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.

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.