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Electric-mobility is a megatrend of the future, especially for the short term advantages. When you think of electric cars, you first think of the lithium-ion traction battery. The fact that every electric car is also equipped with a 12V lead-acid battery is often overlooked.


Lead-acid batteries are part of electric 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 400V! The first electric car with an onboard power system of 800V is the Porsche Taycan (Concept Car Mission E).


People often miss the fact that every electric car is also equipped with a 12V lead-acid battery without exception.

Even today, no electric vehicle can do without a lead-acid battery to supply the power supply of the electrical system. 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 12V consumers in the car.


With regard to technology, depending on the car manufacturer, conventional batteries are used as well as EFB or AGM batteries.

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 electrical system battery in an electric car

1. 12V power supply of electrical system

It goes without saying that a lot of consumers work with 12V 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). It’s apparent that these are not going anywhere as no e-car manufacturer wants to redevelop them, especially considering they don’t take a lot of effort. The 12V electrical system power supply is also set in electric cars. At the moment, the stability, availability and lower risk mean that a lead-acid battery is used in all electric cars for the 12V electrical system. This battery for the electrical system does not act as a starter, but supplies the 12V consumers in the vehicle and, in this way, also supplies the on-board computer, which ultimately controls the high-voltage battery.


2. Safety requires redundancy

The 12V battery for the electrical system 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 assistance systems are supplied with the 12V electrical system battery. The term "Adaptive Cruise Control (ACC)" has established itself in the international automotive industry. Here, the focus is on the safety aspects but also on increasing the driving comfort. Another aspect is the improvement of economic efficiency.



3. High-voltage safety

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 into operation, which draw all the energy from the 12V lead-acid-based electrical system battery.

If the 12V electrical system 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 a change of state

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. The door locking system and the sound system/radio continue to function when the high-voltage system is switched off.


Fun Fact: The 12V electrical system battery allows electric cars to be able to jump start cars with internal combustion engines without the high-voltage battery being connected to the car's power supply. If the jump start takes a long time, of course, it is helpful to activate the high-voltage battery and, in doing so, energise the converter. Otherwise, the 12V electrical system battery may discharge too much, as with jump starters for burners.

Replacing an electrical system battery


Depending on the electric car manufacturer, the electrical system battery must be replaced every 2-3 years during the annual servicing. It is THE safety-relevant component.

Assuming that the high-voltage system would be switched off on the motorway at night for safety reasons at a speed of 130 km/h, the electrical system battery, for example, must continue to reliably supply the vehicle lighting with energy!


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 electrical system battery.


There is a sequence of battery changes that must be observed. Otherwise, short circuits or flying sparks can damage the electronics.

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 identical housing and with 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 pop the bonnet (but do not open it yet), then close the car (let it close) and wait 20 minutes (with the keycard at a safe distance). Only then may you open the bonnet, of course without inserting the keycard. Please note: Many control units in the vehicle already go into standby, if the keycard is only in the proximity of the E-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.

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

Here’s our recommendation after a few test cases in practice: 

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 – now’s 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 shouldn’t be an error message and you can start the electric car as usual.

What else should you keep in mind? 


  • 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 – now’s 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 electrical system 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!
  • 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 only close to the vehicle! 



  • Please note the following when changing batteries without external power supply: Due to the brief absence of the 12V electrical system battery and the resulting voltage interruption, the values of the A and B trip meters and therefore also everything related to them, in particular the current average consumption and the amount of energy consumed since the last reset, are lost on some electric car models of the on-board computer. 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.

Dispose of a used battery correctly.

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