E-mobility is a megatrend of the future, especially for short distances. When people think of electric cars, they first think of the lithium-ion drive battery, but they tend to overlook the fact that 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!


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

Because, as before, no electric vehicle can do without a lead-acid battery to support and supply the on-board network. 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

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

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

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. Carrying 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.


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 e-car manufacturer, the 12V starter and on-board power supply battery 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.

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), 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)!


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 on-board battery by its housing shape and the two terminals to which cables are connected. The terminals of batteries are often coloured red and blue or red and black. The positive terminal is always marked red.

Banner tip: 

We recommend without exception the use of a voltage maintenance device before disconnecting and removing the old battery. This reliably prevents the loss of vehicle data. Through a power supply via the OBD (On Board Diagnostic) connector in the vehicle. 

This way, vehicle data such as navigation settings, telephone book, radio stations, electric seat position, … are retained when the battery is changed. What’s more, this procedure often saves the subsequent deletion of the “voltage interruption” entry in the fault memory.

When changing the battery, it is still important to ensure that the old battery is replaced by an equivalent battery in terms of technology, power class and size.

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

Always disconnect the negative terminal first. Otherwise, flying sparks or even a short circuit may occur.

Proceed as follows:

  • Disconnect the on-board battery from the high-voltage battery. In most cases a plug connection must be pulled apart.
  • 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 present, remove the degassing tube together with the elbow from the battery.
  • If the battery is now exposed, you can remove it.
  • Caution: An electrical system battery weighs between 3 and 15 kilograms. This must be taken into account when replacing the battery.


2. Install the new battery


Once you have removed the old battery, the next step is to install the new one properly. 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 to do it. Connect the on-board battery to the high-voltage battery, usually by pressing a connector together.


Additional tip: 

  • Fit any required terminal adapters from the old to the new battery or replace 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 installed in the interior/passenger compartment.
  • Some vehicles are provided with a tube with an attached angle piece to discharge the battery gases. If this applies to your vehicle, the tube must 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 when the work is finished.


Close the front bonnet, open the car, remove the voltage maintenance device and check the on-board computer display. Normally, no error message appears and you can start the electric car as usual.


 If, in exceptional cases, registration of the new battery in the Battery Energy Management System (BEM) is required, this can be done with our BBST Banner Battery Service Tool.

What else should you keep in mind? 


  • It is essential that you observe the instructions on the battery itself, in 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. This way, when you install it correctly, you ensure that the back-up battery cannot slip 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 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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