting and removing the used battery, you use the Banner Memory Saver. This reliably prevents the loss of vehicle data. Through a power supply via the OBD connector (On Board Diagnostic) in the vehicle. 

When the battery is changed, vehicle data such as navigation settings, telephone book, radio station and electric seat position are … retained.

Which terminal do I attach the cable to first  positive or negative?

Always disconnect the negative terminal first (the earth cable usually hangs onto the negative terminal)! Otherwise, flying sparks or even a short circuit may occur.

Proceed as follows:

  • Unscrew the nut from the negative terminal and disconnect the black connecting cable.
  • Loosen the nut from the positive terminal to remove the red cable.
  • Loosen the screws on the holder system.
  • If the battery is now exposed, you can remove it.
  • Caution: An electrical system battery weighs between 5 and 15 kilograms. This must be taken into account when replacing the battery.


2. Install the new battery


If you have removed the used battery, you should now properly insert the new one. To do this, insert the new battery into the battery compartment. You should then secure the battery using the holding system and tighten the holding screws. Now you can connect the new battery.

3. Connect the new battery


In order to connect the new battery, it is important that you proceed in reverse order. In keeping with this, first attach the red cable to the positive terminal and tighten the nut. Then fasten the black cable to the negative terminal and tighten nut and screw here as well. Now use the terminal grease or terminal spray  nows the right time!


Additional tip: 

  • Replace any existing terminal covers! By doing this, the terminal can be safely covered and protected against short circuits.
  • We recommend the use of a degassing tube for all lead-acid batteries which are installed in the interior/glove compartment.
  • Some vehicles are provided with a tube, with an attached angle piece, for the discharge of the battery gases. If this applies to your vehicle, the tube shall be inserted via the angle piece into the corresponding degassing opening of the battery. If there is a degassing opening on the other side, it must be closed with a sealing plug!
  • Finally, you can replace the battery cover.


4. Close the bonnet after the work is complete


Close the bonnet (or close the boot), open the car and check the electrical system computer display. There shouldnt be an error message and you can start the electric car as usual.

  • It is essential that you observe the instructions on the battery itself as well as the operating instructions and the instructions in the vehicle operating manual. 
  • Always wear protective goggles and gloves when replacing batteries!
  • A holding system makes sense. When correctly installed, this ensures that the electrical system battery cannot slip out of place or fall out.
  • If the battery terminals are corroded, they should be cleaned before installing the new battery. For this you can use a brush or a small wire brush. You can now use the  terminal grease or terminal spray  nows the right time!



  • Make sure that the two terminals are never connected to each other. A resulting short-circuit should have happened by mistake several times.
  • The back-up battery is not always installed at the front, but can also be located in the interior/passenger compartment  or luggage compartment. However, this does not affect the procedure for changing the battery.
  • Fire, sparks, open light and smoking are prohibited during this work.
  • For vehicles with “Keyless Go” systems, it is important to ensure that many control units in the vehicle are already ready when the keycard or key is near the vehicle. 



  • When changing the battery without an external power supply, please note: Due to the short-term absence of the 12 V back-up battery and the resulting voltage interruption, the on-board computer of some e-car models loses the values of trip meters A and B and thus also everything related to them, in particular the current average consumption and the amount of energy consumed since the last reset. If necessary, these values should be noted down beforehand if you need them. After the battery has been replaced, the on-board computer is reset and starts up again in order to determine the average consumption and individualise the forecasts of reach.


Otherwise, the general rule is: Hands off high-voltage components and cables (recognisable by their orange colour)! This is reserved for automotive specialists with the appropriate equipment and additional qualifications (HV high-voltage training). No matter whether it is non-electrotechnical, electrotechnical work or electrotechnical work under voltage (often with nominal voltages of 288 to over 800V)!

More detailed information will be provided later during testing by the author and will be continuously updated for you.


Dispose of a used battery correctly.

As a consumer, you are able to return the old back-up battery free of charge to any Banner sales partner, regardless of the brand. In this way, we can guarantee environmentally friendly recycling.

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