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Get into e-mobility and just drive off - sounds easy, but often it is not.

What do you need to know about the 12V starter and on-board battery?


The Banner experts have briefly noted down a lot that is worth knowing.

1. Charging

Everything important is noted in the article "HOW-TO: Charging Car Batteries".

There are many small causes that contribute to battery failure, such as short trips or additional consumers.


The general rule for battery charging is

Always check the open-circuit voltage with a battery tester or voltmeter. Please ensure that the battery is immediately recharged as soon as the open-circuit voltage limit of 12.5V has been reached. To measure the open-circuit voltage with a battery tester or voltmeter, wait until approx. 5 hours after completion of charging (internally by alternator, externally by charger) or do not discharge for at least 1 hour.

2. Jump starting. Do not jump-start an electric car!

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.

In the "HOW-TO article on jump starting" we show you what you should pay attention to when jump starting a car.


3. Jump-starting - simply push the car.

Do NOT push electric cars! (BEV Battery Electric Vehicle) This cannot work, there is no mechanical clutch! Please also note: The high-voltage system cannot be activated by pushing the car.

4. Battery change

Everything important is noted in the "Basic article on e-mobility". Depending on the e-car manufacturer, the on-board battery must be replaced every 2-3 years during the annual service. It is THE safety-relevant component. Assuming that at night on the motorway at speed 130 the high-voltage system would be switched off for safety reasons, the back-up battery must continue to reliably supply the vehicle lighting with energy, 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.


When replacing the battery without an external power supply, please note: Due to the brief absence of the 12 V on-board battery and the resulting voltage interruption, the on-board computer of some e-car models loses the values of the 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. You should make a note of these values beforehand if you need them. After the battery replacement, the on-board computer is reset and starts again to determine the average consumption and to individualise the range forecast.

5. Briefly about the open day

The following situation is not at all uncommon: When charging the Li-Ion drive battery, the car doors remain open, the heating or interior lights remain 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.


Another banner tip: Take the electric or hybrid car to the workshop for service or repair, then please leave a side window open in the workshop. 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, the workshop visit will now take a little longer than planned.

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)!